Declaration of originality · Web viewThe outcome of the experiment revealed that the compressive...

MODIFICATION AND MODELLING OF OIL WELL CEMENTITIOUS

SYSTEM PERFORMANCE

NOSIKE .N, AMUCHEAZI

A submission presented in partial fulfilment of the requirement of the

University of South Wales for the degree of Doctor of Philosophy

PhD. 2018

Declaration of originality

Declaration of Originality

This is to declare that except where specific reference is made, the work described in this

thesis is the result of work carried out by the candidate. Neither this thesis, nor any part of it

has been presented or is currently submitted in candidature for any other degree at any other

university.

............................................................. ...............................

Nosike N Amucheazi (Date)

(Candidate)

............................................................. ...............................

Dr Daphne O’Doherty (Date)

(Director of studies/supervisor)

............................................................. ...............................

Dr Dr Paul Davies (Date)

(Director of studies/supervisor 2)

N.N Amucheazi (2018) University of South Wales ii

Acknowledgement

Acknowledgement

Firstly, I will like to express my utmost gratitude to God for the wisdom, knowledge and

guidance He offered me during the period of this research programme and especially during

the tedious times.

Secondly, I acknowledge and thank the management and employees of the various

organisations (Cebo Ltd, UK and Impact fluid Ltd, UK) that assisted me in the course of this

research and also, the staff and students of the University of South Wales who participated

immensely in this research.

My special appreciation to my Directors of Studies/Supervisors (Dr Daphne O’Doherty and

Dr P.Davies), my tutors (Prof J.Kinuthia, Prof Abid Abu-Tair, Prof J.Khatib, Dr J.Oti, Dr

J.Bai, Dr F.Hunt, Dr R Robinson) and the Graduate Research Office staff of the University of

South Wales for their extreme support, direction and advice that was useful to accomplish

this research. Additonally, I appreciate the tutors in other departments of the University of

South Wales and other institutions that assited me in this research.

Furthermore, I thank the laboratory officers and my collegues and friends in the University of

South Wales and in other Institutions and places that helped me in various forms during the

period of this research programme.

Finally, this acknowledgement will not be complete without a huge gratitude from me to my

parent and entire family for their tremendous encouragement, prayers and support to see that

my research programme was successful.

God blessings on each and everyone for the contribution and participation to achieve this

research programme.

Thank you all

N.N Amucheazi (2018) University of South Wales iii

Abstract

Abstract The formulation of an oil well cementitious system is in general more complex than that of

conventional cementitious systems for constructing roads, dams, houses, and bridges. To

cement an oil well, various types of cement and admixtures are employed in formulating the

oil well cementitious system. This is to achieve satisfactory performance that will contend

with geological conditions and render required functions in the oil well. However, the

pursued performance is dependent on the properties and dosages of the materials used in

formulating the system, thereby making it a difficult task. Thus, the formulation of an oil well

cementitious system requires several tests, materials, time and cost in order to achieve

satisfactory performance for cementing an oil well. Additionally, there is a growing pressure

for the use of environmental friendly technologies across the globe due to environmental

issues. Portland cement types conventionally used for oil well cementing is one of such

polluting and devastating technology to the environment. The production of Portland cement

entails the use of high energy, natural resources and release of carbon dioxide (CO2) which

contributes to global warming and affects the environment. As such; alternatives to reduce

the adverse impact of cement to the environment are demanded of organisations, prompting

the need to pursue desired performance of oil well cementitious systems for oil well

contructions while also addressing the environmental concern.

The present research set out to achieve the desired performance of an oil well cementitious

system through modification using admixtures and modelling using a mathematical approach.

The research study addresses the mechanisms that influence the performance of oil well

cementitious system, the impact of admixtures (mineral and chemical admixtures) and the

potential of a mathematical model to predict the performance of oil well cementitious system.

The modification of the oil well cementitous system using particular admixtures which

include Metakoalin, Pulverised fuel ash and Superplasticizer and their combinations which is

a new attempt in the modification of oil well cementious systems offers both performance

and environmental benefits for cementing in the construction of oil wells. The development

of a predictive model, which is a key innovation in the current research, endeavours to

simplify and reduce the time, number of tests, materials and cost to formulate and achieve an

oil well cementitous system of desired performance for well construction. .

N.N Amucheazi (2018) University of South Wales iv

Abbreviations and symbols

List of AbbreviationsItem Definition

A Area of cube (m2)AAD Average absolute deviationACI American concrete instituteANOVA Analysis of variance

API American Petroleum Institute

API RP American institute of petroleum recommended practice

ASTM American society for testing and materials

BaSO4 Barium sulphate (Barite)

BS British standard

BS EN Bristish standard European Norm

C Celsius

C2S Dicalcium silicate

C3A Tricalcium aluminate

C3S Tricalcium silicate

C4AF Tetracalcium aluminoferrite

Ca(OH)2 Calcium hydroxide

CaCO3 Calicium carbonate

C-A-H Calcium aluminate hydrate

C-A-S-H Calcium aluminate silicate hydrate

CO2 Carbon dioxide

COV Coefficient of variance

CS Compressive strength (MPa)

C-S-H Calcium silicate hydrate

Ds Density of cementitious system (kg/m3)

dt°C Derivative temperature in Celsius

dw% Derivative weight in percentage

Fc Load applied (N)

Ft Load at fracture (N)

N.N Amucheazi (2018) University of South Wales v


G Gauge length (mm)

GGBS Ground granulated blast furnace slag

GxDifference between specimen length from gauge length at

specific day (mm)

H2CO3 Cabonic acid

HSR High Sulphate Resistance

ISO International organisation for standardization

LOI Loss of ignition

Lx Length change of specimen (mm)

MK Metakaolin

MsMass of cementitious system (cement, admixture, water)

(Kg)

MSR Moderate sulphate resistant

NaCl Sodium chloride

OC Oil well cement

OPC Ordinary Portland cement

PCA Portland cement association

PFA Pulverised Fuel ash

RPM Revolution per minute

MA Mineral admixture

SD Standard deviation

SF Silica fume

SP Super plasticizer

ST Setting time

T Temperature

TS Tensile strength (MPa)

UN United Nation

VsVolume of cementitious system (cement, admixture, water)

(m3)

N.N Amucheazi (2018) University of South Wales vi


w/c Water to cement ratio

WA Water absorption

Wd Weight of dry specimen (g)

WHO World health organisation

Ww Weight of wet specimen(g)

N.N Amucheazi (2018) University of South Wales vii

Contents

ContentsDeclaration of Originality....................................................................................................................... ii

Acknowledgement................................................................................................................................ iii

Abstract................................................................................................................................................. iv

List of Abbreviations.............................................................................................................................v

List of Figures.......................................................................................................................................xii

List of Table..........................................................................................................................................xv

CHAPTER 1: INTRODUCTION..........................................................................................................1

1.1 Background..................................................................................................................................1

1.1.1 Statement of problem..............................................................................................................2

1.1.2 Aims and objectives..............................................................................................................4

1.2 Contribution to knowledge..........................................................................................................5

1.3 Beneficiaries................................................................................................................................7

1.4 Structure of thesis.......................................................................................................................7

CHAPTER 2: LITERATURE REVIEW.......................................................................................................10

2.1 Oil Well Cementing....................................................................................................................10

2.2 Oil Well Cement.........................................................................................................................14

2.2.1 Challenges and Classes of Oil Well Cement........................................................................16

2.3 Admixtures................................................................................................................................21

2.3.1 Chemical admixture............................................................................................................21

2.3.2 Mineral admixtures.............................................................................................................24

2.4. OC system & Admixture Performance......................................................................................27

2.4.1 Density of OC Systems........................................................................................................28

2.4.2 Rheology of OC systems......................................................................................................29

2.4.3 Setting time of OC systems.................................................................................................32

2.4.4 Strength development of OC Systems................................................................................36

2.4.5 Porosity of OC systems.......................................................................................................38

2.4.6 Expansion and Shrinkage of OC systems.............................................................................40

2.4.7 Hydration products and performance of OC systems.........................................................43

2.5 Temperature and OC system performance................................................................................45

N.N Amucheazi (2018) University of South Wales viii

Contents

2.6 Modelling and OC system performance....................................................................................48

CHAPTER 3- MATERIALS......................................................................................................................51

3.1 Cements.....................................................................................................................................51

3.2 Metakaolin (MK)........................................................................................................................52

3.3 Pulverised Fuel Ash (PFA)..........................................................................................................53

3.4 Super-plasticizer (SP).................................................................................................................56

3.5 Water.........................................................................................................................................56

CHAPTER 4: METHODLOGY..................................................................................................................58

4.1 Tests approach...........................................................................................................................58

4.2 Mix design..................................................................................................................................60

4.3 Specimen preparation...............................................................................................................61

4.3 Testing.......................................................................................................................................62

4.3.1 Density................................................................................................................................62

4.3.2 Rheology.............................................................................................................................63

4.3.3 Setting time.........................................................................................................................64

4.3.4 Strength development........................................................................................................65

4.3.5 Water absorption test.........................................................................................................67

4.3.6 Shrinkage test.....................................................................................................................68

4.3.7 Thermo-gravimetric analysis...............................................................................................69

4.3.8 Model development...........................................................................................................70

CHAPTER 5: OC SYSTEM MODIFICATION AND ANALYSIS....................................................73

5.1 Density.......................................................................................................................................73

5.1.1 Density of class G OC system..............................................................................................73

5.1.2 Density of modified OC system...........................................................................................75

5.2 Rheology....................................................................................................................................76

5.2.1 Rheology of Class G OC system...........................................................................................77

5.2.2 Rheology of modified OC system........................................................................................79

5.3 Setting time................................................................................................................................81

5.3.1 Setting time of Class G OC system.......................................................................................81

5.3.2 Setting times of modified OC system..................................................................................83

5.4 Strength development...............................................................................................................84

N.N Amucheazi (2018) University of South Wales ix

Contents

5.4.1 Compressive strength development of Class G OC system.................................................85

5.4.2 Compressive strength development of modified OC system..............................................87

5.4.3 Tensile Strength development of Class G OC system..........................................................92

5.4.4 Tensile Strength development of modified OC system.......................................................94

5.5 Water absorption.......................................................................................................................98

5.5.1 Water absorption of Class G OC system..............................................................................98

5.5.2 Water absorption of modified OC system.........................................................................101

5.6 Shrinkage.................................................................................................................................105

5.6.1 Shrinkage of Class G OC system........................................................................................105

5.6.2 Shrinkage of modified OC system.....................................................................................107

5.7 Hydrate products.....................................................................................................................111

5.7.1 Hydrate products of class G OC system............................................................................111

5.7.2 Hydrate products of modified OC system.........................................................................114

5.8 Performance evaluation and selection of OC system..............................................................119

5.9 The effect of temperature on the setting time and strength performance of an OC system...123

5.9.1 Impact of temperature on the setting time of OC systems...............................................124

5.9.2 Impact of temperature on compressive strength of OC system.......................................126

5.10 Effect of chemical admixture on the setting time and strength performance of OC system...................................................................................................................................................127

5.10.1 Influence of chemical admixture dosage on setting time of OC system..........................128

5.10.2 Influence of chemical admixture on the compressive strength of OC system................131

CHAPTER 6: MODELLING............................................................................................................135

6.1 Model validation..................................................................................................................135

6.2 Model Confidence................................................................................................................139

6.3 Model accuracy....................................................................................................................143

CHAPTER 7: DISCUSSION...................................................................................................................158

7.1 Variations in density of the various OC system........................................................................158

7.2 Variations in rheological properties of the various OC system................................................159

7.3 Variations in setting times of the various OC system...............................................................162

7.4 Variations in strength performance of the various OC system................................................165

7.5 Variations in water absorption of the various OC system........................................................169

N.N Amucheazi (2018) University of South Wales x

Contents

7.6 Variations in shrinkage of the various OC system....................................................................172

7.7 Variations of hydrate product content of the various OC system............................................174

7.8 Variations of OC system performance with temperature and chemical admixture.................177

7.9 Variations in the developed models for the OC systems.........................................................181

7.10 Practical implication...............................................................................................................183

CHAPTER 8: CONCLUSION AND RECOMMENDATION........................................................................186

8.1 Conclusion...............................................................................................................................186

8.2 Recommendations for future work..........................................................................................188

References.........................................................................................................................................190

Appendix...........................................................................................................................................227

N.N Amucheazi (2018) University of South Wales xi

List of figures

List of FiguresFigure 2.1-Oil rig for oil well construction..............................................................................11Figure 2.2-Illustration of oil well casing string........................................................................12Figure 2.3-Illustration of oil well cementing...........................................................................14Figure 2.4 Cement making process..........................................................................................15Figure 2.5- Geothermal gradient of petroleum geology provided by West Virginia University

USA as cited in Malagay-Bay (2013). (Scale: 1km -1000m)...........................................46Figure 4.1 OC system development, tests and model approach..............................................59Figure 4.2 –The Briquette mould dimension...........................................................................67Figure 5.1-Densities of class G OC system at various w/c ratios vs. recommended density

1773-1965kg/m3................................................................................................................74Figure 5.2 Densities of OC-MA systems at 0.44 w/c ratio vs. density range (1773-1965

kg/m3)................................................................................................................................75Figure 5.3-Yield stress of class G OC system at various w/c ratios........................................77Figure 5.4-Plastic viscosity of class G OC system at various w/c ratios.................................78Figure 5.5-Yield stress of OC-MA system at 0.44 w/c ratio...................................................79Figure 5.6-Plastic viscosity of OC-MA system at 0.44 w/c ratio............................................80Figure 5.7-Setting times of class G OC system at various w/c ratios at 23oC.........................82Figure 5.8- Setting times of OC-MK system at 0.44 w/c ratio at 23oC....................................83Figure 5.9-CS of OC system at various w/c ratios at 24 hours................................................85Figure 5.10-CS development of OC system at various w/c ratios...........................................86Figure 5.11-CS of OC-MA system at 0.44 w/c ratio at 24 hours.............................................88Figure 5.12-CS development of OC-MK system at 0.44 w/c ratio..........................................88Figure 5.13-CS development of OC-PFA system at 0.44 w/c ratio.........................................89Figure 5.14-TS of OC system at various w/c ratios at 24 hours..............................................92Figure 5.15-TS development of OC system at various w/c ratios...........................................93Figure 5.16- TS of OC-MA system at 0.44 w/c ratio at 24 hours............................................95Figure 5.17- TS development of OC-MK system at 0.44 w/c ratio.........................................95Figure 5.18 -TS development of OC-PFA system at 0.44 w/c ratio........................................96Figure 5.19-WA of OC system at various w/c ratios at 24 hours............................................99Figure 5.20 -WA of OC system at various w/c ratios for various days...................................99Figure 5.21 -WA of OC-MA system at 0.44 w/c ratio at 24 hours........................................101Figure 5.22-WA of OC-MK systems at 0.44 w/c ratio at various days.................................102Figure 5.23-WA of OC-PFA systems at 0.44 w/c ratio at various days................................102Figure 5.23-Shrinkage of OC system at various w/c ratios at various days..........................106Figure 5.24-Shrinkage of OC-MK system at 0.44 w/c ratio at various days.........................108Figure 5.25-Shrinkage of OC-PFA system at 0.44 w/c ratio at various days........................108Figure 5.26-Weight loss of OC system of different w/c ratios from 50oC-250oC.................111

N.N Amucheazi (2018) University of South Wales xii

List of figures

Figure 5.27-Weight loss of OC system of different w/c ratios from 400oC -550oC..............112Figure 5.28-Weight loss of OC-MK systems at 0.44 w/c ratio from 50oC to 250oC.............114Figure 5.29 -Weight loss of OC-PFA systems at 0.44 w/c ratio from 50oC to 250oC...........115Figure 5.30 -Weight loss of OC-MK systems at 0.44 w/c ratio from 400oC-550oC..............115Figure 5.31 -Weight loss of OC-PFA systems at 0.44 w/c ratio from 400oC -550oC............116Figure 5.32 -Impact of temperature on the ST of OC system modified with 10% and 20% MA

dosages............................................................................................................................125Figure 5.33- Impact of temperature on the CS of OC system modified with 10% and 20%

MA dosages at 24 hours..................................................................................................126Figure 5.35- Setting times of OC0.44, OC-MK and OC-PFA systems incorporating SP

admixture dosages at 42oC..............................................................................................129Figure 5.36 –Setting times of OC0.44, OC-MK and OC-PFA systems incorporating SP

admixture dosages at 60oC..............................................................................................129Figure 5.37 –Compressive strength at 24 hours of OC0.44, OC-MK and OC-PFA systems

incorporating SP admixture dosages at 23oC..................................................................131Figure 5.38-Compressive strength at 24 hours of OC0.44, OC-MK and OC-PFA systems

incorporating SP admixture dosages at 42oC..................................................................132Figure 5.39-Compressive strength at 24 hours of OC0.44, OC-MK and OC-PFA systems

incorporating SP admixture dosages at 60oC..................................................................132Figure 6.1-The R2 and AAD of the setting time values (in minutes) of the OC-MK systems

........................................................................................................................................141Figure 6.2- The R2 and AAD of the compressive strength values (in MPa) of the OC-MK

systems............................................................................................................................141Figure 6.3- The R2 and AAD of the setting time values (in minutes) of the OC-PFA systems

........................................................................................................................................142Figure 6.4- The R2 and AAD of the compressive strength values (in MPa) of the OC-PFA

systems............................................................................................................................142Figure 6.5-Predicted vs. Actual setting time of OC20MK system incorporating 0.6% SP

dosage.............................................................................................................................143Figure 6.6-Predicted vs Actual setting time of OC10PFA system incorporating 0.4% SP

dosage.............................................................................................................................144Figure 6.7- Predicted vs Actual compressive strength at 24 hours of OC20MK system

incorporating 0.6% SP dosage........................................................................................144Figure 6.8- Predicted vs actual compressive strength at 24 hours of OC10PFA system

incorporating 0.4% SP dosage........................................................................................145Figure 6.9- Prediction of the setting time of OPC based systems incorporating MK of

different properties developed by Broni et al (2015) using model in the current study. 146Figure 6.10- Prediction of the 24 hours compressive strength of the class G OC system

developed by Broni et al (2015) using models in the current study...............................146

N.N Amucheazi (2018) University of South Wales xiii

List of figures

Figure 6.11 –Response surface of the setting time of the OC system incorporating 10% MK........................................................................................................................................148

Figure 6.12– Response surface of the setting time of the OC system incorporating 20% MK........................................................................................................................................148

Figure 6.13 – Response surface of the 24 hours compressive strength of the OC system incorporating 10% MK...................................................................................................149

Figure 6.14 – Response surface of the 24 hours compressive strength of the OC system incorporating 20% MK...................................................................................................149

Figure 6.15-Response surface of the setting time of the OC system incorporating 10% PFA........................................................................................................................................150

Figure 6.16 – Response surface of the setting time of the OC system incorporating 20% PFA........................................................................................................................................150

Figure 6.17-Response surface of the 24 hours compressive strength of the OC system incorporating 10% PFA..................................................................................................151

Figure 6.18 – Response surface of the 24 hours compressive strength of the OC system incorporating 20% PFA..................................................................................................151

Figure 6.19 –Same setting time of OC10MK system occurring at different temperatures with different dosages of SP admixture..................................................................................153

Figure 6.20 – Same setting time of OC20MK system occurring at different temperatures with different dosages of SP admixture..................................................................................154

Figure 6.21 – Same compressive strength of OC10MK system at 24 hours occurring at different temperatures with different dosages of SP admixture.....................................154

Figure 6.22– Same compressive strength of OC20MK system at 24 hours occurring at different temperatures with different dosages of SP admixture.....................................155

Figure 6.23 – Same setting time of OC10PFA system occurring at different temperatures with different dosages of SP admixture..................................................................................155

Figure 6.24 – Same setting time of OC20PFA system occurring at different temperatures with different dosages of SP admixture..................................................................................156

Figure 6.25 – Same compressive strength of OC10PFA system occurring at different temperatures with different dosages of SP admixture....................................................156

Figure 6.26 – Same compressive strength of OC20PFA system occurring at different temperatures with different dosages of SP admixture....................................................157

N.N Amucheazi (2018) University of South Wales xiv

List of tables

List of TableTable 2.1-Major chemical compounds of cement (Cemex, 2015)............................................17Table 2.2-Key features of API oil well cement (API Specification 10A,2002).......................20Table 3.1-Oxide composition class G cement material (Cebo ltd), Metakaolin and Pulverised

fuel ash material (Christodoulou, 2000)............................................................................55Table 3.2-Some physical properties of the class G cement material (Cebo ltd), Metakaolin and

Pulverised fuel ash material (Christodoulou, 2000)..........................................................55Table 4.1-Mix design and material composition of the various OC systems...........................61Table 5.1-Performance of the OC systems of various w/c ratio vs. OC0.44 system..............120Table 5.2-Performance of the OC-MK systems at fixed w/c ratio of 0.44 compared to the

OC0.44 system.................................................................................................................121Table 5.3-Performance of the OC-PFA system at fixed w/c ratio of 0.44 compared to the

OC0.44 system.................................................................................................................122Table 6.1-The ANOVA of the Prob>F value of the setting time of the OC-MA system.......137Table 6.2-The ANOVA of the Prob>F value of the compressive strength of the OC-MA

system..............................................................................................................................137Table 6.3-The ANOVA of the significant model terms for the setting time of the OC-MA

system..............................................................................................................................138Table 6.4-The ANOVA of the significant model terms for the compressive strength of the

OC-MA systems..............................................................................................................138Table 6.5-ANOVA for significance of model........................................................................139Table 6.6-Generated quadratic models for the prediction of the OC-MK and OC-PFA systems

Setting time and compressive strength............................................................................139Table 6.7-Performance of the Model in predicting the output variables of the systems........140

N.N Amucheazi (2018) University of South Wales xv

Chapter 1: Introduction

CHAPTER 1: INTRODUCTIONThis chapter explains the background, aim and objectives of the research study, the impact of

the research and the contribution to knowledge. The chapter concludes with a brief

description of the structure of the entire research thesis.

1.1 Background

The production of oil (hydrocarbon) plays an important role in the economy of nations and

global economic structure (Brown et al, 2015). The world’s oil demand and use of oil for

energy has been on the increase since the discovery of oil. According to Yahaba (2010), oil

consumption experienced an increase of 171% from 1965 to 2008 and it’s noted by the U.S

Energy Information Administration (2016) to have increased from 2009 to mid 2016 by

approximately ten (10) million barrel per day. The rate of oil consumption illustrates the high

reliance on oil for energy around the world. Hence, a significant decrease of oil production in

the world may lead to economic challenges; unless a more economical resource for energy is

discovered and adopted as replacement (Shariar, 2011). In this regards, it is essential to ensure

that wells produce oil at their full capacity so as to avoid economic challenges. However, a

vital aspect to ensure that wells produce oil adequately depends so much on the construction

of the oil wells. The construction of oil wells involves a cementing aspect using a

cementitious system which is very important in the entire engineering of the oil well (Drilling

& Completion Committee (DCC), 1995, Mangadlao et al, 2015). Improper formulation of an

oil well cementitious (OC) system for well cementing can adversely affect the construction or

the existence of a well leading to loss or spillage of oil as seen in the case of Gulf of Mexico

deepwater and Montara development incident (Hart, 2010). The occurrence of oil spills

causes a shortage of oil for energy and economic losses. More so, the spillage of oil adversely

affects the environment including the contamination of soils for crops, pollution of ground

water and harm to animals and marine habitants due to its toxicity. As such, to avoid the

economic and environmental issue that can arise from oil production, the oil industries are

investing hugely in developing technologies and materials to improve oil extraction and

minimize environmental hazards.

N.N Amucheazi (2018) University of South Wales 1


In the well cementing process, a steel casing is cemented in the well by pumping a volume of

cementitious system downward through the casing string to return upwards in the annulus -the

space between the formation wall and the steel casings. The purpose of this operation is to

displace the formation fluids from the oil well, provide formation and casing stability, prevent

corrosion of the casings and isolate geological formation zones in the oil well. This operation

is conducted using an impermeable hydraulic material that can prevent migration of formation

agents which can lead to accidents, environmental disaster, unplanned cost, damages (Sugama

2006) and loss of lives. According to Lyons (1993) and Labibzadeh (2010), the purpose of

cementing an oil well is very essential as this protects the well and enables it to produce at its

full potential and this is conducted using the mixture of cement and water to produce a hard

substance over time. The first cements were produced by the early Greeks and Romans from

volcanic ash mixed with slaked lime, but, the technology was lost during the middle age, until

the development of Portland cement in the early 1800’s (Hall, 1976). The development of the

Portland cement since its discovery has played and continues to play very important roles in

several engineering fields including in oil well engineering. For nearly a century, well

cementing has been largely conducted by placing the Portland cementitious system in the

annulus of oil wells until concerns of global warming, environmental issue (Huntzinger and

Eatmon, 2009) and even more performance as oil exploitation shifts to more challenging

fields (Nahm et al, 1993, Shariar 2011, Guo et al 2016).

1.1.1 Statement of problem

Oil fields most recently discovered in parts of the world show the need to site wells in more

hostile conditions characterized by fields with high temperature, pressure, corrosive fluids

among other geological conditions in depths of 2000m and beyond (Gunnar 2008,

Lehmkoster et al, 2015). In this regards, the construction of the well must be adequately

conducted for the purpose of producing oil in these fields. Inadequate construction of an oil

well such as poor hydraulic seal by the cementitious system used for cementing the well can

lead to a devastating end, or loss of pressure that could result to the well not producing at its

full potential (Calvert, 2006). The failure of the oil well cementitious system to provide

cementing functions in the oil well is associated with certain factors that affect the



performance of the cement in the geological depth including mud diffusion, acid attack,

pressure and temperature. However, temperature presents a significant impact on the

performance of cementitious system during and after the construction of the oil well (Gulliot

1990, Shariar, 2011). Temperature influences the setting and strength performance of

cementitious system which presents challenges in the placement of the system and provision

of stability expected of the cementitious system in the oil well (Bezerra et al, 2011; Gulliot,

1990; Kellingray et al, 1990; Noik and Rivereau, 1999). Therefore, an OC system must be

able to contend with the conditions affecting its performance to render required functions in

the oil well before its application. For this reason, an OC system is designed with

consideration of the factors that can affect its performance to render desired functions in the

well. Over the years, several types of admixtures have been introduced for the modification of

the performance of cementitious system for cementing of oil wells due to existing condition in

the geological depth. This is to achieve successful placement of the OC system, provision of

stability by the OC system and long term existence of oil wells. But, the approach to achieve

the desired performance of the OC system for well cementing is not an easy task. According

to Silva (1997) it requires a complex formulation of cement and admixture(s) with numerous

trials and errors to achieve a balance for proper placement, strength and long term

performance of the OC system. Furthermore, the production of cement technology to meet the

need of modern engineering and other industrial purposes present environmental and global

warming concerns. According to Mehta (2001), cement materials such as that used for

cementing oil wells have an adverse impact on the environment. Cement manufacturing

causes air pollution due to the emission of carbon dioxide (CO2), requires lime and other

natural resources for its production and consumes an energy of 5000MJ in the production of

one tonne of cement material, all of which have direct effect on climate change contributing

to environmental issues (Robertson 1974, Higgins, 2007). Hence, industries are required to

adopt technologies that are environmentally friendly for construction works so as to reduce

the adverse impact of cement to the environment (Silva et al, 1997, Christodoulou 2000).

As a result of these challenges and issues associated with the use of Portland cement material,

the use of mineral admixtures (MA), also known as supplementary cementitious materials

offering sustainability and performance benefits are being encouraged for the formulation of



cementitious systems. However, the impact of mineral admixtures on cementitious systems

performance needs to be verified before use. Over the past decade, significant research has

been conducted to characterize the impact of mineral admixtures in ordinary concrete, but

only limited information can be found on the impact of mineral admixtures on oil well

cementitious system and its behaviour with chemical admixtures for well cementing due to

strict performance demands (Shariar, 2011). The properties of the cement material and the

surrounding condition determine the performance of cementitious system. Hence, it is

essential to investigate the effects of MAs and chemical admixtures on the performance of oil

well cement for the formulation of an OC system. The outcome can be beneficial to identify

the influence of a MA and/or a chemical admixture on the OC system performance and to

develop a model to ease the design process of an OC system. This will be valuable for the

selection of the type and dosage of admixtures to achieve sufficient setting time and strength

for placement of an OC system and stability of oil wells in conditions such as high

temperature.

The major consequences that follow if an OC system fails to render the required functions in

the oil well include loss of oil well, blowout incidents or accidents leading to possible loss of

lives, unplanned costs, property damage, environmental damage, penalties and negative

reputations. These demands and consequences that follow for an OC system in cementing oil

wells provide the main motivation for this research

1.1.2 Aims and objectives

The present research work aims to modify and model the performance of an oil well

cementitious system over a range of geological temperature conditions in the oil well.

In order to achieve the aim of the research, the following objectives are set out to be fulfilled:-

Develop a knowledge of the properties of cement material and the influence of such

properties on the performance of cementitious system;



Develop an understanding of the effects of admixtures and oil well conditions

especially temperature effects on the performance of oil well cementitious system;

Investigate the impact of admixtures (mineral and chemical) in modifying the

performance of an oil well cementitious system;

Investigate the effects of temperature on the setting time and strength performance of

an oil well cementitious system modified with admixtures (mineral and chemical);

Develop a model to predict the setting time and strength performance of the OC

system for a selected range of temperatures so as to simplify and reduce the number of

tests and materials to achieve desired performance of OC system for oil well

construction.

1.2 Contribution to knowledge

Research plays a momentous role in establishing ideas, methods, technologies or

improvements to initiate new things or to solve challenges. The current research has taken an

ambitious step in the technology of oil well cement. It details an approach to formulate and

achieve desired performance of OC system which is new -contributing to the knowledge of

how cementitious system are designed for oil well cementing and also exposing the effect of

oil well conditions such as temperature on the performance of cementiitous system and the

environmental concerns associated with cement technology. Furthermore, this research

contributes to knowledge on how some of the challenges to oil well cement can be addressed

by presenting a fundamental understanding of the influence of cement and admixtures

properties to the performance of the OC system. It also establishes a test protocol to screen

and determine appropriate admixture dosage in modifying the cementitious system for oil

well cementing. In general, the knowledge from the present research study will be beneficial

for the efficient design of an OC system to manage the performance and also the

environmental concerns in oil well cementing.

The following are considered the main contribution of the research;



The study explored the performance of oil well cementitious systems incorporating

supplementary cementitious system. The outcome is a contribution towards

understanding the means by which mineral admixtures alter the performance of oil

well cementitious system. This will aid in the knowledge of selecting the admixtures

and their suitable dosage to achieve the desired performance of the OC system that can

withstand the challenges presented by the oil well conditions during and after

construction of the oil well.

The study explored the impact of chemical admixtures on neat and modified oil well

cementitious system using MA. The outcome revealed that the impact of chemical

admixture to the performance of OC system differs with the constituents of the cement

material and the condition of the surrounding such as temperature which the system is

exposed to. The outcome provides knowledge on how multiple admixtures can be

applied in the design of an OC system for optimal performance.

The study developed mathematical models and multiple domain response surfaces of

cement, admixture(s) and temperature relationship. This will contribute to the

knowledge of simplifying the design process of an OC system and reducing the

required experimental tests to achieve the desired performance with various

admixtures and well condition. The parameters (cement, admixture and temperature)

also contribute knowledge in understanding the performance of an OC system in the

relationship between cement material and admixture dosages in different temperature

degree.

Finally the study contributes knowledge to the development of environmental friendly

cement technology with the ability to meet current and future needs in oil well

cementing. The technology contributes knowledge on how environmental concerns of

pollution, high energy consumption and resource depletion associated with cement

technology can be reduced using environemental friendly materials for cementitous

system develeopment.



1.3 Beneficiaries

There are certain interest groups that will benefit from the current research, these include:

Oil Industries: by adopting environmental friendly materials for the modification of

cementitious system to achieve adequate performance for required cementing function

in the oil well.

Oil well cementing engineers/ designers: adopting and applying the materials and

model used in the current study will ease the design process of formulating

cementitious system for cementing oil wells characterised by various conditions

Cement material and admixture producers- the knowledge from the research study will

be beneficial for the production of admixtures with suitable properties to modify the

performance of oil well cementitious system as required for desired purpose.

Cement users and community: as the research provides a base for the re-use of waste

and by-products for cement purposes thereby initiating economic and environmental

benefits for the community

Research community: an approach to the pursuit of sustainability in cement

technology using environmental friendly materials (natural, waste or by-products) to

achieve required performance by cement in engineering and a means of managing

global warming and other environmental challenges

1.4 Structure of thesis

The other chapters of this research work based on investigation, modification and modelling

of oil well cementitious system performance are structured with the following focus

accordingly:

Chapter 2: This chapter is a review of literatures to develop an understanding of oil well

cementitious system practices including modelling of cementitious system performance. The

current state of technology used for oil well cementing is critically reviewed in this chapter. N.N Amucheazi (2018) University of South Wales 7


More so, resources and information regarding admixtures and cementitious system

performance are explored. The chapter also summarizes the challenges present in oil well

cementing, solutions adopted and the gaps that need attention before embarking on the

investigation undertaken in this study

Chapter 3: This chapter presents the materials used in the research work providing details of

the materials including the source of the materials, the chemical (Oxide) composition and

some physical properties of the materials and the reason for choosing each of the material for

the study.

Chapter 4: This chapter presents the composition of the designed specimens that were

studied and the tests that were conducted in the research work. The development processes of

the specimens investigated in the research work are described in the chapter as well as the

method for the tests. The tests described in this chapter include density, rheology, setting

time, compressive strength, tensile strength, water absorption, shrinkage and thermo-

gravimetric analysis for hydrate content as well as the model development approach.

Chapter 5: This chapter presents the results obtained from the outcome of the modified oil

well cementitious system tests in this research study. The analysis of the findings from the

results is also presented in this chapter for the various types of specimens investigated in the

study.

Chapter 6: This chapter presents the development and analysis of the mathematical model to

predict the setting time and compressive strength performance of the various systems

examined in this research study. It also presents the the analysis of the generated response

surface and contours for the setting time and compressive strength performanc of the systems

studied in this research.

Chapter 7: This chapter presents the discussion of the findings from the analysed test results.

It explains the correlation and comparison of outcomes obtained from the various specimen



examined in the study. Additionally, it discusses the overall performance and environmental

implication of the materials to oil well cementitious system development.

Chapter 8: This is the final chapter of the research work. The chapter presents the

conclusions and proffered recommendations from the current study and provides area for

further research to be conducted.


Chapter 2-Literature review

CHAPTER 2: LITERATURE REVIEWThis chapter reviews existing literature on oil well construction and cementing process,

cement material and its properties, types of admixtures and the performance of cementitious

system. This is to establish the status of current knowledge and ideas on the present study so

as to provide the intellectual context for the research.

2.1 Oil Well Cementing

Oil well cementing is an aspect of the entire process of constructing an oil well. The

construction of an oil well entails the engineering for the exploitation of oil (hydrocarbon)

resources beneath the earth and sea surface. Different types of oil wells are constructed for the

extraction of oil which depend on the geological characteristics of the field bearing the oil

resource. The different types of oil wells are vertical, horizontal and directional wells

(Mbendi, 2012).

Typically, the initial aspect in constructing an oil well is locating the suitable and closest area

to erect the rig (Figure 2.1) to reach the oil reservoir. This is carried out to reduce cost and

operation time to complete the oil well. Locating the suitable position for the construction of

an oil well is conducted by studying the geological formation using seismic survey. With the

findings obtained from the seismic survey, a suitable location for the well is sited and the rig

is erected for the aspect of constructing the oil well commencing with drilling operations,

(Hartley 2011, Natural Gas Org, 2011).



Figure 2.1-Oil rig for oil well construction

The drilling aspect in the construction of an oil well is conducted with the use of a drill bit

that has irregular edges, attached to a drilling string (pipes and collars) to bore the geological

formations with drilling fluid (mud) which is pumped through the string to remove the

cuttings (rock particles) as the hole of the well is being made to a target depth. When the

target depth in the construction of the well has been achieved, metal tubes known as casings

are then installed, commencing with the casing of largest diameter so as to allow additional

tubes to be fitted in the course of constructing the well to further depth or to the depth of the

discovered oil. The initial casings are known as the conductor casing followed by the surface,

intimidate and production casings (Figure 2.2), (Hartley 2011, Natural Gas Org, 2011).



Figure 2.2-Illustration of oil well casing string

After the aspect of installing the casing tubes, the cementing of the oil well is carried out

using a mixture of cement and water known as the cementitious system (Figure 2.3). The

cementing aspect of an oil well is very essential for the complete construction and long term

existence of oil wells (Lyon, 1993). Usually, cementing is conducted for different formation

zones such as the aquifer zones or weak formation zones as the drilling and casing installation

proceeds. In the cementing process, a volume of cementitious system is pumped down into

the well through the casing string and up to fill the annular space –the space between the

formation wall and the casings. The cementing job begins with the primary cementing



operation. This is the first cementing job that is conducted and it is expected to provide the

required functions in the well including sealing the annular space. However, if the primary

cementing job is poor or faulty, then a secondary cementing job (remedial cementing) is

required and conducted (DCC, 1995). A sufficient quantity of OC system which is a mixture

of cement and water is prepared for the cementing job so as to fill the annulus of the well. The

cementitious system after pumping is then allowed for some time to transform from fluid to

solid state to seal the formation zone and provide other required functions for further

construction operation or production of oil (Teja, 2009). The functions of the OC system

include:-

displacing drilling fluid and formation fluid from the annulus;

preventing the contamination of the aquifer;

isolating different geological formation zones and preventing the migration of

formation fluids to the surface;

providing stability to the oil well ;

preventing the casing string from shock loads in deeper drilling and from buckling;

protecting the casing string from corrosive fluids;

plugging and abandoning a well and

enabling the well to produce at full potential (Nahm et al, 1993, Shariar 2011, Tipton

2013).



Figure 2.3-Illustration of oil well cementing

2.2 Oil Well Cement

According to Ogbonna and Iseghohi (2009), the major choice of material used for cementing

oil wells is the Portland cement. The Portland cement has a long history commencing from

the Babylonians. According to Kirca (2005), in the early development of cement, the

Babylonians used clay as cement due to its binding capability. The act of using clay as cement

was later advanced by the discovery of lime and gypsum by the Egyptians and was used for

building huge structures such as the pyramids. Further improvements were made to cement

technology by the Greeks and finally the Romans developed cement that produced structures

of remarkable durability (Hall, 1976; Kirca, 2005). The word “cement” traces to the Romans,

who used the term ‘Opus Caementicium’ to describe masonry resembling modern concrete

that was made from crushed rock with burnt lime cement, (Zhijun, 2012). However, it was not

until 1824 that the Portland cement was developed by Joseph Aspdin and is now used in



various engineering fields including the construction of road, dams, buildings of magnificent

height (Buckely, 2001) and wells.

The Portland cement material mainly used for the cementing of oil wells is primarily made

from limestone, clay minerals and gypsum materials. The making of Portland cement entails

that the various materials are burnt together in a high energy consuming and high temperature

process which emits carbon dioxide while enabling the primary ingredients to chemically

combine into new chemical compounds in small particles (Figure 2.4). These chemicals are

then able to react in the presence of water and transform the fluid solution of the cement and

water to solid (Lafarge, 2015). The reaction of the chemicals of the cement with water is

known as hydration and the rate of hydration may vary depending on the particle

characteristics of the cement. Thus, the properties of cement define the performance of a

cementitious system among other factors such as pressure, acidic fluid and temperature that

can alter its performance and are used for classifying cement into grades or types (API

Specification 10A, 2002) as detilaed in table 1.1.


Clay & other materials

Lime stone

Cement

Gypsum

Clinker (stony residues)

Raw mix of materials

Grinding Klin(Burning)

Crushing of all

Materials

Figure 2.4 Cement making process


2.2.1 Challenges and Classes of Oil Well Cement

Oil well cements provide the base ingredient in the cementitious systems that are pumped into

the casing tubes and out from the base to fill the annulus in oil well construction (Detriot et al,

1981, Calvert, 2006). Initially, the available types of cements were sufficient to provide the

functions required in the annular space during construction and for long term existence of an

oil well to produce the discovered oil. However, with an increase in the demand of energy, the

exploitation of oil to meet demands migrated to fields characterised by oil resources in more

hostile geological formations and the stringent performance criteria for functions required of

the available cement in these conditions are not being satisfied (Shariar, 2011, Guo et al

2016).

In the annulus of the oil well, Portland cement material used for cementing the well is

exposed to several geological conditions which have an adverse effect on its performance.

These geological conditions include mud, acidic fluid, pressure and temperature (Tipton

2013).

The cementitious systems for cementing oil wells are sometimes pumped to depths beyond

2000m with temperature rising as the depth increases. At such geological depths of 2000m

and beyond, the temperature in an oil well is above 23oC and the performance of the

cementitious system used for cementing of the oil well can be adversely affected including the

setting and strength performance for placement and the provision of stability to the well

(Gunnar 2008). The cementitious system may also be subject to acid attack from geological

fluids and high pressures reaching over 200MPa depending on the density of the fluids above

it (Joshi and Lohita, 1997). Due to these challenges, improvements were made to cement for

oil well construction resulting in various cements which are regulated by the American

Petroleum Institute (API) Standardization Committee (API 2002). The API committee

specifies the requirement for the cements used for cementing of oil wells and the chemical

and physical properties of these cements distinguish each cement type from another. This is

because; the chemical and physical properties of a cement material influence the performance

of the cementitious system (API, 2002). The major chemicals that influence the performance

of cements are; Tricalcium aluminate (C3A), Tricalcium silicate (C3S), Dicalcium silicate



(C2S) and Tetracalcium aluminoferrite (C4AF) which are summarized in Tables 2.1 and the

physical properties that distinguish cements are the size and shape of the particles of cement

materials. Mainly, the physical properties of cement influence the rate of chemical reactions

via the particles surface area interactions. According to the United States Department of

Transport (1990), the particle surface area and chemical compound of cements determines the

performance of a cementitious system. Greater particle surface area increases the available

surface for interaction amongst the cement particles leading to high chemical bonding for

hardening of the system. Additionally, the types and proportion of the chemicals of the

cements can slow down or increase the transformation process rate of the system from fluid to

solid state and influence the strength of the system including the porosity, shrinkage, and long

term performance of the system (El-Enein et al 2015; Portland Cement Association PCA

2006; ACI Comm. 225R 1985).

Table 1.1-Major chemical compounds of cement (Cemex, 2015)Compounds Influence

Tricalcium aluminate.

Ca3Al2O6 or C3A

React rapidly with water and evolve high heat leading to quick

transformation of the cementitious system from fluid to solid state.

The reaction is controlled by adding gypsum in the cement making

process

Tricalcium silicate.

Ca3SiO5 or C3S

Reacts rapidly with water, evolving high heat in thier reaction and

are responsible for the early strength gain of the cementitious

system

Dicalcium silicate.

Ca2SiO4 or C2S

Reacts slowly with water and evolves heat during thier reaction

which is dissipated before a significant temperature rise occurs due

to thier slow reactivity. They contributes to the strength of the

cementitious system at later ages

Tetracalcium

aluminoferrite.

Ca4Al2Fe2O10 or

C4AF

React rapidly with water but do not evolve much heat or contribute

much to the strength of the cementitious system.



The different types of oil well cements as regulated by the API are Ordinary (O), Moderate

sulphate resistant (MSR) and High Sulphate Resistance (HSR) which are classed into eight

grades ranging from class A to H based on their properties and capabilities (Table 2.2) (API

Specification 10A, 2002). The class represents the range of geological depth, pressure,

temperature and acidic condition the oil well cement type(s) can be used to cement. For

instance, the Class A cement is recommended for use in milder, less demanding oil well

conditions when special properties of another class of cement is not required. The class B

cement is recommended for cementing to a similar depth as the Class A cement but has an

advanced capability of resisting acid attacks (API Specification 10A, 2002). The Class C

cement is used when high strengths are desired and also in emergency repairs due to its

smaller particle size (US Army Corps of Engineers, 1984, API Specification 10A, 2002). The

Class G and H cements are the most commonly used oil well cements especially for

cementing deeper, acidic, higher pressure and higher temperature oil wells (Lafarge, 2009).

The basic difference between the class G and H cement is their particle surface areas, with the

class H cement particles coarser than that of the class G cement and thus require a lesser

quantity of water (Nelson et al, 2006). However, despite these improvements, the challenge of

temperature among other conditions on the performance of oil well cement continues to exist

which is due to the exploitation of oil in fields characterised with reserves in depths of 2000m

and beyond to meet energy demand (Guo et al 2016).

In addition to the performance challenge of the OC system, the production of cement has an

adverse impact on the environment. Cement production entails the use of several natural

resources, high energy consumption and emission of unfriendly gases into the atmosphere.

Approximately 1.65 tonnes of limestone and 0.4 tonnes of clay are quarried for the production

of 1 tonne of cement, about 5000MJ of energy is required to generate a temperature of

approximately 1500oC to fuse various chemical elements into 1 tonne of cement clinker, while

the production of a tonne of cement results in the release of 1 tonne of CO2 into the

atmosphere which is detrimental to the environment by contributing to global warming, (Adak

et al, 2007). In this regards, industries are encouraged to adopt alternatives to reduce the use

and adverse impact of cement technology for the preservation of the environment

(Christodoulou, 2000).



One of the key challenges in the pursuit of alternative cement technology for several cement

works including the cementing of an oil well is the concern of performance. Several factors

which include the geological conditions, cement properties and mix water ratio influence the

performance of an OC system (Souza, 2012). Inadequate performance of an OC system in the

oil well can lead to oil spill, environmental pollution and other incidents (Braune, 2012). For

these reasons, alternatives that can offer the performance required of cement in the oil well

are being pursued. According to Popovics (1992), certain modifications can be made to the

performance of cementitious system for the construction of an oil well. But, the approach to

achieve the desired performance of the OC system is not an easy task. This entails the

incorporation or use of admixture(s) to partially replace the cement material so as to achieve a

system that can contend with conditions such as temperature among others that degrade the

performance of OC system in the well. According to Silva (1997) the process requires a

complex formulation of the cement and admixture(s) with numerous trails and errors to

achieve a balance for desired performance of the OC system.



Table 2.2-Key features of API oil well cement (API Specification 10A,2002)Cement class A B C D E F G H

Recommended water to cement (w/c) ratio,

0.46 0.46 0.56 0.38 0.38 0.38 0.44 0.38

Recommended range of depth, m (ft)

0 to 1830 (0 to 6000)

0 to 1830 (0 to 6000)

0 to 1830 (0 to 6000)

1830 to 3050 (6000 to 10000)

3050 to 4270 (10000 to 14000

3050 to 4880 (10000 to 16000)

0 to 2440 (0 to 8000)

0 to 2440 (0 to 8000)

Availability O grade compatible with ASTM C 150, Type 1 Portland cement

MSR and HSR grades comparable with ASTM C 150, Type II

O, MSR and HSR grades comparable with ASTM C 150, Type III

MSR and HSR grades MSR and HSR grades

MSR and HSR grades

MSR and HSR grades

MSR and HSR grades

Cost Lower cost Lower cost More costly than Ordinary Portland cement

More costly than Ordinary Portland cement





Other features Intended for use when special properties are not required.

1) Intended for use when condition require moderate or high sulphate resistance. 2) Lower C3A content than Class A.

1) Intended for use when conditions require high early strength. 2) the C3S content and surface area are relatively high.

1) Required under conditions of moderately high temperatures and pressure. 2) Retarded cement and retardation is achieved by reducing C3A, and increasing the particle size of the content.

1) Required under conditions of moderately high temperatures and pressures.2) Retarded cement and retardation is achieved by reducing C3S and C3A, and increasing the particle size of the cement grains.

1) Required under conditions of extremely high temperatures and pressures. 2) Retarded cement and retardation is achieved by reducing C3S and C3A, and increasing the particle size of the cement grains.

1) Basic well cement. 2) Thickening Times controllable with additives to prevent loss of circulation up to 120o C

1) Basic well cement. 2) Surface area is coarser than that of Class G. 3) Thickening Times controllable with additives to prevent loss of circulation up to 230o C

O - Ordinary, MSR - medium sulphate resistance, HSR - high sulphate resistance



2.3 Admixtures

Often, OC systems are exposed to formation fluids attack, pressures and especially temperature

in the annulus of wells. Temperature compels severe requirements on the performance of OC

systems as it alters the performance of the systems (Shariar, 2011). Mainly, an OC system is a

mixture of cement material and water to provide the required functions in the oil well, but due

to the conditions (temperature, acidic fluid and pressure) in the well, the performance of the

OC system can be adversely affected, as such, requiring modifications with the use of

admixtures for adequate performance.

Admixtures are ingredients other than cement or water that are added before or during the

mixing of the cement and water for cementitious system in order to achieve a performance

suitable for purpose. Admixtures are classified by their influence on the performance of the

cementitious system. The effect of admixtures on cementitious systems depends on the

properties of the cement material and that of the admixture which include the physical nature,

chemical compositions and specific gravity of the materials. Also, the water to cement ratio,

mixing approach, material proportions and surrounding condition such as temperature

contribute to the overall performance of the system (Nelson et al., 1990; 2006). Admixtures

can be manufactured or natural and are categorized as chemical admixtures and mineral

admixtures which are also known as supplementary cementitious material.

According to Schlumberger (2012), the performance of OC systems in the construction of oil

wells depends on the chemical and physical properties of the cement and admixtures for the

system. A wide variety of admixtures are available and used for modifying the performance of

cementitious systems such as for extending the setting time, increasing the strength, reducing

porosity, altering density and enhancing durability of the system among others, (PCA, 2016).

2.3.1 Chemical admixture

Chemical admixtures are ingredients that are manufactured to be added to a cementitious

system to alter its performance for a given purpose (Suryakanta, 2016). Chemical admixtures

can be in the form of a liquid or solid powder. They are several types of chemical admixture;



however typical chemical admixtures for OC systems can be categorized into the following

groups;

Set accelerators: these are types of admixtures that shorten the setting time of a cementitious

system or duration it would have taken a system to transform from fluid state to solid state. In

the cementing of oil wells, this implies a decrease to the solidification period of an OC system

to become a stiff substance. The use of accelerators in cement based systems is mainly

advantageous in conditions that can delay the setting of a system (PCA, 2016).

Set retarders: these are chemical admixtures used to delay the setting time of cementitious

systems. Retarders are used to alter the setting time of systems especially in conditions that are

hot by extending the setting period of the system. An increase in temperature initiates early

solidification which affects the placement of the cementitious system. Thus, this type of

admixture is introduced to enhance and enable placement of the OC system while extending

the setting period. (PCA, 2016)

Weighting agents: these are admixtures with a higher specific gravity than cement which are

introduced to increase the density of cementitious system for displacement of mud or formation

fluid in high pressure oil wells. More so, they may have slow reactivity so as to use low mix

water content for additional density increase of the system for well cementing purpose

(Petrowiki, 2015).

Extenders: these are set of admixtures with lesser specific gravity than cement. They are

introduced to decrease the density of a cementitious system for cementing of zones which are

weak. (Petrowiki, 2015)

Dispersants: dispersants are admixtures introduced in a cement based system to reduce

viscosity. They are used to enhance the flow of the cementitious system. They are mainly

beneficial in the placement of the OC system by reducing viscosity while the cementitious

system is pumped into the oil well. The improvement of flow decreases the pump energy

required to place the system in the oil well and reduces the frictional pressure that is exerted on

the geological formation that are weak to prevent loss of circulation or cement escape into the

formation (Petrowiki, 2015).



Fluid-loss control agents: these types of admixture are used to maintain the appropriate fluid

quantity in a cementitious system so that the performance of the system remains within the

desired range. The performance variation of the system can be highly affected by the content of

water in the system. For instance, if the content of water for the system is higher than intended,

the setting time, loss of fluids, porosity and permeability of the system tends to increase while

the viscosity, strength and density of the system decreases. If the case is reversed with the

water lower than intended, the opposite tends to occur with the performance of the system,

(Petrowiki, 2015).

Others (speciality admixtures): these are set of chemical admixtures which include agents such

as strength retrogression prevention agents, expansion agents, antifoaming agents which can be

used to introduce foaming to OC system to be used in weak formations. They are also used to

stabilize foaming of ultra-lightweight oil well cementitious systems since foaming can be

detrimental (Ramachandran, 1995)

The OC system may also incorporate one or more chemical admixture to achieve the desired

purpose. Similarly, some chemical admixtures can be used to enhance multiple performance of

an OC system for the required performance. Most chemical admixtures are able to render the

function of dispersants to an OC system while providing their basic function to the system. For

instance, retarding admixtures which are basically used for extending the time which OC

system transforms from fluid to solid state can also be used as a viscosity modifying agent to

decrease friction of the system in the oil well. Also, fluid loss admixtures that are particularly

developed to prevent loss of the OC system to the geological formation at its fluid state, can

also be used to retain the water in the system for adequate reactions of chemicals and

performance of the cement (Nelson et al, 2006). Additionally, advancements in chemical

admixture technologies for cements have led to the development of various dispersants with

the ability to render multiple improvements to the performance of cementitious systems (PCA,

2016). These dispersants, meant mainly for controlling the viscosity of cementitious system

can also substitute as water reducing agents to reduce the mix water content for formulating a

cementitious system. They may also initiate higher strength performance, delay setting time,

reduce porosity and shrinkage of a system with more presence of cement than water (PCA,

2016)



2.3.2 Mineral admixtures

In addition to chemical admixtures, mineral admixtures (MA) can be added in the formulation

of cementitious systems to achieve the desired performance. MAs are natural minerals,

industrial by-products or waste materials with cementitious characteristics (Owaid et al, 2012).

MAs vary in cementitious characteristics due to their chemical compositions and physical

properties and as such influence the performance of systems differently in relation to the type

and dosage used (Toutanji et al, 2004). MAs are commonly used to alter the performance of

ordinary concrete systems or cementitious system for cement works such as dams and bridge

construction (PCA, 2016). They are accepted as a sustainable approach to reduce the adverse

impact of cement material on the environment such as the emission of CO2, energy

consumption and resource depletion (Christodoulou, 2000; Shariar, 2011). In this regards,

industries are encouraged to use MAs as a strategy to safeguard the environment

(Christodoulou, 2000).

In the construction of oil wells, the Portland cement which has an adverse impact on the

environment is the conventional material used for cementing of oil wells (Ogbonna and

Iseghohi, 2009). The ability to withstand continuous contact with water, in addition to the

ability to set and harden with greater strength has made Portland cement distinctive and the

main cement utilized in oil well constructions (Central Asia Cement, CAC 2008). However,

considering the adverse impact of cement to the environment, it is advised to balance the

reliance on cement to reduce environmental issues associated with the product (Abubakar,

2013). Abubakar (2013) suggested that industrial by-products and minerals with cementitious

properties (MAs) are some of the materials that can be used to initiate a balance in the use of

cement both for performance and the environment. Some of the available MAs include Barite,

Bentonite, Dolomite, Gypsum, Lime stone, Kaolin, coal fly ash, Slag and Silica sand.

Barite: Barite is a type of mineral that is formed of barium sulphate (BaSO4), (Hanor, 2000).

Barite is used for a wide range of industrial purposes due to its high specific gravity. In oil

industries, it is commonly used in the construction of oil wells to increase the density of an OC

system for well cementing purposes (Vaaidehi, 2016).



Bentonite: Bentonites is an absorbent alumina-silicate mineral consisting mainly of

montromorillonite. When water is introduced to bentonite, the water molecules enter between

the clay plates, forcing them apart. While the plates are dispersed, the bentonite system

becomes quite fluid. Bentonite is useful in oil well construction; its property of swelling on

contact with water makes it useful as a sealant. It is also useful for delaying the setting of

cementitious systems. (Hosterman and Patterson, 1992)

Coal fly ash: Caol fly ash is a product from the end use of Coal mineral. Coal is a sedimentary

rock usually occurring in rock strata. It is primarily composed of carbon along with other

elements and commonly used as fuel for electricity generation. Its end use as fuel for electricity

results in ash known as pulverised fuel ash (PFA) bearing cementitious properties and can be

used to modify the performance of cementitious system such as improving its flow (Blander

and Sinha, 2011).

Dolomite: Dolomite is an anhydrous carbonate mineral composed of calcium magnesium

carbonate. It is the primary component of the sedimentary rock known as Dolo-stone. It is

beneficial in neutralizing acid effects that degrade cement performance, (Hobart 2015)

Gypsum: gypsum is a soft mineral composed of calcium sulphate. It is moderately water

soluble. Due to its chemical characteristics, it is beneficial in cement and cementitious system

to manage the effect of tricalcium aluminate (C3A) on transforming a system from fluid to solid

(Klien and Dana 1985).

Kaolin: kaolin is a form of clay mineral which is refined to obtain metakoalin consisting

principally of aluminate silicate (Pohl, 2011). It is useful in a cementitious system where it is

used to supplement the chemicals to alter the strength of a system among other performance

(Kinuthia 1997 and Kinuthia et al., 1999).

Limestone: these are sedimentary rocks of calcium carbonate composite in different crystal

forms (Kranjc, 2006). Limestone mineral have several uses which include mainly the making

of cement used for oil well cementing and as aggregate to modify the performance of a system.

This is because, the Limestone can harden and it is durable (Lucas 2003)



Slag: Slags are a by-product of metal which have been separated from the actual iron ore

during processing, also referred to as ground granulated blast-furnace slags (GGBS). They are

a glass like product consisting of silicon dioxide and other metal oxides (Mikey, 1999). Ground

granulated blast-furnace slags are used in combination with Portland cement to produce less

permeable cementitious system with better durability while developing strength slowly over a

longer period than neat Portland cement system (Konstantin, 2006).

Silica fume: silica fume also known as micro silica is a by-product of silicon metal as a result

of using coal to reduce high purity quartz in an electric arc furnace. It possesses cementitious

properties, consisting mainly of silicon dioxides SiO2 in fine particles. This makes it very

useful in improving reactivity for performance in cementitious systems (PCA, 2016).

Among the various MAs, metakaolin (MK), slags (GGBS), pulverised fuel ash (PFA) and

silica (SF) are commonly used (Khatib and Hibbert., 2005; Nelson et al., 2006; PCA, 2016).

The partial replacements of cement using these MAs offer substantial benefits in regards to

modifying the mechanical performance of a cementitious system for a given purpose and the

reduction of CO2 emission including the amount of energy used in producing cement (Mehta

2001; Christodoulou, 2000; Higgins, 2007; Khatib, 2008; Torgal and Jalali, 2011). MAs such

as PFA, slag and silica are reported to have huge impact in reducing the energy consumption

and CO2 released in manufacturing cement (Norchem, 2011). This is because, they are

industrial by-products. For instance, the PFA material which is a by-product of coal used in a

coal fired electricity generating plant, is reported not to need further processing before it can be

used as a MA, thereby requiring no use of energy or release of CO2 to obtain its finished

product compared to cement (Sear, 2004). The MK material is different as it is obtained from

the calcinations of kaolin clay. The calcinations process to obtain a tonne of MK from kaolin

clay requires a temperature between 600oC to 800oC which consumes less energy with lower

CO2 emission of 0.07 tonne (Michel de Spot and Wojtarowicz, 2003; Torgal and Jalali, 2011)

than that of cement production which requires a temperature of 1500oC generated by 5000MJ

of energy with an emission of one (1) tonne of CO2 for a tonne of cement (Higgins 2007,

Srinivan et al, 2014). However, the impact of MAs on cementitious system needs to be

established before use due to their properties.



Over the past few decades, numerous studies have been conducted to characterize the influence

of MAs in ordinary concrete systems (Alvarez, 2006; Antiohis et al 2006; Brook 2000; Ferrari

et al, 2001; PCA 2016), but only limited information can be found on the use of MA or the

application of MA(s) and chemical admixture(s) in modifying the performance of OC system

for oil well conditions such as temperature (Shariar, 2011).

2.4. OC system & Admixture Performance

The performance of an OC system is influenced by several factors including the well condition,

properties of the cement, mix water ratio and preparation approach (Salam 2013, Aldred et al,

1998, Sasaki 1986). Therefore, designing a cementitious system requires consideration of the

various factors that influence its performance for successful cementing job. According to

Salehi et al (2009), an OC system is designed to render desired functions in the oil well with

the view of existing geological conditions. As such, designing a cementitious system for the

purpose of cementing a well, demands adequate consideration of the inherent characteristics of

the oil well. These include temperature and other geological conditions that can adversely

affect the performance of the OC system.

Furthermore, it is vital to establish that the performance of an OC system is adequate to render

required functions in the well. OC systems are expected to satisfy certain criteria which include

short term and long term criteria. The short term criteria for an OC system include the ability of

the system to be placed over sufficient period of time in extreme levels of geological conditions

to enable proper placement of the system in the oil well and the provision of stability to the

casing string through developing adequate strength (Shariar, 2011). The long term criteria for

an OC system include possessing low permeability to prevent the migration of formation fluid

and resist acid attacks for long term existence of the oil well for satisfactory production of oil

(Shariar, 2011). The short term criteria are very vital for the success of cementing an oil well

and this is characterised by the placement (setting time) and provision of stability to the oil

well (strength) by the OC system among others such as density and rheology. The long term

criteria are characterised by the porosity, shrinkage and resistance of acid attack by the OC

system for the duration of the oil production.



2.4.1 Density of OC Systems

The density of an OC system for cementing oil wells ranges from 1773 kg/m3 (110 lb/ft3) to

1965 kg/m3 (123 lb/ft3) based on the water to cement ratio (w/c) and the class of the API

cement (Hossain and Al-Majed 2015, Shahriar, 2011, API specification 10A, 2002 ). During

the cementing of oil wells, a sensitive objective that requires crucial attention is the

management of the pressure in the annular space of the well at all times and depths (Aldred et

al, 1998). The annular pressure in an oil well is a factor of the density of the formation and

drilling fluids that exert hydrostatic pressure in the annulus of the oil well. As such, the density

of the OC system is required to be adequate to displace the fluids from the well (Guillot et al

1999). According to Mustaq (2013), for proper cementing to take place, the density of the OC

system should be satisfactory to avoid the diffusion of solid particles from the annular fluids in

dynamic or static conditions that can compromise its integrity. In this regards, it is desirable to

increase the density of the OC system above that of the annular fluid in order to minimize the

diffusion of annular fluid particle in high pressure conditions and in other situations, lower

density may be adopted to achieve a balanced pressure for proper displacement of annular

fluids from the well.

The desired density of an OC system can be achieved through incorporating admixtures with a

different specific gravity compared to that of cement. The incorporation of an admixture in the

system is another approach to achieve the desired density of system. Various density modifying

admixtures exist for altering the density of OC systems. Weighting agents or admixtures

increase the density of cementitious system to achieve heavier system, while extenders are

materials with lower specific gravity than cement and are introduced to decrease the density of

systems. These set of admixtures may also influence other performance charateristics of a

cementitious system.

Admixtures such as lime and barite with a relatively higher specific gravity compared to that of

cement material and with finely divided solid particles are some of the materials used in

increasing the density of an OC system, (Miller, 2009; Ariffin, 2009). Hematite, Ilmenite and

micro sand are other types of weighting agents, but only barite and hematite have related N.N Amucheazi (2018) University of South Wales 28


API/ISO standards (Nelson et al., 2006). This is due to the influence of the admixtures to OC

systems. It has been reported that barite is a less efficient weighting admixture than ilmentite

but it was found that ilmentite is less favourable to the consistency of OC system before setting

compared to barite material, thus making barite a preferable weighting agent to ilmentite

(Saasen and Log, 1996, Nelson et al., 2006).

In the case of achieving a low density OC system, bentonite and silica can be introduced to

decrease the density of a system. They possess specific gravity that is lower than that of a

cement material (Buck 1992). As such, adding their dosages as partial replacement of cement

directly decreases the density of a cementitious system, however, caution must be exercised on

the dosages of the admixture. This is because; they can influence other performance

charateristics of the system. For instance, bentonite can alter the reactivity of cementitious

system to a detrimental end by excessively extending the transformation of the system to solid

(Buck 1992), while silica can adversely affect the strength performance of systems depending

on the dosage incorporated (Ajay et al, 2012). Metakoalin, pulverised fuel ash, slags, dolomite

among others have lesser specific density than cement and can also be used to decrease the

density of a system, but with caution due to other influence to the performance of a

cementitious system.

Additionally, the desired density of a system can be achieved by adjusting the water to cement

ratio for the system (Al-hadithi, 2000, Hendrick 2009). Due to the difference in the density of

water and cement, altering their specific ratio alters the density of the system. However, the

increase in mix water can result in bleeding (sedimentation) of the system which can affect

various performance characteristics of the system (Yang, 2015). Sedimentation refers to the

segregation where free water in the system rests above the surface of a freshly prepared system.

On the other hand, the increase of cement can alter the rate of chemical reactions and adversely

affect various performance charateristics of the system (Yang 2015).

2.4.2 Rheology of OC systems

The understanding of the rheology of an OC system is essential in an oil well construction to

identify the potential of placing of the system in the oil well, (Shariar, 2011). Rheology is the

science of the deformation and flow of materials (Banfill, 2003; Quanji, 2010). According to N.N Amucheazi (2018) University of South Wales 29


Barnes et al (1989), rheology can be used to study the flow properties of materials whose

behaviour is visco-elastic comprising of fluid and solid materials. The emphasis on flow means

rheology is concerned with the relationship between shear stress and strain. In shear flows, the

visco-elastic material flows in response to a shear stress to produce a velocity gradient referred

to as the shear rate, which is equivalent to the rate of increase of the shear strain (Banfill,

2013).

In oil well construction, rheology is used to determine the flow properties of an OC system

such as the yield stress, plastic viscosity, etc. A study of the rheological properties of an OC

system is important in assuring that a formulated OC system can be placed in the well to fill the

annular space (Shahriar, 2011). Rheological studies identify the frictional pressure of an OC

system that will be exerted on the formation wall. According to Aldred et al (1998), if the

frictional pressure due to the viscosity of the system is above the pore pressure, the system

could rupture the wall and escape into the formation. Guillot (1990) added that several factors

such as the mix water to cement ratio, the chemical compositions of the cement material, the

preparation approach, and the surrounding conditions affect the viscosity of systems to flow in

the oil well, therefore making it necessary to identify the rheological properties before

placement. According to Reddy et al (2014) and Suryakanta (2014), the nature of lubrication is

the most vital factor that affects the rheological behaviour of cementitious system and water is

found to have a lubricating effect that enables a cementitious system to flow. More so, the

physical properties of the cement material characterised by the size and shape of the cement

particle can influence the rheological behaviour of an OC system (Berg, 1979, Barnes et al

1989). According to Berg (1979), the rheological properties including yield stress and plastic

viscosity of an OC system usually increases with finer cement particles. Barnes et al (1989)

explained that the influence of particles to the yield stress and plastic viscosity of OC system is

a function of inter-surface contact volume presented by the particles.

Numerous studies on the impact of admixtures on the rheology of cementitious system have

been conducted and the outcomes reveal that the presence of admixture alter the rheological

behaviour of a cementitious system. Su et al (1991) reported that adding admixtures such fine



polymer powders to a cementitious system changes the rheological behaviour by improving the

flow of the system. Chougnet et al (2006) reported a similar outcome that the partial

replacement of cement with fine polymer particle increases the flow of the cementitious system

and enhances its ability to be placed. However, the impact of admixtures characterised by finer

particles than cement to improve the flow of a system is not always the case as certain

admixtures with finer particles than cement have been found to decrease the flow of the system

and requiring an increase in mix water (Ferraris et al, 2001, Sabir et 2001). Adding fine MK

particles in cementitious system has been reported to have adverse effects on the flow of

cementitious system (Sabir et al. 2001). The most common reason for the poor flow behaviour

is that the addition of MAs with finer particles than cement increases the particle surface

interactions, thereby increasing friction and the reaction of chemicals to transform the system

into solid in reduced time and requiring an increase of the mix water to dilute the reaction

(Barnes et al, 1989). Studies using silica have also been found to show such adverse impact on

the flow of cementitious system. The additions of fine silica particles in cementitious system

influenced a demand for more mix water to achieve the desired flow (Memon et al, 2013).

However, in other cases, it is reported that the use of a fine MA admixture can reduce the mix

water demand and increase the flow of the system depending on the properties of the MA

(Lange et al 1997, Ferraris et al, 2001). Lange et al (1997) studied the flow of a cementitious

system incorporating increasing content of fine GGBS. The results revealed that for a given

flow rate, the maximum content of GGBS in the cementitious system decreased the quantity of

mix water used to obtain the particular flow rate with neat cement system. Ramachandran,

(1995) explained the theory to which some fine MAs, especially PFA material, improve the

flow of cementitious system, as the function of the spherical particles of such MAs which

effortlessly roll, thereby decreasing inter particle surface contact and friction. Bapat (2012)

acknowledged that spheres like that of the PFA particles, amongst the various shapes offers the

minimum surface contact area in a given volume. Sakai et al (1997) noted that the spherical

shape of such MAs is responsible for the low water demand and the improved rheological

behaviour.

Furthermore, the addition of water-reducing chemical admixtures such as sulfonate,

polycarboxylate and hydroxycarboxylic agents can enhance the fluidity and rheology of



cementitious system for cementing of an oil well (Talyor 1997). Shariar (2011) investigated the

impact of various water reducing chemical admixtures on the rheology of oil well cement. The

investigation included the use 0.5% and 1% of lignosulfonate and polycarboxylate water

reducing admixtures dosage by weight of cement in an OC system. The results revealed that

the water reducers improved the flow of the OC system, however, the degree of improvement

differed with the type and dosages of the water reducing chemical admixture. The

polycarboxylate water reducing admixture was found to decrease the yield stress and plastic

viscosity of the system more than that of lignosulfonate admixture indicating an increase in the

flow of the system (Shariar, 2011).

2.4.3 Setting time of OC systems

Setting time of a cementitious system is defined as the duration it takes the system to transform

from a fluid state to a solid (rigid) state (Smith, 1990). The proper placement of a cementitious

system for an operation such as in oil wells is accomplished with adequate knowledge of the

setting time of the system (Struble et al, 2011). According to Nonat and Mutin (1992) and

Brook et al (2000), the setting of cementitious systems results from two fundamental stages:

the coagulation establishing the contacts between the cement particles and the subsequent

formation of the hydrates in the contact zones making the coagulation structure rigid.

Before setting, a cementitious system is plastic and behaves like a fluid, and after setting the

cementitious system becomes solid transforming into a harder substance. According to Struble

et al (2011), the transition of a system from fluid to solid state is a gradual and progressive

process over time. In the setting process of cementitious systems, an initial set and final set

occurs (Slag Cement Association 2016, ASTM 2001). The initial set of a cementitious system

marks the end of the period in which the fresh cementitious system is workable or flow-able

while the final setting time marks the period the system is sufficiently stiff to resist a definite

pressure (Sirinivas 2016, Slag Cement Association 2016). In general, setting is recognized to

reflect the micro-structural changes that take place during the hydration of the cementitious

system (Struble et al, 2011). Taylor (1997) summarized setting as an occurrence that takes



place during the acceleratory period of hydration which typically occurs after a few hours after

the pre-induction (mixing) period and dormant period. According to George (1998), PCA

(2016) and Brunjes (2016) the setting time of a system is affected by various factors such as

the mix water to cement ratio, the chemical and physical properties of the cement material and

the surrounding condition the system is exposed to.

In the construction of wells, an OC system is placed and allowed to achieve initial set before

further drilling operation to a deeper depth is commenced (George, 1998). The time required

for an OC system to set in a well construction ranges from five (5) to eight (8) hours (300 to

480 minutes) for the surface casing zone and twelve (12) to twenty four (24) hours (720 to

1440 minutes) for the intermediate and the production casing zones (Cairo university, 2012;

Schlumberger, 2016). However, due to the surrounding conditions such as varying temperature

degrees in the well, the setting of the OC system can be adversely altered (Zhang et al, 2010).

Settings that occur below the desired time as a result of high temperature in an oil well can

have devastating consequence on the placement of the system due to loss of circulation, while

settings that occur above the desired time can present huge financial implications due to the

loss of productivity time (Shariar, 2011). These challenges create the importance to manage the

setting time of an OC system during the construction of the oil well.

For instance, the setting time of a cementitious system can be increased by reducing the

proportion of tri-calcium aluminate (C3A). Setting times of up to 6 hours at a temperature of

21°C and 4 hours at a temperature of 93°C can be achieved with Portland cement types that

includes no C3A compared to Class C cements (Ramachandran, 1984). The mix water to

cement ratio can also be used to alter the setting of cementitious system (Hu et al, 2014, Nili et

al 2013). Hu et al (2014) and Nili et al (2013) reported that the lower the water to cement ratio,

the higher the rate of hydration which leads to a decreased setting time in setting, while higher

water to cement ratios decreases the rate of hydration resulting in an increase in the setting time

of the system.

Retarding admixtures can also be used to alter and increase the setting time of a cementitious

system, especially in elevated temperature condition (Ramachandran, 1984). Retarding

admixtures alter the hydration of the cementitious systems to extend their setting time (Ramya,



2008). For instance, carbohydrates such as sugar alter and extend the setting time of

cementitious systems, thus they can be used as retarding admixtures (Bentz et al., 1994).

However, these are not commonly used in well cementing because of its sensitivity with small

variations in its concentration to render a different setting performance for cementitious

systems (Nelson et al 1990; Bermudez, 2007). According to Bermudez, (2007), a percentage of

5% of sugar and below is commonly used and acts as retarder, while a percentage above 5%

acts as accelerator to the cementitious system. Lignosulfonates and hydroxycarboxylic acids

are additional types of retarders that are used in extending the setting time of systems. They are

found to be more effective for OC systems or cements with low C3A contents (Nelson et al.,

1990). The process by which retarding admixtures extend the setting of cementitious system is

still not properly understood but it is identified that such agents bind calcium ion (Taylor,

1997) thereby reducing the rate of hydration (Kamsuwan and Srikhirin, 2010). Billingham et al

(2005) studied the effect of retarders on cementitous system and identified that the chemical

activities of the retarders influenced and decreased the initial rate of hydration.

Other types of retarding admixtures exist that are used for several cement and concrete jobs

especially for cement jobs above ground. Ground granulated blast-furnace slag (GGBS),

pulverised fuel ash (PFA) among others are some of the agents used as retarders in cement and

concrete job. Ge (2003) conducted a test on the settings of cementitious system incorporating

15% of PFA and 20% of GGBS admixture. The setting times of the cementitious systems were

found to increase with partial replacement of the cement material with PFA and GGBS

material. According to Thomas (2007), the influence of PFA material on the setting

performance of cementitious systems is related to the chemical and physical properties of the

PFA material. The calcium content of a PFA material is the best indicator of how the PFA

material will influence the setting performance of the system. The lower the calcium content of

the PFA, the lower the amount of calcium available to bond with aluminate and silicate oxides

for the setting of the system. Additionally, Thomas (2007) added that other chemical properties

such as carbon in the PFA material can adversely affect the setting of the system. According to

Brunjes (2016), the presence of 3% or more of un-burnt carbon in the PFA material can

prevent the stiffening or hardening of cementitious system.



Unlike retarders, sodium chloride (NaCl) is used to shorten the setting time of cementitious

systems (Haliburton, 2017; Eric et al, 2016; Bett, 2010; Nelson et al, 1990). Although there are

concerns of the effect of NaCl on corrosion, it was found that corrosion increases with salinity

up to about 5% of NaCl, and above 5%, the solubility of oxygen reduces in the water resulting

in decrease of corrison rate (Brondel et al, 1994). Nelson et al (1990) studied the effect of 10%

to 30% NaCl incorporated by weight of water for cementitious systems and found that it

shortened the transformation time of the system from fluid to solid. NaCl is also used to offset

the delay that is initiated by other admixtures such as dispersants admixtures due to its

accelerating effect in the setting of a cementitous system. Nelson et al (1990) attributed the

accelerating effect to the chemical nature of such admixtures and thier concentration. Salts of

carbonates, aluminates, nitrates, sulphates, as well as alkaline bases such as sodium hydroxide

and potassium hydroxide, accelerate the setting of cementitious system (Nelson et al., 1990;

2006). Glycerin contents of 26% by volume or less are also found to accelerate the setting of

Class G oil well cementitious systems due to thier chemical characteristics (Saasen et al.,

1991). More so, the partial replacement of cement using MK material was found by Siddique

and Kaur (2001) and Shekarchi et al, (2010) to be responsible for the decrease in setting time

of cementitious systems for cement works above ground. Shekarchi et al (2010) found that the

setting time of a cementitious system decreased with an increase of MK dosage from 5% to

15% used to partially replace cement for a system. Although, in some cases, the presence of

MK was reported to extend the setting time of a cementitious system, it was identified that the

properties and dosage of the MK material in the system was responsible for the outcome

(Brooks 2000, Rashad, 2015). Brooks (2000) and Rashad (2015) explained that the dispersion

effect provided by the MK particles on the cement particles and the low dosage of the cement

in the system influenced the retardation in setting time.

The addition of the various types of admixture(s) in a cementitious system to control the setting

time of the system is beneficial in cement works to save operation cost and time. Nonetheless,

admixtures must be carefully selected which require tests to determine their influence on the

setting of the system. This is because; the effects of these admixtures depend so much on their

chemical and physical properties, dosage, interrelationships and the properties of the cement

material. N.N Amucheazi (2018) University of South Wales 35


2.4.4 Strength development of OC Systems

The strength of a hardened OC system is essential in the construction of oil wells. The strength

of an OC system secures the casings and provides stability to the well by preventing vibration

effects to the casing during construction and collapse of the formation in the well (Tipton

2013). OC systems are required to attain a minimum compressive strength of 3.4 MPa and

tensile strength of 0.34 MPa to support the casing string in oil wells (Backe et al 2001). The

importance of the strength of an OC system in the annular spaces is well documented (Myers et

al, 2005, Mojtaba et al 2010, Syahrir et al 2013). The adequate strength of an OC system offers

stability while withstanding shocks from further drilling operations, fracturing or perforation

operations for the production of oil from the well (Ridha et al, 2013). Furthermore, the

adequate strength of an OC system is important in the cementing of multilateral junctions, a

technique which has evolved as an economic means for increasing reservoir productivity and

reducing costs of developing wells (Blanco et al 2002). However, the strength performance of

an OC system is not without challenge as it is affected by various factors.

The strength performance of an OC system is influenced by factors such as the water to cement

ratio, the properties of the cement, the presence of admixture(s) and the surrounding conditions

(Buntoro and Rubiandini, 2000; Saidin et al., 2008; Mojtaba et al 2010). It was found by

Buntoro and Rubiandini (2000) that the presence of magnesium oxide caused an increase in

shear bond strength while decreasing the compressive strength of an OC system at 150oC. In

another case, the partial replacement of cement by 3% to 5% of magnesium oxide at a

temperature of about 250oC enabled an OC system to achieve a desirable shear bond and

compressive strength performance (Buntoro and Rubiandini, 2000; Saidin et al., 2008). The

difference in the outcome was associated to the condition of the surrounding the modified OC

system was exposed to.

Multiple admixtures can also be used to achieve the desired strength performance for

cementitious systems (Mala et al, 2013; Erdem and Kirca, 2008). A modified cementitious

system incorporating 3% of magnesium oxide and 35% of silica by weight of cement was



found by Carathers and Crook (1987) to have a higher shear bond and compressive strength

over three days. They attributed the result to the high pozzolanic activity of the silica material.

Olu (2016) studied the effect of Rice husk ash (RKA) on the strength performance of MK

modified cementitious system. The addition of the 5% RKA material resulted in an increase in

the strength performance of the 15% MK modified system. The occurrence was reported as the

combined pozzolanic effect of the RKA and MK in the system to increase the strength

performance of the system compared to that of neat cementitious system. But, increasing the

ratio of the RKA material to equate with the MK dosage in the system was observed to result in

a decrease of the strength performance of the MK modified system and was attributed to the

unburnt carbon from the RKA material. Trabelsi and Al-Samarraie (1999) noted that the

properties of admixtures can also alter the porosity of the cementitious system which can

contribute to a strength increase if reduced or strength decrease if increased. Trabelsi and Al-

Samarraie (1999) explained that admixtures of finer particles than cement enhance contact

between the cementitious particles and increase reactions for strength. Rao (2003) added that

the strength performance of modified cementitious system is not only dependent on the

physical properties of the admixtures incorporated but also on hydration time or duration.

In regards to the admixture and hydration time effect, Mirza (2002) examined the effect of

pulverised fuel ash (PFA) on a cementitious system for a cement job above ground. The study

compared the compressive strength performance of a modified cement system incorporating

PFA to that of the neat cement system bearing equivalent water to cement ratio for twenty eight

days. The outcome showed the PFA modified cement system had a lower strength than the neat

cement system at twenty eight days. Mirza (2002) associated the strength performance of the

modified cement system to the presence of the PFA material for the twenty eight days period.

But, beyond twenty days, other studies have reported a higher compressive strength with

systems modified with PFA material compared to the neat cement system which is attributed to

the formation of hydrates from the pozzolanic reaction (Thomas, 2007).

Other studies have also been conducted on the effect of water and admixture dosage on the

strength performance of an OC system. Dahab and Omar (1989), studied the influence of water

on the strength performance of an OC system using sea water, fresh water and distilled water.

They deduced from their study that the ratio and type of water used in preparing a cementitious



system affects the strength performance of the OC system due to the pH level of the water.

Hence, a different strength performance is achieved depending on the ratio and water used.

Yogendran et al, (1991) also established that the ratio of mix water and admixture dosage in

the system influences strength performance. Their evaluation on the effect of mix water to

cement ratio of 0.38 and silica dosages of 5% to 25% on a cementitious system indicated that

the strength performance of the system increased. However, the highest strengths of the system

was observed with Silica dosages of 10% and 15% while dosages below or above 10% and

15% silica had lesser strength. The outcome was attributed to adequate hydration with

available moisture for the system incorporating 10% and 15% of silica material (Yogendran et

al, 1991). Heba (2011) also identified a similar outcome with silica fume dosages on the

strength performance of a cementitious system with a water to cement ratio of 0.42. Heba

(2011) concluded that the dosage of admixture for a cementitious system can improve the

strength of a system but a dosage too high can adversely affect the strength at the fixed water to

cement ratio.

2.4.5 Porosity of OC systems

Porosity affects the long term performance of an OC system for the existence of oil wells

(Hearn et al, 1994). The volume and connectivity of pores which are holes of different sizes in

a system affect not only the strength performance of cementitious systems, but also its

durability. The long term performance of a cementitious system in hostile conditions depends

largely on its transport properties, which are influenced by its pore structure (Hooton et al,

1993; Hearn et al, 1994). Three main mechanisms govern transport in cementitious systems

which are permeability, diffusion and absorption. According to Hearn et al (1994),

permeability is the measure of the flow of fluids under a pressure gradient, while diffusion is

the movement of ions due to a concentration gradient and absorption can be described as the

ability to take in water by means of capillary suction. These three mechanisms are greatly

influenced by the amount of the pore spaces in the cementitious system and the connection of

the pores to each other. Hearn et al (1994) established further that capillary absorption is

responsible for the initial ingress of water and other formation fluids that saturate the



cementitious system. More so, the higher the amount of water absorbed by a cementitious

system, the less durable the cementitious system becomes. As such, performance or durability

of a cementitious system is directly related to its absorption of fluids (Hooton et al, 1993;

Bentz et al 2001) which is due to the volume and connectivity of the pores.

The porous nature of a hardened OC system is influenced by various factors which include the

water to cement ratio (w/c), the degree of hydration, the shape and sizes of the cement particles

and the properties of the type of admixtures incorporated (Nail, 1997, Kim et al 2014).

Additionally, the surrounding conditions such as temperature can also influence the porous

nature of cementitious system. According to Hagar (2013), an increase in temperature has a

detrimental effect to the pore structure of cementitious systems which can adversely lead to

unwanted fluid absorption and affect the long term performance of the system. The effect on

pore structure from an increase in temperature was attributed to a change in the rate of

chemical reaction which varies with temperature depending on the water to cement ratio for the

system (Hagar, 2013). Komonen and Penttala (2001, 2004) studied the porosity of cementitious

system under elevated temperatures and found the volume of pores to double when the curing

temperature was increased from 20°C to 1000°C. Based on the outcome of the experiments,

they argued that high temperatures accelerated the hardening of the cementitious system. This

resulted to a coarse system leaving a high pore volume which allows ingress of unwanted fluid

and leads to inadequate performance of the cementitious system (Komonen and Penttala,

2001).

It is important to manage the volume and connectivity of pores in an OC system because of the

migration of formation fluids that are acidic in the well which can adversely affect the

durability of the cementitious system for the long term existence of the well (PCA, 2016).

Formation fluids that are acidic deteriorate the performance of OC systems due to their pH

level that is lower than that of cementitious system (PCA, 2016). As such, admixtures are used

to reduce the pore spaces that can occur in an OC system. Polymeric latexes have been found

to produce a good bond with the cement material for an OC system to disallow the migration of

formation fluid through the reduction of the system permeability (Nakayama and Beaudoin,

1987; Su et al, 1991). Also, as found by Khatib et al (2012), the porosity and pore size of



cementitious system can be reduced using mineral admixtures. Khatib et al (2012) studied the

the effect of PFA and Gypsum to replace 25% of cement material with a w/c of 0.5 for a

cementitious system. They found from the study that the porosity and pore size of the system

was reduced compared to the neat cement system when the PFA dosage was 85% and the

Gypsum was 15% for the 25% partial replacement dosage of the cement material (Khatib et al ,

2012). In another study, Gingos and Mohammed (2011) investigated the impact of three

different PFA replacement dosages (10%, 20% and 30%) on the porosity of cementitious

system. They found that replacing cement with a dosage of 20% PFA at a w/c ratio of 0.5

resulted in the least amount of water absorbed through a period of 28 days. The study

established the reactivity and finer particle sizes of the 20% PFA dosage and the particular w/c

ratio as responsible factors to the outcome. A similar finding was made by Nuruddin et al

(2009) on the effect of admixture dosages on the porosity of a cementitious system. Nuruddin

et al (2009) studied the effect of Rice husk ash (RHA) from 5% to 20% dosage as partial

replacement of cement for cementitous system. They identified from their study that the

dosages of the RHA decreased the porosity of the system. However, the system with RHA

dosage of 5% was found to have the lowest pore volume. Nuruddin et al (2009) attributed the

outcome to change in the reactivity of the system with the dosage of the admixture in the

system. Furthermore, Castro et al (2011) studied the influence of mix water content on the

water absorption of the cementitious system using water to cement ratios of 0.35 0.40, 0.45 and

0.50. They found that the percentage of water absorbed by the system increased with the

increase of mix water to cement ratio for the cementitious system. Thus, the various studies

(Gingos and Mohammed, 2011; Shafiq 2009; Castro et al 2011) imply that the dosage of

admixture and the mix water content for a cementitious system can increase or decrease the

porosity and/or water absorption of a system depending on their ratio(s) to the cement material.

2.4.6 Expansion and Shrinkage of OC systems

Formation fluid migration through space in the annulus of an oil well can compromise the

integrity of the well and lead to a devastating end (Beirute and Watters, 1998, Khandka 2007).

As such, expansion and shrinkage of an OC system are crucial for the long term existence of an



oil well (Ametek, 2008). An OC system expands after its placement in the annulus and this

expansion enables it to properly seal off the formation and contribute to the stability of the well

(Backe et al, 1998). The expansion of an OC system occurs due to hydration heat and it is

preferable in oil well construction to commence after the system has hardened so as to prevent

fracturing or creation of pores in the OC system. More so, if an OC system expands before it

has hardened, the expected isolation to be rendered by the system in the annulus becomes

affected. This is because casing strings undergo expansion due to temperatures in the annulus

of the oil well. Afterwards, the temperature in the well is decreased by drilling fluid (mud)

circulation from further drilling and this can lead to a contraction of the casings resulting in a

micro annulus between the casings and the system (Buntoro and Rubiandini, 2000). However,

this can be properly addressed using admixtures that initiate expansion to the system after

setting. Gypsum and magnesium oxides are some of the commonly used materials that can be

added during or after production of the cement to manage a possible contraction challenge

(Agzamov et al, 2001). However, over time, as hydration of the OC system continues, the

system begins to shrink leaving a micro annular space between the casings or the formation

wall and the cementitious system (Dusseault, 2000).

Shrinkage of the cementitious system is the reduction of the system volume emanating from

the reaction of cement and water over a hydration period (Zhang et al, 2010). The shrinkage of

a system can jeopardise the existence of the well, however, this depends on the degree of

shrinkage as a low shrinkage is believed to lessen the risk of formation fluid migration in the

well (Backe, 1998). According to Cowan (2012), the shrinkage of an OC system is preferred to

be lower than 2% from its initial dimension. For this reason, it is important to manage the

shrinkage of an OC system.

In managing the shrinkage of OC system, it is important to understand the factors that

influence its shrinkage (Garber, 2016). Different factors influence the shrinkage of systems

which includes internal and external factors such as the water to cement ratio, the properties of

the cement material, the characteristics of the admixtures and the condition of the surrounding

the system is exposed to, such as temperature etc. Internally, the factors that affect the

shrinking of a system is ascribed to the reaction of the cement chemicals with water which



leads to loss of water, thereby decrease in size of the system and is refered to as autogenous

shrinkage (Garba, 2016). Externally, the shrinkage of the system is ascribed to the reaction of

the hydration products of the system with CO2 occuring in the surrounding regarded as

chemical shirnkage or the loss water during reaction of the system affected by the condition of

the surrounding known as drying shrinkage of the system (Garba, 2016). According to

Chenevert and Shrestha, (1991), the shrinkage of a cementitious system can be managed and

reduced by decreasing the mix water content for the system or incorporating an admixture such

as sodium chloride, silica, bentonite, or sodium silicate. According to Guneyisi (2010), Idiart

(2009) and Justnes (1995), increasing or decreasing the water to cement ratio influences the

rate of chemical reaction of the system to shrink. Idiart (2009) reviewed the effect of water to

cement (w/c) ratio on a cementitious system shrinkage using w/c of 0.26, 0.45, 0.55 and 0.65

and found that the higher the ratio of cement to water, the greater the shrinkage of the

cementitious system at early days due to rate of hydration while as curing days progresses, the

higher water ratio results in an increase of the amount of water lost from the system and thus

potentiality to suffer higher shrinkage at later ages. The loss of free water causes little to no

shrinkage, but the loss of water from the capillaries, held by hydrostatic tension causes

significantly larger shrinkage (Idiart 2009).

Mokarema et al (2005) associated the shrinkage of a cementitious system to the pore spaces

and hydration of the system. They found that replacing cement partially with GGBS reduced

the volume of shrinkage of the system. This reduction in shrinkage was induced by the addition

of GGBS in the cementitious system and was associated with the reduction of the pore volume

as result of the finer particle of the GGBS (Mokarema et al, 2005). In another study, the

addition of MK was found to reduce the shrinkage of a cementitious system due to an increase

in the hydration products from the chemical reactions and the finer particles of the MK material

(Siddique, 2009). Bonakadar et al (2005) reported on the effect of MK and Silica fume dosages

of 5%, 10% and 15% on the shrinkage of a cementitious system and concluded similarly that

the finer particles and hydration products as a result of the admixtures adjust the pores thereby

reducing the voids which the system is suppose to shrink into.

Backe (1998) highlighted that retarding admixtures are also useful in reducing the shrinkage of

cementitious systems provided that the strength performance of the system is satisfactory.



Using PFA which is reported to have a retarding effect on the setting period of cementitious

systems was found to reduce the shrinkage (Keck and Riggs 1997). Partially replacing cement

with PFA improves the particle packing density of the system while also adjusting the

hydration products of the system (Zhou, 2012). Because of the change to the density and

hydration product of the system, the potential pore spaces are refined to smaller diameters

thereby reducing the volume of possible shrinkage (Shafiq, 2013; Zhou, 2012).

2.4.7 Hydration products and performance of OC systems

Hydration products are chemical compounds that are formed from the reaction of chemicals of

cement in the presence of water. According to Thomas (2015), hydration products are different

types of ionic species that emanate from dissolved cement chemicals in water which increase

over time, saturate the pores of the cementitious system and become energetically favourable to

precipitate into new solids. The hydration products of a cementitious system are different in

their compounds and among the various products are the calcium aluminate hydrates (C-A-H),

calcium silicate hydrates (C-S-H) and calcium hydroxide (Ca(OH)2) which have major a

impact on the performance of a cementitious system.

C-A-H products comprising of Tricalcium Aluminate (C3A) and the Tetracalcium

Aluminoferrite (C4AF) products are crystalline in nature and have major effect on the early age

performance of a cementitious system (Portland Cement Association, 1997). The C-A-H

products, particularly the C3A react most rapidly at the beginning of hydration leading up to the

rigidity of the fresh cementitious system (Portland Cement Association, 1997). According to

Michaux et al (1989) and Ropp (2013) C-A-H contributes to the hardening of cementitious

systems and can adversely affect the rheology and setting of a system. In this regards, the

aluminate content in cement is controlled so as to prevent premature stiffening of a system. For

this reason, gypsum is added to cement to manage the effect of aluminates (Michaux et al,

1989).

The C-S-H is largely amorphous as a gel comprising of Tricalcium Silicate (C3S) and

Dicalcium Silicate (C2S) products (Diamond, 1976). According to Larbi (1993) and Jennings

and Tennis (1994), most of the time-dependent performance of a hardened cementitious system



is controlled by the C-S-H products. It influences the strength and durability performance of a

system as their formation fills the pores in the system and reduces porosity and permeability

(Hussain et al 1994 and Shashi, 2002). Also, the C-S-H product contributes to the setting of

cementitious system, particularly the C3S as it hydrates faster than the C2S product. The C2S

product mainly influences the performance of the cementitious system at the hardened stage

from seven (7) days and for longer period of time (Michaux et al, 1989)

The Ca(OH)2, also known as Portlandite, saturates the cementitious system aqueous phase and

is responsible for raising the pH level of a system (Zhang and Bachu, 2011). The Ca(OH)2 is

found to be beneficial in cementitious systems in occurrence of carbonation but can also be

unfavourable to the durability of a system when bi-carbonation occurs (Chi at al, 2002). Zhang

and Bachu (2011) explained the process of carbonation involving CO2 and Ca(OH)2 in a

hydrated system that is beneficial to OC system. This is due to the formation of calcite

(CaCO3) resulting from the reaction of CO2 and Ca(OH)2 which reduces porosity thereby

improving the performance of the OC system. However, bi-carbonation resulting from CaCO3

and carbonic acid (H2CO3) reaction presents a detrimental effect to the performance of

cementitious system. This is because the calcite is dissolved, resulting in leaching which

compromises the integrity of a cementitious system (Crow et al, 2010). According to Eleni et al

(2014), the rate of carbonation or bi-carbonation is likely to be higher in a cementitious system

with higher water to cement ratio and pore volume and requires consideration for performance.

Omosebi et al (2016) and (Kutchko, 2007) added further, that the occurrence of carbonation or

bi carbonation is strongly dependent on the presence of CO2 or carbonic acid and the content of

available Ca(OH)2 and needs to be managed for the performance of the system.

Compared to neat cement systems, the performance of cements incorporating admixtures such

as ground granulated blast furnace slag (GGBS), pulverised fuel ash (PFA) and silica fume

(SF) and metakolin are found to be durable ( Amudhavalli and Mathew 2004, King 2012 and

Gopalakrishan et al, 2001). The durability of cementitious systems incorporating these

admixtures especially in acidic environment is due to the combined effect of reduced porosity

and reduced calcium hydroxide in the cementitious system. Based on findings by Schiessl

(1998), Gopalakrishan et al (2001) and Oti et al (2009), a cementitious system modified with



either GGBS or PFA or both MAs which are alumina silicates reacts with the Ca(OH)2 to form

additional C-A-H and C-S-H products in the cementitious system. The formation of these

additional hydrate products improves the density of the cementitious system and refines it

pores (Schiessl 1998 and Gopalakrishan et al, 2001). The refinement of the pores decreases the

permeability of the cementitious system, meaning it becomes more difficult for fluids to move

through the pore structure of the system. This makes the system less susceptible to

deterioration. The finding is also supported by the study conducted by Feldman et al (1991) on

the influence of admixtures such as PFA on hydration products of cementitious system.

Feldman et al (1991) conducted a study on the hydration product of a neat cement system and a

PFA-cement system using a thermo-gravimetric (TG) instrument at various w/c ratios with a

focus on the variation of Ca(OH)2 content over time. The study revealed that Ca(OH)2 content

was lower in the PFA-cement system compared to that of the neat cement system over the

study period. Hussain et al (1994) also conducted a TG analysis on a cementitious system

incorporating PFA after hydration period of 180 days. The study revealed a reduction in

Ca(OH)2 content by 53% in comparison to that of neat cement system for the period. The

partial replacement of cement using MK was also found to increase the content of C-A-H and

C-S-H and was reported to be the result of a pozzolanic reaction between the chemicals of the

cement and MK material (El-Diadamony et al, 2016; Ayub et al 2013; Pradip and Ajay 2013).

A study using the molybdate method to investigate the amount of C-S-H product in a

cementitious system over time through un-reacted SiO2 was also conducted (Hubbert et al,

2001). This study found a reduction in the amount of SiO2 in the modified cementitious system

ranging between 6-9% while that of the neat cement system was 15%. The low un-reacted SiO2

revealed a reaction with Ca(OH)2 to form additional C-S-H product in the modified system

(Hubbert et al, 2001). The formation and benefits of hydration products in a cementitious

system makes it worth considering in the development of cementitious system to achieve

durable performance for long term existence of the oil well.

2.5 Temperature and OC system performance

The performance of an OC system during the cementing of oil wells is vital in achieving a

successful cementing job. Several factors affect the performance of an OC system during the



cementing process in oil wells construction including formation fluids, pressure and

temperature. However, amongst the various conditions inherent in the oil well, temperature has

the more drastic effect on the performance of an OC system (Guillot, 1990). The temperature

in an oil well increases with geological depth and can exceed 200oC (Gunnar, 2008). According

to the West Virgina University as cited in Malagay (2013), the temperature increase in an oil

well differs with location. For instance, in the oil producing area of Nigeria, the geothermal

(temperature) gradient ranges from 13oC to 33oC per kilometer (Emujakporue and Ekine,

2014), while in Iran it ranges from 22.6oC to 31oC per kilometer (Mohadeseh and Bahram,

2010). However, the average temperature increase of an oil well with geological depth

commencing from the surface temperature is noted to increase by 2.5oC per 100m or 25oC per

1km (Figure 2.5) (West Virgina University, cited in Malagay, 2013). The effect of temperature

on the performance of an OC system which includes setting and strength performance required

for placement and stability provisions also depends on the type of cement material, admixtures

and w/c ratio used in the formulating the OC system.

Figure 2.5- Geothermal gradient of petroleum geology provided by West Virginia University USA as cited in Malagay-Bay (2013). (Scale: 1km -1000m)



According to Ge (2003), the setting duration of a cementitious system that influences the

placement of a system decreases with an increase in temperature. From experiments conducted

by Guillot, (1990) on the effect of temperature on an OC system, it was identified that the

placement of an OC system is affected with an increase in temperature (Guillot, 1990).

Comparing the effects of temperature and pressure on the placement of an OC system, it was

reported by Guillot, (1990) that the effects of pressure are insignificant to the placement of OC

system due to low compressibility while temperature has huge effects on placement due to the

transformation of the system from fluid to solid state. A similar finding was made by

Kellingray et al (1990) using a modified consistometer to study the solidification of an OC

system at a high temperature of 119oC and pressure of 81.1 MPa. It was found from their study

that temperature had considerable effects on the placement of OC systems.

The effect of temperature on the strength performance of an oil well cementitious system has

also been studied (Bezerra et al 2011, Noik and Rivereau 1999). Bezerra et al (2011) studied

the performance of an OC system in comparison to an OC system incorporating silica dosages

of 0, 18% and 36% at temperatures of 30°C, 100°C, 120°C, 180°C and 230°C. The aim of the

study was to identify the effect of temperature on the strength performance of a neat OC

system and that of an OC system modified with silica. The outcome of the experiment revealed

that the compressive strength of the OC modified with 18% silica recorded the highest

compressive strengths compared to that of the neat system and the highest strength was

observed at 180oC. The compressive strengths of the OC system incorporating 18% silica were

higher than that of the neat OC system for the various temperatures examined ranging from

2.34% to 41.64% at 12 hours. The findings reveal that temperature can have a detrimental

impact on the strength performance of a neat OC system and that an admixture based on its

dosage applied can be beneficial in managing the effect of temperature on the strength

performance of an OC system. The temperatures used in the study were based on some of the

temperatures encountered in oil well construction with geological depths (Figure 2.5).

Komonen and Penttala (2001) also identified compressive strength loss with an increase in

temperature for cementitious system. From their study, it was noted that cementitious systems

exposed to a temperature between 50oC to 120oC can experience a similar loss of compressive

strength as that caused by a temperature of about 400oC due to inadequate hydration.



Furthermore, it was found that temperature adversely affects the durability of an OC system

through coarsening of the pore structure leading to a higher volume of pores in the system. The

porosity of a cementitious system was found to increase with an elevation of temperature from

20oC to 1000oC which affects strength performance or allows ingress of fluids, including acids

that can compromise the integrity of a cementitious system (Komonen and Penttala, 2001).

These effects of temperature including the change in rate of hydration and porosity that

influence the performance of an OC system present a huge challenge in the cementing of oil

wells. According to Shariar (2011) the effects of well conditions, such as temperature, leads to

several tests and errors, including material wastes and cost to design an OC system. Such an

exhaustive task is carried out to achieve an adequate OC system performance for cementing an

oil well. This is because, the improper design of an OC system can lead to a disastrous end of

the oil well, pollution of the environment, high cost and loss of oil and other resources. Thus,

knowledge of the effect of temperature on the performance of an OC system can aid in

developing a model to simplify the process of designing or formulating an OC system with

adequate performance for cementing of oil wells with varying temperature conditions.

2.6 Modelling and OC system performance

Models can be mathematical or theoretical and are used to predict outcomes from tests or

observations and to reduce the number of investigation to determine outcomes (Stroud, 2001).

Over the past decade, several types of models have been developed to predict the performance

of OC systems (Shariar 2011, Salam et al 2013; 2014) however; these models differ in purpose

due to their different focus. Some of the recent models for an OC system have been focused on

the placement and stability provision of an OC system in an oil well due to challenges posed by

the oil well conditions such as increasing temperature with geological depth in the construction

of a well (Shariar 2011; Guo et al 2016). Developing a model of the setting time and strength

performance of an OC system can be beneficial in oil well cementing as this can simplify the

complexity in designing an OC system to achieve adequate placement and stability provisions

under specific oil well conditions like temperatures. Models are used to predict observation and

can be mathematical to predict performance such as that of cementitious system (Banfill, 2003; N.N Amucheazi (2018) University of South Wales 48


Nehdi 1998 and Guillot, 1990). According to El-Chabib and Nehdi (2005), models such as

those used for cement performance are empirical equation derived from the analysis of

observations within test limits. However, in cases where several effects are present in test

observations, a mathematical expression to include the multiple effects is required.

Regression, a form of mathematics which analyzes the effects of one or several variable(s) on a

product can aid in such cases where one or more parameter(s) have effect on a product.

Regression attempts to equate the relationship between independent parameter(s) and

dependent parameters (Statsoft 2010). The relationship between the parameters may be linear

or non-linear (Stroud, 2001). According to Cohen, et al (2003), multiple regressions explains

the effects of a single or multiple parameter(s) on the output (dependent) product with or

without considering the effects of other parameter(s). The use of multiple regression analysis is

common and has been applied in various aspect of engineering including cement performance.

Different types of softwares are also available to simplify the application of multiple

regressions to develop models for various products in engineering. Software packages such

SPSS, Microsoft excel or design expert are some of the packages that can analyse and develop

regression models. However, the details provided by particular software may differ from

details provided by another depending on the package of the software. For instance, Design

Expert provides a detailed graphical representation of the parameter(s) effects on a product

while also providing a statistical analysis of the effect of the parameter(s) unlike some of the

other softwares. Several models have also been developed using Design Expert for cement

performance and other products for various operations in the field of engineering (Salam et al,

2014; Oke et al, 2014; Falode et al, 2013 and Shariar 2011).

Shariar (2011) developed a mathematical model using Design Expert to predict the rheological

properties of an OC system. The mathematical model was focused on predicting the effect of

MK, SF, PFA and RHA on the rheological properties of an OC system in temperatures of 23oC,

41oC and 60oC existing in oil wells that can impact the placement of systems. Shariar (2011)

found from the study that the equation of the mathematical model for each system differed to

predict the rheological properties of each type of system. This was due to the different impact

of each admixture on the rheological properties of the OC system. Shariar (2011) also

investigated the various regression orders to predict the rheological properties of the OC



system and found the second order to offer the best prediction. Shariar (2011) attributed the

performance of the second order to predict the output as a result of the non-linear nature of the

observed outcomes of the rheological properties of the OC systems. However, the rheological

models developed by Shariar (2011) using Design Expert did not include time. Rheological

properties of an OC system can aid in understanding the placement behaviour of OC system,

but, the placement of OC system requires knowledge of the time the system will set so as to

place the system on-time and save cost or avoid financial implication from undesired time of

setting of a system in the construction of oil well.

Falode et al (2013) investigated the strength performance of OC systems and developed a

mathematical model using Design Expert to predict the strength performance of an OC system.

The model was focused on the effect of various admixtures including retarders, accelerator,

anti-foam and dispersant on the strength performance of an OC system. Falode at al (2003)

found that the mathematical model for the strength prediction of each system differed to be

able to predict the observation from their investigation. However, the investigation and the

model developed by Falode et al (2013) for the strength performance of OC system did not

include the effect of well condition such as temperatures. The knowledge of the effect of

admixture on the strength performance of an OC system is vital, however, the condition of the

oil well such as temperatures can alter the strength performance of an OC system to render

needed stability to the well thereby making it important to be studied for adequate performance

and stability functions of a system in the oil well.

Thus, the use of Design Expert can be adopted to develop a multi-domain model from the

experimental outcome of OC system performance such as the setting time and strength

performance at 24 hours for OC system modified with admixtures to contend with

temperatures in oil well. This can simplify the design of an OC system for adequate placement

and stability provision among other required performance for functions in the oil well. The

model will also be beneficial in reducing the time, amount of tests, use of material and cost in

achieving adequate performance in the formulation of OC systems.


Chapter 3-Materials

CHAPTER 3- MATERIALS

This chapter presents the details of the materials used in the current research study; the

source of the material, reasons for choosing each material and the properties of the

materials. The materials used for the study include Class G Portland cement (OC), Mineral

admixtures which include Metakaolin (MK) and Pulverised fuel ash (PFA), a Super-

plasticizers (SP) chemical admixture and water. The details of the materials are presented

below.

3.1 Cements

The Portland cement type selected for this study was the Class G oil well cement type

conforming to API Spec 10A. The selection of this cement type was due to its sulphate

resistance characteristics and ability to cement at deeper depths than other cements. More so,

it is the conventional cement type used for cementing of oil wells (Ogbonna and Iseghohi,

2009). The class G cement used in this study was supplied by Cebo Limited, UK. The

chemical (oxide) composition and physical properties of the cements are presented in Tables

3.1 and 3.2.

The use of cement as a binder is well established (Neville, 1995, Brooks et al 2000, Nelson et

2006) and this study employed the class G cement type as the control based on its

conventional use for oil well cementing. The class G cement was used as a control to assess

the performance of other formulated cementitious systems in the current research. Several

studies have been conducted on the use of MA to modify the performance of cement for

above ground works but only limited study can be found on cement incorporating MA and

other admixture to modify performance of a system for cementing of oil wells (Shairar,

2011). This current study is based on using MA and chemical admixture to modify the

performance of cement for cementing oil wells so as to contribute some knowledge to the

technology of oil well cementitous system. More so, the use of cements incorporating MA is

seen as way of reducing the adverse impact of cement to the environment such as cutting

down on energy usage and carbon dioxide emitted in cement production (Kumar et al, 2014).


Chapter 3-Materials

As noted by Stajanca and Estokova (2012), with the energy crisis that is experienced as

energy demand increases, it is necessary to imbibe methods that will decrease energy usage

including the reduction of CO2 while pursing performance in cement technology.

According to Bye (1999) and Little et al (2000), cement is a material which binds various

solid bodies together after contact with water to transform it to a hard substance. It develops

rigidity after contact with water and then starts to increase in strength due to chemical

reactions and it can also harden in water (Eglinton 1987). The cements products such as the

Portland cement type can be manufactured using different process which include the wet and

the dry process (Bye 1999, Neville 1995). However, regardless of the process adopted for the

manufacturing of Portland cement materials, the manufacturing of the cement requires high

degree of temperatures between the ranges of 1350oC to 1500oC using about 5000MJ of

energy which is a huge concern for the environment (Robertson 1974; Higgins 2007). More

so, green house gases are emitted during the manufacturing of cement product amidst the

quantity of raw materials that are used for making the cement (Bye 1999, Neville 1995,

Higgins 2007). It is reported that about one and half (1.5) tonnes of limestone and clay

material are quarried to produce one (1) tonne of Portland cement and one tonne of CO2 is

emitted into the atmosphere for the production of one tonne of cement which all affect the

environement (Higgins, 2007). As such industries are encouraged to reduce the reliance on

cement by adopting alternatives that can reduce the adverse impact of cement to the

environment while pursing performance (Christodoulou, 2000)

3.2 Metakaolin (MK)

Metakaolin obtained from processing kaolin clay is one of the mineral admixtures that was

selected for use in this research. The selection of Metakaolin material for this research study

was based on its chemical (oxide) and physical properties. Its chemical and physical

properties differ from that of cement with high alumina and silicate composition and a high

particle surface area. Some of the relevant oxide and physical properties of the metakaolin

material are presented in Table 3.1 and 3.2. Also the selection considered the environmental

aspect as well as the availability of the MK material. From investigations of Michel de Spot

and Wojtarowicz (2003) and Torgal and Jalali (2011), the environmental impact of MK is


Chapter 3-Materials

outlined to consume less energy to generate a heat of about 600oC to 800oC with a CO2

emission of 0.07 tonne for the production of one tonne of Metakaolin compared to the high

energy consumption (5000MJ) and one tonne of CO2 emitted to manufacture a tonne of

cement. On the availability aspect, kaolin clay used for producing Metakolin is found to be

available in large quantities of billions of tonnes in most countries including oil producing

countries such as the United States of America, Britain, Iran, Mexico, Brazil, Nigeria etc

(Virta 2002). For instance, in Nigeria, one of the oil producing countries, large deposits of

kaolin clay are scattered in different parts of the country. According to Foraminifera (2012),

Nigeria has an estimated reserve of about two billion (2,000,000,000) metric tonnes of kaolin

deposits. Recently, more efforts have been geared towards developing the solid mineral

sector of Nigeria which includes mining kaolin mineral that will create business

opportunities. The market for Kaolin is large and is viewed as sustainable because of the

numerous applications of the product including in cement technology. Most of the countries

produce, import or export and process kaolin mineral to Metakaolin for use in cement

technology (Virta 2002). Kaolin can be supplied in raw or processed form depending on the

requirements and can be supplied or transported as dry powder, semi-dry noodle or as liquid

slurry. In the case of this research, a dry powder Metakaolin processed from Kaolin Clay was

used. The dry powder Metkaolin material has also been used by other researchers in cement

technology (Khatib et al, 2014; Bai et al, 2007; Kinuthia et al, 2000). The material was

supplied by Imerys Minerals Ltd UK, formerly known as English China Clay (ECC)

International Ltd, UK.

3.3 Pulverised Fuel Ash (PFA)

Pulverised fuel ash (PFA) is a by-product of coal fired electricity generating plants. It is a

finely divided residue resulting from the combustion of ground or powered coal which is

transported from the firebox through the boiler by the flue gases. Physically, PFA is a

powdery material, composed mostly of silica with mostly silt-sized and clay-sized glassy

spherical particles (ACI committee 116, 2013). According to ASTM C 618 (2016), two

classes of PFA are available which are differentiated by the proportion of chemicals (oxides)

or type of coal used in the production of the PFA material. The amount of calcium, silica,


Chapter 3-Materials

alumina and iron content possessed by the ash define the PFA material type which include

class F and class C PFA materials (ASTM C618, 2005). The calcium content (also known as

lime) of the Class F PFA is usually less than 20% (Karthik, 2008; Jatale et al, 2013) and the

class F PFA material requires an alkaline activator or cement agent for reaction and is usually

recommended to be used in sulphate bearing environment. For the class C PFA, the burning

of the younger lignite or sub-bituminous coal produces the Class C PFA material. The class C

and the class F PFA materials are both pozzolans because of their chemical compositions

expect that class C PFA has self cementing capabilities. The class C PFA material contains

more than 20% of calcium content which can enable it to harden and gain strength over time.

In this case, class C PFA material can do without an activator unlike the Class F PFA

material to harden (Suresh and Sundaramoorthy, 2014).

The class F PFA was selected for this research study. The selection of the Class F PFA

material for this research was based on its chemical and physical properties (Table 3.1 and

3.2). The class F PFA lacks self cementing capability due to its low calcium chemical

composition and possess spherical shaped particles which is unique among the various

available MAs (Bapat 2012; Ramachandran, 1995). Also the environmental benefit of PFA

which entails no use of energy and CO2 emission for its product as well as the availability in

some of the cement manufacturing countries and oil producing countries was considered in

its selection. Class F PFA is found to be available in large quantities in some of the cement

and oil producing countries such as United States of America, Mexico and Nigeria. For

instance, Nigeria which is one of the oil producing nations in the world and experiencing a

rise in cement production could benefit from the impact on Class F PFA in oil well cement

technology. Nigeria holds an estimated coal reserve of about 2 billon metric tons (Amadi

2012). Over the past decades, Nigeria coal production declined but major emphasis on power

generation has initiated a rise in the production of coal in Nigeria. Thermal coal, which is the

type of coal mined in Nigeria, is used in electricity generation and other energy raising

processes. Currently pulverised fuel ash is obtained from the combustion of coal at the Oji

River thermal station in Nigeria (Amadi, 2012) with plans of additional coal power plant to

be constructed. Thus, the Coal power plant will provide numerous advantages including

industrial by-products such as pulverised fuel ash that can be put into use in cement


Chapter 3-Materials

technology. The Class F PFA used in this research study has been supplied by Aberthaw

Works- Lafarge Company in Wales, United Kingdom.

Table 3.1-Oxide composition of the class G cement material (Cebo ltd), Metakaolin and Pulverised fuel ash material (Christodoulou, 2000)

Oxide (%) Class G cement MK PFA

CaO 64.2 0.07 1.4

SiO2 21.6 52.1 49.8

Fe2 O3 4.9 4.32 9.3

Al2 O3 3.3 41.0 26.4

SO3 2.2 0.01 0.8

MgO 1.1 0.19 1.4

K2O - 0.63 3.5

Na2O 0.41 0.26 1.5

TiO2 - 0.81 1.0

Loss of Ignition (LOI) - 0.6 4.9

Table 3.2-Some physical properties of the class G cement material (Cebo ltd), Metakaolin and Pulverised fuel ash material (Christodoulou, 2000)

Physical properties Class G cement MK PFA

Bulk density (kg/m3) 1505 300 1000

Specific gravity 3.15 2.5 2.3

Specific surface (m2/kg) 385 12000 300-600


Chapter 3-Materials

3.4 Super-plasticizer (SP)

A super plasticizer chemical admixture code named “ADVA 340” by Grace company,

developed with a modified synthetic carboxylated polymer and complying with BS EN 934-

2, BS 5075-3 and ASTM C494 Type F specifications was selected for this study. The super

plasticizer admixture was selected because of the enhancement it renders to a cementitious

system for placement while offering plasticity effect which allows for mid to high range

water reduction leading to considerable improvement in compressive strength, im-

permeability and durability of a cementitious system. Its use is efficient with the various

classes of Portland cements, including cement with sulphate resisting capability (Grace

2016). However, the magnitude of the effect of chemical admixture is governed by the

quantity of the product used, the water to cement ratio, and the characteristics of the cement

and constituent materials for the cementitious system. Hence, this makes it necessary to

assess the influence of admixtures to determine the desired dosage for cementitious system

formulation such as its workability, set characteristics, compressive strength and shrinkage.

The effects of over dosing chemical admixtures, depend on the type used and function of the

degree of overdose which may increase the level of workability and may induce the on-set of

segregation and extend setting of the cementitious system above desired times. The ADVA

340 SP admixture is recommended by the supplier to be used between 0.2% to 1% by weight

of cement for the cementitious system (Grace 2016). However, in this research, the dosage

range of 0.2% to 1% of the ADVA 340 SP admixture was applied by weight of mix water as

guided by API recommended practice 10B-2 for the use of liquid admixture for oil well

cementitious system (API RP 10B-2, 2005). The ADVA 340 SP chemical admixture selected

for this research study was supplied by the manufacturer- Grace Construction Products Ltd,

United Kingdom.

3.5 Water

Water is a universal solvent and the most abundant molecule on the earth’s surface,

constituting about 70% of the planet’s surface (UN water report, 2016). In nature, its

existence is in liquid, solid and gaseous states. It is a colourless, tasteless and an odourless


Chapter 3-Materials

substance at standard temperature and pressure (UN water report, 2016). Tap water devoid of

any contamination and complying with standards for drinking water as recommended by the

World Health Organisation (WHO 2008) was used for this study. The selection of the tap

water was important in order to avoid any possible contamination from non-trusted water

sources that could affect outcomes in the study. The tap water used for this study was

supplied by the University of South Wales water network.


Chapter 4-Methodology

CHAPTER 4: METHODLOGY

This chapter describes the formulation process for the various oil well cementitious systems

that were developed and tested in this research study. It also contains the experimental

procedures followed in conducting the tests and investigating the various OC system

performance charateristics as outlined by the American Petroleum Institute (API) and the

American Society for Testing and Materials (ASTM) standards. The experimental tests

conducted for the study include; density, rheology, setting time, compressive strength, tensile

strength, water absorption, shrinkage, thermo-gravimetric measurement of hydrate product

of the formulated oil well cementitious systems. The chapter also includes the approach

adopted for the development of a model to ease the design process of an OC system with

particular focus on setting and compressive strength performance under elevated

temperature.

4.1 Tests approach

The approach for this research study to modify and model the performance of the OC system

which include the density, rheology, setting time, compressive strength, water absorption,

shrinkage and hydration involved; formulation of the OC systems, testing of the various OC

systems performance, selection of OC systems with satisfactory performance, investigation of

temperature effect on setting time and compressive strength of the selected OC systems and

modelling the setting time and compressive strength of selected OC systems. These aspects

were carried out to fulfil the aim of the research.

According to Buntoro and Rubiandini (2000), the type and dosage of admixture used in

formulating a cementitious system affects the performance of the system. Typically, the class

G Portland oil well cement is used for oil well cementing (Ogbonna and Iseghohi, 2009).

Thus, the study employed the class G cement as the control, based on its conventional use for

oil well cementing. Hence, to achieve the purpose of the current study, the performance of

class G cementitious system with different water to cement ratio was investigated to identify

the influence of mix water ratio on the performance of OC system. Also the class G cement at



a fixed water to cement ratio of 0.44 was modified to develop other OC systems by the

addition of two mineral admixtures. The modification involved using the proportions of 10%,

20%, 30%, 40% and 50% of the selected mineral admixture (MK and PFA) to partially

replace the class G cement to develop OC systems. This was to investigate and determine the

influence of the selected mineral admixtures in modifying the performance of an OC system

for condition in the oil well. The control was used to evaluate the performance of the

modified OC systems using the two different types of mineral admixtures (MK and PFA)

adopted for the study. Figure 4.1 presents the systematic format adopted for the preparation

and experimental tests in this study.


Model Development

Hardened state investigations-Strength developments-Shrinkage-Water absorption- Hydration-Thermogravimetry

Plastic state investigations-Density-Rheology-Setting times

Setting time & compressive strength

investigations at selected temperatures

Evaluation &

Selection

Mixing

Specimen preparations

Figure 4.1 OC system development, tests and model approach


4.2 Mix design

At the preliminary stage of the research work, it was found necessary to look at the trends of

designing oil well cements before formulating the various OC systems for the current study.

For oil well cementitious systems developed using the class G cement material, the water to

cement ratio (w/c) of 0.44 was found to be the standard w/c ratio used for designing class G

OC systems (API, 2002). The design standards of OC systems are specified by the American

Petroleum Institute for successful cementing of oil wells (API Specification 10A, 2002). As

such, the w/c ratio of 0.44 was adopted as the control design to evaluate the performance of

other developed OC system.

The mixtures in this study were conducted to modify the performance of an OC system for

the desired performance under oil well condition. Therefore, different w/c ratios specifically

for class G OC system and particular dosages of 10%, 20%, 30%, 40% and 50% MK or PFA

were used to design and partially replace the class G cement for OC system with fixed w/c

ratio, while adopting the standard w/c ratio (0.44) for class G OC system as the control. The

selected w/c ratio and dosages for the MA were to enable the full range of possible

performance emanating as a result of either water ratio and MK or PFA presence in the OC

system to be identified.

For the purpose of the study, after the identification of the performance of the various OC

systems designed at various w/c ratio and OC system incorporating MA, only the w/c ratio

and dosage of MK or PFA that improved the performance of the OC system and satisfied the

API requirement were selected for setting time and compressive strength performance

investigation under elevated temperatures and further modification using the super plasticizer

chemical admixture (SP). This was to establish the performance of the modified OC systems

at some of the different temperatures that exist in oil wells. The outcome was later used for

the development of models to simplify the design process of an OC system for cementing oil

wells with such temperature conditions. Table 4.1 shows the details of the mix composition

of the various OC systems formulated and tested in this research study for improved

performance compared to the control system.



Table 4.1-Mix design and material composition of the various OC systems

System code Class G cement

(OC)

Metakaolin

(MK)

Pulverised fuel

ash (PFA)

w/c Super-plasticizer (SP)

OC0.36 100% - - 0.36

0.2% , 0.4%,

0.6%, 0.8% and

1%

For selected Systems

OC0.40 100% - - 0.40

OC0.44 100% - - 0.44

OC0.48 100% - - 0.48

OC0.52 100% - - 0.52

OC10MK 90% 10% - 0.44

OC20MK 80% 20% - 0.44

OC30MK 70% 30% - 0.44

OC40MK 60% 40% - 0.44

OC50MK 50% 50% - 0.44

OC10PFA 90% - 10% 0.44

OC20PFA 80% - 20% 0.44

OC30PFA 70% - 30% 0.44

OC40PFA 60% - 40% 0.44

OC50PFA 50% - 50% 0.44

Note: OC0.44 was adopted as the control system in the research study based on API specification

4.3 Specimen preparation

Before mixing and testing any of the oil well cementitious system (specimens) in this

research, samples of the materials to be used which comprised of Class G cement, metakaolin

and pulverised fuel ash were extracted from their specific container using a clean suitable

device to avoid contamination. The extracted material samples were packaged and labelled as

sufficient to formulate the cementitious system specimen for tests. For the extraction of the

chemical admixture (SP) from its container to be incorporated in the cementitious systems,

stirring of the chemical admixture was conducted prior to extraction from its storage

container. This is because, most liquid admixtures are solutions and suspensions or separation

of the chemical ingredients can occur in the storage or when left in static state. Active



ingredients of the chemical admixture may hover to the top or settle at the base of the

container. As such, the extraction of the chemical admixture (SP) was done by stirring and

extracting sufficient amount from the middle of the container, using a clean and dry device.

Also, the materials for the entire test were stored in the same location to avoid contamination

from other products in the storage and with labels for identification. For mixing water, care

was taken to avoid contamination while taking sufficient amount from the source.

The measurement of the proportions (dosages) of the materials to be mixed was conducted

using an electronic weighing device. The weighing device was fixed to the unit of gram (g)

for the mass of each material proportion (i.e Class G cement, MK, PFA, SP and water) for the

OC system specimen. The device was accurate to ±1% of reading for measurements at 10g or

above up to the full scale of the weighing device. For measurements less than 10g; the device

was accurate to ±0.1% of reading. The weight balance was fixed to two (2) decimal places

precision at a minimum. For the mixing of the OC systems (specimens), a Hamilton beach

(HMO 200) bench top mixer was used for the preparation of specimens. The mixing device

for the preparation of the cementitious systems had a bottom blade for blending and a

container capacity of one (1) litre. The mixing container and blade are constructed of

corrosion-resistant material and the mixing blender assembly is constructed so that the blade

is separate from the drive mechanism as specified by API for preparation of OC system for

tests (API 10B-2, 2012).

4.3 Testing

4.3.1 Density

The importance of the OC system density test is to determine the adequate density to

minimize diffusion of drilling mud and the displacement formation fluid by the cement in its

fluid-like state. The test was conducted using a fluid density balance as specified by API

10B-2. Firstly, the fluid density balance was calibrated using water before introducing the

prepared OC system in the device for measurement. The formulated OC systems were mixed

and poured into the cup of the fluid density balance after calibration. The lid of the balance

cup was then used to cover the cup and pushed downward into the cup until surface contact



was made between the outer skirt of the lid and the upper edge of the cup. This allowed any

excess OC system to spill from the cup and was wiped off to keep the cup clean and to

achieve accurate density result of the cementitious system. The rider was then slid across the

graduated arm till a balance was established from the level bubble. The density of the OC

system specimen(s) was determined with equation 4.1 using the fluid density balance

instrument.

Ds=MsVs ....................... (Equation 4.1)

4.3.2 Rheology

Rheological tests determine the flow properties of visco-elastic fluids such as oil well

cements. In the present study, the various OC system specimens were prepared according to

API-10B (American Petroleum Institute, 2005). The preparation of the OC system paste for

the rheological test commenced with weighing and blending the materials (cement and

mineral admixture) in their specified proportions before adding into mixing fluids (water).

The various OC systems were added into the mixing water in not more than 15 seconds for

uniformity. All mixing was carried out at ambient temperature. After the OC systems

preparation, each specimen was homogenized at a rotational speed of 150 rpm for 20 minutes

in an atmospheric consistometer. The atmospheric consistometer apparatus used for the

homogenization was a Fann 165AT concentric cylinder consistometer. After the

homogenization, the specimens were transferred immediately to the test vessel of the

viscometer for measurement. The torque response, commencing from 5.11 - 511s-1 for each

rotational speed from 3 - 300rpm provided by the viscometer respectively was recorded.

However, only the response from 10 - 511s-1 was adopted for the study. This is because of the

controversy regarding the guarantee of flow readings at speeds below 6rpm for cementitious

system as there is frequently poor repeatability of reading at such lower speed (Gandelman et

al 2004). The Bingham plastic model (using the slope of the downward curve) was adopted to

determine the rheological properties (yield stress and plastic viscosity) of the various OC

systems after the measurement. The choice was due to the wide use of the Bingham plastic



model for the study of the rheological properties of oil well cementitious systems in the oil

industries (Guillot, 1990, Shariar 2011).

4.3.3 Setting time

The setting time tests were undertaken according to ASTM C191 08 to achieve the duration it

takes for the OC system to become stiff. The reaction of the mixed cement with water forms

chemical compounds which transform the cementitious systems from fluid or plastic state to

a solid capable of withstanding substantial loads. ASTM C191 08 specifies the procedure for

determination of the setting time.

The setting time test was carried out with the use of a Vicat apparatus as specified by ASTM

C191 08, which comprises of a hollow ring, a base plate and a 1mm diameter needle for

penetration attached to a movable rod. The tests were conducted by preparing neat OC

systems with different water to cement ratios and OC systems incorporating particular MA

materials at a fixed water to cement ratio. The Vicat mould with height of 40±1mm and a

glass base plate was then completely filled with the prepared OC system, smoothened to level

with the top of the mould and allowed to settle in exposed condition for 30 minutes before

penetration tests were commenced. The tests were conducted at three different temperatures

of 23oC, 42oC, and 60oC. For the elevated temperatures, the mould with the prepared OC

system samples were transferred into an oven at the test temperature, and removed at specific

time intervals for tests. The mould with the OC system specimens was properly positioned

underneath the vicat needle periodically and penetration was performed on the specific OC

system by allowing the 1mm Vicat needle to settle into it. The Vicat initial time of setting is

the time elapsed from when the cement and water was mixed including the 30 minutes

interval before commencing penetrations to the time when the penetration was measured to

be 6±mm from the bottom of the mould. The same approach was adopted for the selected OC

system incorporating chemical admixture (SP) to investigate the specimen setting times at the

various temperature degrees.



4.3.4 Strength development

4.3.4.1 Unconfined Compressive strength

The unconfined compressive strength test of the various formulated OC systems in this study

were conducted according to ASTM C109. The unconfined compressive strength test was

adopted against the confined strength test commonly used for OC system due to finding made

by Rahman et al (2010). According to Rahman et al (2010), confined test approach influences

the system to achieve strengths or performance that might not be obtainable if the system is

unconfined and exposed to surrounding condition that will have impact on it. Thus, the OC

systems were prepared and poured into 50mm x 50mm cube moulds with a thin layer of oil

applied to the interior surface of the moulds to prevent the OC system specimens from being

damaged during removal for testing. Each specimen was puddled sufficiently into the mould

with a puddle rod and the excess cementitious system was removed to ensure the specimens

were uniform in the mould. Each formulated cementitious system specimen comprised of at

least three test samples for curing and testing.

For the strength development over days, the formulated OC system were initially cured at

atmospheric temperature for 24 hour and the hardened cementitious cubes were then removed

from the mould before they were placed in a curing tank and maintained at a curing

temperature of 23oC. The curing tank is designed so that the sample is completely submerged

in water. The OC system cubes were raised off the bottom of the tank to allow water to

completely circulate around the specimen during the curing period. At the curing duration or

age at which the cube specimens were to be tested, they were removed from the water tank

and compressed to failure to determine the compressive strength.

For the strength performance of selected OC systems under elevated temperature and

optimized with chemical admixtures, the prepared OC system were poured into the moulds

and immediately transferred to ovens to be cured at selected temperature. The selected

temperatures were 23oC, 42oC and 60oC. They were cured for twenty four (24) hours at each

specific temperature degree. At 24 hours, the OC system specimens were removed from the

oven, de-moulded and compressed to failure to determine the strength performance.



The cementitious system cubes were tested immediately after removal from the curing tank

and oven using the Dension testing machine. Measurements of the cube specimen height and

calculation of the minimum surface area in contact with the test platens was conducted and

the test performed in accordance with ASTM C109 specification. Consideration was also

taken regarding the compressive strength load frame by allowing some gap to fit the cube

specimen properly for testing. The compressive strengths (CS) was calculated as the load (Fc)

required to fracture the cementitious systems cubes divided by the area of the cube specimen

(Ac) in contact with the platen of the loading frame. The average compressive strength was

recorded for all specimens for each particular OC system specimen tested.

CS = FcAc .......................Equation 4.2

4.3.4.2 Unconfined Tensile strength

The unconfined tensile strength test was carried out according to ASTM C190. All

cementitious systems were prepared in their various proportions and mixed using the

Hamilton beach bench-top mixer. At the end of mixing, the cementitious system specimens

were transferred immediately into a Briquette mould whose interior surface had been lightly

oiled to ease removal of the specimen after curing for tests. The mould type bears two

shoulders and a gauge in between so the hardened cementitious systems can be gripped for

the splitting tensile test. The dimension of the Briquette mould is provided in Figure 4.2. All

cementitious systems were allowed to harden in the mould for 24 hours after which they were

de-moulded and transferred to a well controlled curing bath with water at a temperature of

23oC. The Hounsfield H10KM universal testing machine was used for conducting the tests.

The machine has two crossheads; one is adjusted for the length of the specimen and the other

driven to apply tension to the test specimen. The tensile test process involved placing the test

specimen in the testing machine and slowly extending it until it fractured (Ft). The Tensile

strengths (TS) of the various specimens were then calculated as the load (Ft) required to



fracture the specimen divided by the cross sectional area of the cube specimen (At). Equation

4.3 was used to calculate the tensile strength (TS) of each particular specimen after the test.

TS = FtAt .......................Equation 4.3

Figure 4.2 –The Briquette mould dimension

4.3.5 Water absorption test

The water absorption test is used to determine the amount of water absorbed under specified

conditions. Factors affecting water absorption include: the admixture used in formulating a

cementitious system, temperature and length of exposure. Basheer et al (2001), defined water

absorption as the mechanism by which liquids are transported in solids due to pores in the

solid. They further identified basic parameters for consideration in the water absorption of

cementitious system among which is the mass of water required to saturate the cementitous

system.

In the current research, the water absorption of the OC system cube specimens of equal

dimension of 50mm for all sides were determined in accordance with ASTM C642. In this

regard, the specimens were prepared and poured into moulds and left for 24 hours under

atmospheric temperature before removal. The specimens were then transferred to a water

chamber and allowed to age for the specific duration to be tested being 1, 7, 14 and 28 days.

At the specific test date, the specimen were transferred to an oven and dried (in 105 oC for 24

hours) to constant mass and allowed to cool to ambient temperature. Immediately upon


75mm

42.5mm25mm

50mm


cooling, the specimens were weighed to achieve the dry mass (Wd). The specimens were then

placed in a water tank that has the capacity to submerge the whole specimen to reach

equilibrium. After 24 hours, the specimens were removed from the tank and the surface water

on the specimens wiped off with a damp cloth and the specimens were reweighed (Ww). The

percentage of water absorption (Wa) expressed as increase in percentage of weight was then

calculated using Equation 4.4.

Wa = [Ww−Wd ]Wd × 100% .......................Equation 4.4

4.3.6 Shrinkage test

The shrinkage test is used to determine the change in length of the cementitious systems over

time when the specimens were exposed to specific condition. For the purpose of the current

study, drying shrinkage was carried out to indentify the difference in the change of length of

the various OC systems over time. The shrinkage of the various formulated OC system

specimens was determined in accordance with ASTM C157 for drying shrinkage. The

various specimens were prepared using the standard method and cast in prism moulds

(dimension 25×25×285mm) with a piece of metal inserted into each end to determine the

specimen length at different times. The OC system specimens were allowed to set for 24

hours after which they were de-moulded and placed in lime saturated water. After 30 minutes

in the lime water, an initial length reading as the guage length (L) was taken for all

specimens. The specimens were then cured in limewater for another 24 hour before they were

removed and cured in a place with temperature of 23oC for another 24 hours. After allowing

the specimens cure for 24 hours, the first length of the prism specimens to determine thier

drying shrinkage were then taken being the 3rd day with further measurement of the prism

specimens carried out on other days which include 7 days, 14 days and 28 days. The drying

shrinkage (S) was determined as Lx/L coverted to percentage as detailed in equation 4.5.

Where Lx is the difference between the intial length reading taken after curing the specimen

in lime water for 30 minutes (guage length) and the length reading at any particular day, and

L is guage length of the specimens.



Shrinkage (S) = LxL × 100%...............Equation 4.5

4.3.7 Thermo-gravimetric analysis

The Thermo-gravimetric (TG) method was used to determine the content of hydrate product

in the various OC system paste specimens, specifically the calcium aluminate and silicate

hydrate products and the calcium hydroxide Ca(OH)2 products. A TG2950 HR thermo

gravimetric analyser was used for the study. The thermo gravimetric analyser (TA

instrument) was fitted to a thermal analysis controller for plotting and analysing data as well

as for outputting the thermo-gravimetric test results. The heating regime process comprised

of Hi-res from room temperature to 500oC and then a ramp with a heating rate of 10°C per

minute up to maximum 1000°C, in an inert argon gas atmosphere. Firstly, the OC system

specimens were prepared, and then cured at ambient temperature in a chamber to the testing

days (1, 7, 14 and 28 days). On the specific day of testing, the specimens were dried in the

oven at a temperature of 40°C to preserve any phases that were combustible above this

temperature. After drying, a small mortar and a pestle was used to crush each specimen to a

powder for testing. During the TG test, a specimen size of 10mg was used and heating carried

out in algon pans fitted with a lid. Weight loss and temperature increase were logged

continuously during the test and the plots of the TG weight loss (%) and the derivative weight

loss produced with respect to temperature (dw%/dt°C) were analysed. The weight loss of the

OC system specimens between the temperature range of 50oC to 250oC signifies the

dehydration of calcium aluminate and silicate hydrates (C-A-H and C-S-H) or (C-A-S-H),

while the weight loss between 400oC to 550oC indicates the decomposition of Ca(OH)2 in the

various OC systems. The hydrate contents were determined from the weight of the various

OC systems at the temperature ranges.



4.3.8 Model development The current research seeks to develop a model to simplify the design process of OC systems

for proper placement and stability provision in the oil well. In the current work, the Design-

Expert 6.0.8 statistical software from Stat-Ease Inc was adopted for the development of the

models. The selection of the Design Expert software for the model was due to its successful

application by other researchers to model the performance of cementitious system (Shariar

2011, Falode et al 2013, Salam et al 2014). The regression tool of the Design Expert software

for historical data and response surface display was chosen to model the effects of the

independent parameters (Temperature, MA and SP) on the dependent variable (Setting time

and strength performance). Various regression orders (linear and non-linear) were attempted

in the development of the model to predict the dependent variables commencing with the first

(linear) order. However, the second order (non linear) approach was found with its equation

to offer a substantial result that replicate the outcome in the tests. Therefore, the second order

regression approach was adopted to model the effects of temperatures, MA and chemical

admixture on the setting time and strength performance at 24 hours of the OC system using

the relationship presented in equation 4.6

ST= β0 + β1T + β2 T2+ β3M + β4M2 + β5C + β6C2+ β7TM + β8TC + β9MC + e......Equation 4.6

CS= β0 + β1T + β2 T2+ β3M + β4M2 + β5C + β6C2+ β7TM + β8TC + β9MC + e......Equation 4.7

Where

ST is the setting time dependent variable

CS is the compressive strength dependent variable

β is the regression coefficient,

T is temperature,

M is the dosage of specific (MK or PFA) mineral admixture

C is the dosage of SP chemical admixture

e is the residual error



In developing the model, the regression analysis involved 54 data points of the setting time

and strength performance outcomes from the test for the neat OC system and the OC system

incorporating MK or PFA dosages and five different dosage of the SP at three different

temperature degrees (23oC, 42oC and 60oC). The coefficients of the various model parameters

were determined using the least squares approach. Also, the relationship of the independent

parameters and the dependent variable were also generated and presented in the analysis of

variance (ANOVA) in terms of F and probability values (Prob.>F). The probability value

(Prob.>F) indicates the level of certainty that the observed outcome in the test was obtained

by a genuine effect from the parameter(s) rather than chance. The effect of the parameters on

the dependent variable is considered to be highly significant when the Probability value

(Prob.>F) is less than “0.05” and the F value is high (Howard and Wright, 2008). Moreso, the

standard deviation (SD) and the R square (R2) values of the output values for the model were

provided by the software. The standard deviation presents how much a set of data tend to

vary from the best line of fit (Stroud, 2001) while the R2 of a model is the percent or

proportion of variance in the output variable that is predictable from the input parameters

(Glantz et al 1990). It provides a measure of how well observed outcomes are replicated by

the model. R2 is determined by the pair wise correlations among all the variables, including

correlations of the independent parameters with each other as well as with the dependent

variable. For instance, if the model R2 is 0.90 or (90%), the variance of its errors is 90% less

for the variance of the output variable.

Furthermore, the performance of the developed models in predicting the setting times and

strength performance at 24 hours of the OC systems was evaluated using the average absolute

percentage deviation (AAD %). The average absolute percentage deviation was determined

using equation 4.7. The AAD% measures the prediction accuracy of a model from the actual

value expressed in percentage.

AAD% = Σ ( Residual/ Actual )

N × 100.......................Equation 4.7

Where

AAD%= Average absolute percentage deviation



Residual=Actual values ̶ Predicted values (in absolute terms)

Actual= Actual value

N= Number of data points measured


Chapter 5-Modification and Analysis

CHAPTER 5: OC SYSTEM MODIFICATION AND ANALYSIS This chapter presents the test results of the class G OC systems with different w/c ratios and

the modified OC system using MA (MK or PFA) at a fixed w/c ratio of 0.44 compared to the

results of the control system- the class G OC system of w/c-0.44 (OC0.44). Also, the results

of the influence of the super-plasticizer chemical admixture (SP) on the selected OC systems

based on satisfactory performance and the model of the setting time and strength

performance of the OC systems are presented in this chapter. The analysis of the results in

the various sections in this chapter are as follows; density, rheology, setting time,

compressive strength, tensile strength, water absorption, shrinkage, thermo-gravimetric

analyses for hydrate products and model development/analysis for prediction of the

performances of the OC system.

5.1 Density

This section presents the results and analysis of the modification effect of mix water ratio to

the density of the neat class G OC system and the modification effect of a MA (MK and PFA)

to the density of the OC system. Also, the comparison of the densities of the various systems

in regards to satisfying the density range for an OC system, as recommended by API is

presented in this section.

5.1.1 Density of class G OC system

The density of an OC system for oil well construction varies depending on the type of cement

material and the w/c ratio. The density of OC systems commonly used as recommended by

API for cementing of oil wells ranges from 1773 kg/m3 to 1960 kg/m3 (Sharair, 2011, API

specification 10A, 2002). Usually the Class G cement is formulated with a w/c ratio of 0.44

to achieve a satisfactory density in oil well construction (API specification 10A, 2002). As

such, the effect of mix water ratio on the density of class G OC system was investigated in

this study. The findings can aid in understanding the influence of water in modifying an OC

system. For this purpose, various systems using the class G cement material were prepared

with different water to cement ratios (w/c). This was to determine the density of the systems

and establish the change as a result of mix water ratio.



Figure 5.1 shows the result of the densities obtained for the various class G OC systems

designed with different w/c ratios compared to the control (OC0.44) system. The results

include the densities of class G OC system designed with the following w/c ratio; 0.36, 0.40,

0.44, 0.48 and 0.52.

OC0.36 OC0.40 OC0.44 OC0.48 OC0.521550

1650

1750

1850

1950

2050

1965

1773

OC systems

Dens

ity (k

g/m

³)

Figure 5.1-Densities of class G OC system at various w/c ratios vs. recommended density 1773-1965kg/m3

From the density results presented in Figure 5.1, it was found that the density of the class G

OC system decreased with the increase of mix water to cement ratio. The decreasing density

with increase of mix water to cement ratio was also found to deviate slightly from a linear

order. The outcome can be ascribed to bleeding occurence with increase of mix water to

cement ratio as noted by Yang (2015). The change in density with increase of mix water

content was also found to agree with the influence of mix water content on the density of

cementitous systems as recorded by Hossain and Al-Majed (2015) and Mustaq (2013). The

density results of the various systems with different w/c ratio indicate that the higher the ratio

of mix water, the lower the density of the particular system



In regards to satisfying the recommended density range of OC systems which varies between

1773 kg/m3 to 1960 kg/m3, the OC0.40 and OC0.48 systems along with the OC0.44 system

were found to satisfy the recommended density.

5.1.2 Density of modified OC system

The density of the modified OC systems comprising of Class G cement partially replaced

with MK and PFA is presented in this section. The w/c ratio was fixed (w/c=0.44) for the

purpose and the class G cement was partially replaced with 10%, 20%, 30%, 40% and 50%

of either MK or PFA material. Figures 5.2, depicts the densities of the modified OC system

using MK material and the densities of the modified OC system using PFA material.

OC0.44 OC10MA OC20MA OC30MA OC40MA OC50MA1550

1650

1750

1850

1950

2050

OC.44

OC-MA systems

Dens

ity (k

g/m

³)

Figure 5.2 Densities of OC-MA systems at 0.44 w/c ratio vs. density range (1773-1965 kg/m3)

From Figure 5.2, the densities of the various OC-MK and OC-PFA systems can be seen to

vary with the increase of their dosages to the cement material content for the system. The

partial replacement of the class G cement with MK material initiated a decrease to the density

of the system compared to the OC0.44 system. The density of the OC-MK system was found



to decrease as the dosage of the MK material increased in the modification of the system

from a density of 1850kg/m3 for the 10MK to 1650 kg/m3 for the 50MK. All densities

obtained with the presence of MK in the system were found to be lower than that of the

OC0.44 system whose density was recorded as 1900 kg/m3. It was also observed amongst the

OC-MK systems that only the OC10MK and OC20MK systems recorded densities that were

within the recommended density range for OC systems. The decreasing trend with increase of

MK in the system also agreed with the findings of Bu et al (2016). However, there was slight

difference in the outcome with dosage of MK which could be due to the different type of

cement, the presence of chemical admixture and the relative density of the MK that was used.

For the OC-PFA systems, as depicted in Figure 5.2, the outcome of the density of the OC-

PFA systems was observed to have a similar trend as the system modified with MK dosages.

Introducing the PFA dosages in the system resulted to a system with a lower density when

compared to the OC0.44 system but with higher densities than the OC-MK systems for the

particular dosages. For the OC system of OC10PFA, a density of 1880 kg/m3 was obtained.

Further increases of PFA dosage in the system was observed to cause further decreases to the

density of the OC system down to 1710 kg/m3 for OC50PFA. The density results indicate that

the presence of PFA will decrease the density of an OC system at the adopted w/c ratio

(0.44). Furthermore, it was found from the density results that amongst all the OC-PFA

systems, only the OC10PFA, OC20PFA, OC30PFA systems had densities that were within

the density range. The OC system modified with 40% and 50% PFA dosages where found to

have densities below the least density range of OC systems.

From the density results of the modified class G OC systems using the MAs, the outcome can

be attributed to the specific gravity of the MAs which are lower than that of cement. This can

be supported with reports of Saasen and Log (1996) and Nelson (2006) on the impact of MAs

on the density of an OC system. They noted that MAs including PFA alters the density of

systems due to their specific gravity which is different from that of cement.

5.2 Rheology

This sub section presents the results and analysis of the modification effect of the mix water

ratio to the rheological properties (yield stress and plastic viscosity) of the class G OC



system and the effect of MK and PFA to the rheological properties of the OC system. The

yield stress (Ys) of visco elastic fluids such as cement based systems is noted as the point

which the visco-elastic fluid begins to deform or flow under shear, while the plastic viscosity

characterises the resistance to flow. The yield stress and plastic viscosity of the various OC

systems were determined using the Bingham approach.

5.2.1 Rheology of Class G OC systemThe flow behaviour of an OC system is very important for the proper placement of systems in

the annulus of oil wells. In this section, the effect of mix water ratio to the rheological

properties of the class G OC system is presented and analysed in Figure 5.3 (yield stress) and

Figure 5.4 (plastic viscosity).

OC0.36 OC0.40 OC0.44 OC0.48 OC0.520

7

14

21

28

35

OC systems

Yield

stres

s (Pa

)

Figure 5.3-Yield stress of class G OC system at various w/c ratios



OC0.36 OC0.40 OC0.44 OC0.48 OC0.520

0.05

0.1

0.15

0.2

OC systems

Plasti

c visc

osity

(Pa.s

)

Figure 5.4-Plastic viscosity of class G OC system at various w/c ratios

From Figure 5.3, it can be observed that the yield stress of the class G OC system decreases

with an increase of mix water for the system. The yield stress of the OC0.36 system which

was observed to be 17.4Pa was found to be the highest, while the OC0.52 system was

observed to record the lowest yield stress value with 7.3Pa. The outcome confirms that the

increase of mix water to cement ratio for the development of class G OC system, influences

the yield stress of the system to a lesser value to flow.

For the plastic viscosity of the neat class G OC system as shown in Figure 5.4, the outcome

was found to follow a similar pattern to the yield stress outcome. The increase of mix water

to cement ratio was found to result in a decrease in the plastic viscosity for the various class

G OC systems. The class G OC system designed with the lowest mix water ratio (OC0.36

system) was found to have the highest plastic viscosity, while the class G OC system

designed with the highest ratio of mix water to cement (OC0.52 system) recorded the lowest

plastic viscosity among the various system. This indicates that water can alter the flow

behaviour of OC system depending on the ratio to cement in the development of class G OC



system. In general, the finding suggests that the higher the presence of mix water for a

cementitious system, the higher the potential of the particular system to flow.

5.2.2 Rheology of modified OC system

The results of the rheological properties of the modified OC system using MK and PFA

material are presented in this section. The yield stress and plastic viscosity results of the

modified OC systems were compared to that of the control system (OC0.44) to evaluate the

impact of the MA types and their dosages on the flow behaviour of OC system. Figures 5.5

depict the yield stress results of the OC-MK system and OC-PFA system compared to the

OC0.44 system, while Figure 5.5 shows the plastic viscosity results of the OC-MK and OC-

PFA system accordingly.


7

14

21

28

35

OC0.44

OC-MK

OC-PFA

OC-MA systems

Yield

stres

s (Pa

)

Figure 5.5-Yield stress of OC-MA system at 0.44 w/c ratio




0.05

0.1

0.15

0.2

OC0.44

OC-MK

OC-PFA

OC-MA systems

Plasti

c visc

osity

(Pa.s

)

Figure 5.6-Plastic viscosity of OC-MA system at 0.44 w/c ratio

From the rheological performance results shown in Figure 5.5, the yield stress of the OC-MK

system was observed to increase with an increase in the MK dosage. However only the

results of the OC systems modified with 10% and 20% MK dosages could be obtained. Other

OC-MK systems could not be measured due to their stiffness that resisted the rotor of the

viscometer instrument to rotate and take readings. Comparing the yield stress of the OC-MK

system to the yield stress of the OC0.44 system, the yield stress of the OC-MK systems were

found to be higher than that of the OC0.44 system rising to 33Pa for OC20MK system

compared to that of the OC0.44 which was 12Pa.

In the case of the OC system modified with PFA material, Figure 5.5 shows the yield stress

result with increase of PFA dosage in the OC system. The yield stress of the OC system was

found to decrease with increase of PFA dosage in the system reducing to 10.4Pa for the

OC50PFA system compared to 12Pa for the OC0.44 system.

For the plastic viscosity of the OC systems modified with MK and PFA material, Figure 5.6

shows the plastic viscosity of the OC-MK and OC-PFA system followed a similar trend to



that of the yield stress. The plastic viscosity of the OC-MK systems were observed to

increase with an increase of MK dosage while the plastic viscosities of the OC-PFA systems

were found to decrease with increase of PFA dosage in the system. Comparing the plastic

viscosity of the OC-MK systems and OC-PFA systems to the OC0.44 system, the plastic

viscosity of the OC-MK system were found to be higher than that of the OC0.44 system,

while the plastic viscosity of the OC-PFA systems were below that of the OC0.44 system.

Only the plastic viscosity of the OC10MK and OC20MK system were determined due to the

stiffness of the OC-MK systems with further increase of MK in the system. The plastic

viscosities of the OC system modified with MK material were observed to be higher than that

of the OC0.44 system whilst the plastic viscosity of the OC-PFA systems were lower than

that of the OC0.44 system. The plastic viscosity of the OC10PFA was found to be 0.07Pa.s

while the OC50PFA system recorded a plastic viscosity of 0.05Pa.s compared to that of the

OC0.44 system with 0.075Pa.s.

5.3 Setting time

This section presents the results and analysis of the effect of mix water ratio to the setting

time of neat class G OC system and the effect of MA types (MK and PFA) on the setting time

of an OC system at ambient temperature. Also, a comparison of the setting times of the

various systems to satisfy the required time for OC system to set at a temperature (23oC)

experienced around the surface casing zone in the oil well is presented in this section. The

expected time for OC systems to set at 23oC ranges from 300 minutes (5 hours) to 480

minutes (8 hours), (Cairo university, 2012; Schlumberger, 2016).

5.3.1 Setting time of Class G OC system

The time for an OC system to set is very crucial in the construction of an oil well due to

performance, time and cost challenges. In this section, the effect of mix water ratio on the

setting time of the class G OC system is presented. Also, the evaluation of the various class G

OC systems for proper placement in the oil well using the required setting times for an OC

system at the surface casing zone is presented in this section. Figure 5.7 depicts the setting

times of the various class G OC systems of different w/c ratio compared to the OC0.44

system.



OC0.36 OC0.40 OC0.44 OC0.48 OC0.520

120

240

360

480

600

720

300

480

OC systems

Setti

ng ti

me (

minu

tes)

Figure 5.7-Setting times of class G OC system at various w/c ratios at 23oC

From the setting time results presented in Figure 5.7, it can be seen that the setting times of

the class G OC system increases with increase of the ratio of mix water to cement for the

system ranging from 325 minutes for the OC0.36 system to 597 minutes for the OC0.52

system.

For the required time for OC system to set at surface casing zone as specified by API (Cairo

University, 2012; Schlumberger, 2016), the OC0.36, OC0.40 and OC0.44 systems were

found to meet the setting time requirement. The outcome of the setting time of the systems

can be attributed to the water to cement ratio adopted for the formulation of each system. Hu

et al (2014) and Nili et al (2013) noted that the ratio of water to cement can alter the setting

period of cementitious systems and from the study of Nili et al (2013), a similar impact with

an increase of water ratio increasing the setting time of cementitious systems as found with

the class G OC system in this study was observed. Comparing the setting times of the various

OC systems to that of the OC0.44 with a setting time of 425 minutes, the result indicate that



the lower the ratio of mix water to cement, the shorter the time it takes the system set, while

the higher the ratio of the mix water to cement, the longer the time it takes the system to set.

5.3.2 Setting times of modified OC system

OC systems are expected to set over a sufficient time to enable proper placement to be

conducted in the oil well. In this section, the setting time of the OC system as a result of the

presence of MAs in the system is presented. Also, the evaluation of the systems for

satisfactory placement in the oil well using the required setting times for systems at surface

casing zone is presented. Figure 5.8 depicts the setting times of OC-MK and OC-PFA

systems compared to the OC 0.44 as well as the evaluation to satisfy the setting requirement.

.


120

240

360

480

600

720

300

480

OC0...

OC-MA systems

Setti

ng ti

me(

minu

tes)

Figure 5.8- Setting times of OC-MK system at 0.44 w/c ratio at 23oC

From Figure 5.8, the setting times of the OC-MK systems were found to be shorter than the

setting time of the OC0.44 system. Modifying the OC system with the partial replacement of

cement using the dosage of 10% MK, was found to reduce the setting time of the OC10MK

system compared to the OC0.44 system. Increasing the dosage of MK to cement for the OC

system was found to further reduce the setting time of the OC system. For the setting time



requirement of OC system, the OC10MK and OC20MK system were found to meet the

required setting times for temperature experienced around surface casing. From the outcome,

it can be concluded that the presence and increase of MK dosage accelerates the setting time

of the OC system.

For the OC-PFA systems, Figure 5.8 shows the setting time of the OC-PFA system to differ

from that of the OC-MK compared to the OC0.44 system. The result showed that partially

replacing cement with PFA material lengthens the setting time of the OC system from the

438 minutes for the OC10PFA system to 653 minutes for the OC50PFA system. Comparing

the setting time results of the OC-PFA systems to that of the OC0.44 system, the results agree

with the findings of Ge (2003) on the effect of PFA on the setting time of cementitious

system. The finding of Ge (2003) showed that, the higher the dosage of PFA material in an

ordinary Portland cement system, the longer it took the system to set compared to the

reference system or systems with lower PFA dosage. Furthermore, for the required time for

the OC system to set at the temperature experienced around the surface casing zone, only the

OC10PFA and OC20PFA system were found to satisfy the setting time requirement.

With regards to the different setting time observed for the OC-MK and OC-PFA systems, it is

noted that the properties of cement material can influence the setting duration of a cement

based systems (Hu et al, 2014). The setting times showed that the MK and PFA material

affects the setting of OC system differently. While the presence of MK resulted to shorter

setting time for the OC system, the presence of PFA resulted in increase of setting times for

the OC system. Thus, the outcomes can be attributed to the chemical and physical properties

of the MAs and thier dosages at the fixed water to cement ratio adopted for the system.

5.4 Strength development

This section presents the results and analysis of the effect of mix water ratio on the strength

development of neat class G OC systems and the effect of the MA types (MK and PFA) on the

strength development of the OC system at various days. OC systems are expected to achieve

a minimum compressive strength of 3.4MPa and tensile strength of 0.34MPa within 24 hours

to support the casings in the oil well for further well construction operations (Backe et al,



2001). The evaluations of the strength performance of the various OC systems to meet the

minimum strength requirement are also presented in this section.

5.4.1 Compressive strength development of Class G OC systemThe unconfined compressive strength (CS) of class G OC systems formulated with different

water to cement ratio (w/c) are presented in this section. The purpose of varying the w/c ratio

of the OC systems is to identify the effect of mix water ratio to the strength development of

OC system. Given the required performance of an OC system, the CS of the various OC

systems was measured at 24 hours to determine their strength performance to satisfy the CS

requirement. The results of the CS at 24 hours and the CS development of the various

systems at various days up to 28 days are presented in Figure 5.9 and Figure 5.10

respectively.

OC0.36 OC0.40 OC0.44 OC0.48 OC0.520

3

6

9

12

15

18

3.4

OC systems

Com

pres

sive s

treng

th (M

Pa)

Figure 5.9-CS of OC system at various w/c ratios at 24 hours



0 7 14 21 280

10

20

30

40

50

60

70

80

3.4

OC0.36OC0.40OC0.44

Curing period (days)

Com

pres

sive s

treng

th (M

Pa)

Figure 5.10-CS development of OC system at various w/c ratios

From Figure 5.9, the results show that the compressive strength attained by the class G OC

systems prepared with different water to cement ratio decrease such that the OC0.36, OC

0.40 and OC0.44 system attained strengths above the minimum compressive strength

required at 24 hours for OC systems. The results indicate that the higher the ratio of mix

water to cement for the OC system, the lower the compressive strength of the OC system for

the range of w/c ratio examined. The OC0.52 system was found not to record any strength at

24 hours which took the longest time before setting. Thus, this could be due to a very low

strength attained by the OC0.52 system at 24 hours beyond the capturing range of the

measurement device.

From Figure 5.10, the CS development of the various class G OC systems from one (1) day

up to twenty eight (28) days is shown. The CS was investigated at different days to identify

the strength development pattern of the various systems. From the results, the CS

development of the class G OC systems for 1 day (24 hours), 7 days, 14 day and 28 days

showed the compressive strength of the various systems as increasing with curing days. The

increase in strength of the various OC systems was also discovered to differ between days as

it was found that the difference in strength increase at 7 days from 1 day (24 hours) was the



highest compared to other days from the previous day. This was ascertained using

‘percentage increase’ which was determined as ‘the difference between the strength at a

particular day from the previous day’ divided by the ‘previous day’ and multiplied by

hundred. The outcome showed that with curing days, the increase in strength for the various

systems became slower. For instance, the strength of the OC0.36 system was found to

increase by 268% at 7 days from the strength attained at 1 day (24 hours). At 14 days from 7

days, the strength increased by 13%, while at 28 days from 14 days, the strength increased by

12% which is lesser than the strength increase observed at 7 days from 1 day and 14 days

from 7 days indicating a slower development in strength with curing days.

Also, it was found that the CS of the class G OC systems prepared with a lower mix water

ratio than the w/c of the OC0.44 system produced higher strengths than the OC0.44 system,

while the OC system designed with higher mix water ratio had lower CS than the OC0.44

system. Thus, the outcome can be attributed to the influence of water to cement ratio adopted

for the various OC system. Bell (1996) reported that if mix water ratio is above the optimum

consistency ratio for a cementitious system, then a low strength is produced over curing age.

The CS outcome of the various OC systems corresponds to findings of Dahab and Omar

(1989) on the influence of mix water ratio to cementitious system. They deduced from their

study that higher mix water content above consistency limit reduces the strength performance

of the cementitious system.

5.4.2 Compressive strength development of modified OC system

The unconfined compressive strength (CS) developments of an OC system modified with a

MA (MK and PFA) are presented in this section. The evaluation of the compressive strength

performance of the OC-MA systems to satisfy the minimum strength requirement (3.4MPa)

at 24 hours is also presented. Figure 5.11 presents the CS results of the OC-MK and OC-PFA

systems at 24 hours while Figure 5.12 and 5.13 presents the CS development of the OC-MK

and OC-PFA system respectively.




3

6

9

12

15

18

3.4

OC0.44

OC-MA systems

Com

pres

sive s

treng

th (M

Pa)

Figure 5.11-CS of OC-MA system at 0.44 w/c ratio at 24 hours

0 7 14 21 280

10

20

30

40

50

60

70

80

3.4

OC0.44OC10MKOC20MK


Com

pres

sive s

treng

th (M

Pa)

Figure 5.12-CS development of OC-MK system at 0.44 w/c ratio



0 7 14 21 280

10

20

30

40

50

60

70

80

3.4

OC0.44OC10PFAOC20PFAOC30PFA


Com

pres

sive s

treng

th (M

Pa)

Figure 5.13-CS development of OC-PFA system at 0.44 w/c ratio

From Figures 5.11, the compressive strength of the OC system modified with dosages of

10%, 20%, 30% and 40% MK were found to satisfy the CS required of OC system at 24

hours whilst the OC50MK system was observed to have compressive strength less than the

CS required at 24 hours. Compared to the OC0.44 system, the CS results reveal greater

strength gains by the OC system incorporating 10%, 20%, and 30% MK than the CS of the

OC0.44 system. The OC40MK and OC50MK system were observed to have strengths below

the OC0.44 system at 24 hours. Comparing the various results of the CS of the OC-MK

system, it indicates that the MK dosages of 10%, 20% and 30% improves the CS of the class

G OC system at 24 hours at the adopted w/c ratio.

In the case of the strength development of the OC-MK system, Figure 5.12 shows the results

of the OC-MK systems compared to that of the OC0.44 system from 1 day (24 hours) up to

28 days. The strength development of the OC-MK system can be found to vary with

increasing dosage of MK in the OC system. The OC30MK system was observed to have the



highest strength gain across the various days compared to the other OC-MK systems and the

OC0.44 system. The OC10MK and OC20MK systems were also observed to have a CS

greater than that of the OC0.44 system and the OC systems modified with 40% and 50% MK

dosages were found to have a CS lower than that of the OC0.44 system for all days up to 28

days. It is also observed from the results that the compressive strength of the OC20MK

system was greater than that of OC10MK system for the various days. The strength

development of the various OC-MK systems showed similarity with the CS outcome of the

systems at 24 hours compared to the OC0.44 system. Though the OC system modified with

40% MK had a similar CS to the OC0.44 system at 7days, from 14days, the CS of the

OC40MK was observed to differ from the CS of the OC0.44 system. The compressive

strength development of the OC40MK system and the OC50MK system remained below the

CS development of the OC0.44 system across the various days. The CS development of the

OC system can be attributed to the properties of the MK and the water demand of the OC-

MK system as a result of the increase in MK dosage for the system. The OC system modified

with 50% MK was observed to be in granular form which is due to the influence of the high

presence of MK for the system at the w/c ratio. The outcome of the CS performance of the

OC-MK systems can be supported by findings of silica material effect on the CS of a

cementitious system from the study of Heba (2011). The effect of the silica material dosage

on cementitious system indicated that at a certain dosage of the material presence in the

system, the compressive strength of the cementitious system increased. But with further

increases of silica material in the system, the compressive strength of the system was found to

decrease at the same w/c ratio (Heba, 2011). Heba (2011) concluded that the percentage of

admixture for a cementitious system can improve the strength of a cementitious system but a

dosage too high can adversely affect the strength performance at the same w/c ratio. Also, the

impact of the MK dosages on the strength of the system agrees with the findings made by

Wild et al (1996). It was found from the study of Wild et al (1996) that the partial

replacement of ordinary Portland cement with 10% and 20% MK dosages increased the

strength of the cementiotus system at a w/c of 0.5. However, the results obtainded by Wild et

al (1996) were above the strengths obtained in the current study which can be ascribed to the

w/c ratio and the presence of other admixture incorporated in the system by Wild et al (1996)



For the OC-PFA systems, Figure 5.11 shows the compressive strengths of the OC-PFA

systems compared to that of the OC0.44 system and the OC-MK system at 24 hours. Unlike

the CS performance of the OC-MK system at 24 hours, the results show the various OC-PFA

systems to have lower strengths than the OC0.44 system at 24 hours with the compressive

strength of the system decreasing with increasing dosage of PFA material in the system. The

outcome can be attributed to the chemical and physical properties of the PFA material as

found and argued by Mirza (2002). Mirza (2002) noted that the properties of PFA alter the

chemical reaction of a cementitious system for strength. This is evident from the decreasing

CS trend with the presence of PFA material. The CS of the OC system modified with the

10% PFA material dosage was found to have the highest CS compared to the other OC-PFA

systems while the OC system modified with a dosage of 50% PFA material had the least

compressive strength. This indicates that the high strength attained by the OC10PFA system

compared to the other OC-PFA systems is due to the high presence of the class G cement in

the system against the low dosage of the PFA material to offset the chemical reactions in the

system. Furthermore, only the OC systems modified with dosages of 10%, 20% and 30%

PFA material were found to satisfy the compressive strength required at 24 hours for OC

systems.

Figure 5.13 shows the CS development of the OC-PFA systems compared to the OC0.44

system from one day (24 hours) up to 28 days. The result shows the compressive strength of

the various OC-PFA systems to increase with curing days. However, compared to the OC0.44

system, the CS development of the various OC-PFA systems remained lower than that of the

OC0.44 system for the curing period of 28 days. The result can be ascribed to the presence of

PFA material in the OC system resulting in a slow strength gain within the 28 day period.

The result is in accordance with findings of the effect of PFA on the strength development of

cementitious system by Mirza (2002). Mirza (2002) established that partial replacement of

cement with PFA material influenced the low strength gain of the cementitious system for the

period of 28 days. Mirza (2002) attributed the outcome from the study to the rate of chemical

reactions due to the chemical and physical properties of the PFA material which differed

from that of the cement material. More so, Buntoro and Rubiandini (2000) noted from their

study that the properties and dosages of admixture can alter the strength performance of OC

system. Amongst the OC-PFA systems, the OC10PFA systems was found to have the highest



CS at the various days while the OC50PFA system had the lowest CS at the various days up

to 28 days.

5.4.3 Tensile Strength development of Class G OC system

The tensile strength development (TS) of the class G OC system prepared with different w/c

ratio is presented in this section. The purpose of varying the water to cement ratio (w/c) is to

identify the effect of mix water ratio to the tensile strength development of an OC system.

Also, OC systems are required to achieve a minimum tensile strength of 0.34 MPa within 24

hours in construction of oil wells (Backe et al, 2001). As such, the TS of the various OC

systems was measured at 24 hours to identify the TS performance of the OC systems to

satisfy the requirement. The results of the TS at 24 hours and the TS development of the

various class G OC systems are presented in Figure 5.14 and Figure 5.15 respectively.

OC0.36 OC0.40 OC0.44 OC0.48 OC0.520

0.4

0.8

1.2

1.6

0.34

OC systems

Tens

ile st

reng

th (M

Pa)

Figure 5.14-TS of OC system at various w/c ratios at 24 hours



0 7 14 21 280

1

2

3

4

5

6

0.34

OC0.36OC0.40OC0.44


Tens

ile st

reng

th (M

Pa)

Figure 5.15-TS development of OC system at various w/c ratios

Form the results presented in Figures 5.14, the tensile strength of the various class G OC

systems prepared with different w/c ratio can be found to decrease with an increase in the mix

water ratio for the OC system. For the OC0.48 and OC0.52 system, a tensile strength could

not be recorded which can be associated to very weak tensile strengths beyond the capability

of the measurement device to capture readings. As such, the OC0.48 and OC0.52 systems fell

short of the expected TS for OC system at 24 hours. The OC0.36, OC0.40 and the OC0.44

systems were found to attain tensile strengths above the required TS at 24 hours which can be

ascribed to the ratio of water to cement for the reaction of the chemicals. The impact of the

water to cement ratio is already highlighted from the works of Dahab and Omar (1989) who

showed that the mix water ratio above the consistency limit for cementitious system reduces

the strength performance of the particular cement system.

Figure 5.15 shows the tensile strength development of the various OC systems with different

w/c ratio from one day (24 hours) up to 28 days. It can be seen from the result that the tensile

strength of the various OC systems prepared with different w/c ratio increased with curing

days. The maximum tensile strength of the various OC systems was recorded at the 28 th day.



For the influence of w/c ratio, it was observed that the OC system prepared with the lowest

mix water ratio (OC0.36) recorded the highest tensile strength compared to the OC0.44

system and other class G OC systems strength. The OC system formulated with the highest

mix water ratio (OC0.52) was observed to have the lowest tensile strength amongst the

various OC systems up to 28 days. The TS development pattern was observed to be similar to

that of the compressive strength for various OC systems. High strength gains were identified

at 7 days from the strengths recorded at the initial day (24 hours) for the various systems. The

strength gain at 7 days from 1 day (24 hours) was also found be the highest compared to other

days. The least strength gain for the various OC systems was observed between 14 days and

28 days indicating a much slower strength development for the period for the various class G

OC systems. The outcome suggests that increasing the mix water ratio for OC system

formulation with the w/c ratios considered in this study could decrease or slow the

development of the tensile strength of the OC system.

5.4.4 Tensile Strength development of modified OC system

The tensile strength (TS) developments of an OC system modified with a MA (MK and PFA)

are presented in this section. The evaluation of the tensile strength performance of the OC-

MA systems to satisfy the minimum tensile strength requirement (0.34MPa) at 24 hours is

also presented. A fixed w/c ratio was used for the various systems to enable proper

identification of the effect of MK or PFA to the TS of the OC system. The TS results of the

modified OC systems were compared to the TS of the OC0.44 system. Figure 5.16 presents

the TS results at 24 hours of the OC-MK and OC-PFA systems while Figures 5.17 and 5.18

present the TS development of the OC-MK system and the OC-PFA system respectively.




0.4

0.8

1.2

1.6

0.34

OC0...

OC-MA systems

Tens

ile st

reng

th (M

Pa)

Figure 5.16- TS of OC-MA system at 0.44 w/c ratio at 24 hours

0 7 14 21 280

1

2

3

4

5

6

0.34

OC0.44OC10MKOC20MK


Tens

ile st

reng

th (M

Pa)

Figure 5.17- TS development of OC-MK system at 0.44 w/c ratio



0 7 14 21 280

1

2

3

4

5

6

0.34

OC0.44OC10PFAOC20PFAOC30PFA


Tens

ile st

reng

th (M

pa)

Figure 5.18 -TS development of OC-PFA system at 0.44 w/c ratio

From Figures 5.16, the tensile strength of the various OC-MK systems was found to vary

with an increase in the MK dosage for the OC system. The OC10MK, OC20MK and

OC30MK systems were found to have tensile strengths higher than that of the OC0.44 system

at 24 hours, while the OC40MK and OC50MK systems had tensile strengths lower than the

OC0.44 system at 24 hours. From the results, it was found that OC system modified with

10%, 20%, 30% and 40% MK dosage satisfied the tensile strength requirement for OC

system at 24 hours while the OC50MK system fell short of the requirement. The TS outcome

of the OC-MK system can be attributed to the w/c ratio and the properties of the MK.

In Figure 5.17, the TS development of the OC-MK system compared to the OC0.44 system

from one day (24 hours) up to 28 days is presented. From the results, it was observed that the

various OC systems increased in tensile strength with curing days. The TS developments of

the various OC-MK systems were found to follow a similar pattern as that of the compressive

strength of the systems. The OC10MK, OC20MK and OC30MK systems were found to

achieve a greater TS than the OC0.44 system at the various days up to 28 days. The

OC40MK system was found to have tensile strengths slightly lower than that of the OC0.44



system at 1day (24 hours) and 7 days, but from 14 days onwards, a clear difference in tensile

strength was observed between the OC0.44 system and the OC40MK system. The TS of the

OC40MK system compared to the OC0.44 system from 14 days was found to be clearly

lower than the TS of the OC0.44 system. The OC50MK system was found to have the lowest

tensile strength amongst the various OC-MK systems at the various days which can be

attributed to its reactivity with the w/c ratio selected for the system design. The outcome can

be supported by studies of Heba (2011). It was identified from studies of Heba (2011) that the

dosage of silica material improved the strength of cementitious system but high dosage of

silica decreased the strength of the system at same w/c ratio (Heba, 2011). The outcome was

attributed to the rate of chemical reaction with available mix water for the dosages of the

silica and cement material for the systems.

Figure 5.16 also shows the tensile strength performance of the OC-PFA systems compared to

the OC0.44 system at 24 hours. From the results, it was found that the increase of PFA

dosage for the OC system resulted in a lower tensile strength at 24 hours compared to that of

the OC0.44 system. The result indicate that OC10PFA, OC20PFA and OC30PFA systems

satisfied the TS requirement for OC systems at 24 hours while the OC40PFA and OC50PFA

systems, fell short of the requirement with not recording a tensile strength at 24 hours from

the test which can be ascribed to very low strengths below the capturing range of the

measurement device. The TS performance of the OC-PFA system can be attributed to the

properties of the PFA material as recorded by Mirza (2002). Mirza (2002) observed that the

presence of PFA produce lower strength at earlier days compared to neat cement system due

to chemicals and physical properties of the PFA that differ from that of the cement.

Figure 5.17 shows the tensile strength development of the OC-PFA system compared to the

OC0.44 system from one day (24 hours) up to 28 days. The results show the tensile strength

of the various OC-PFA systems to increase with curing days but remained lower than the TS

of the OC0.44 system for the curing duration examined. The highest gain in tensile strength

for the various OC-PFA systems were recorded at 7 days from one day. At 14 days, the

tensile strength gains were found to be lower than the observed strength gain at 7 days from

one day. The low gains were more obvious at 28 days from 14 days. More so, it was

identified that the OC10PFA and OC20PFA systems had the highest tensile strengths



compared to other OC-PFA systems. This can be ascribed to the high presence of the class G

cement to PFA material for the OC system thereby influencing the rate of chemical reactions

for tensile strength of the system. It can be observed that the OC50PFA system recorded the

lowest tensile strength which can be attributed to increased influence of PFA with high

dosage to alter the chemical reaction of the system. The results indicate that the presence of

PFA material in OC system within 28 days period at the w/c ratio and the PFA dosages

studied would offer lower tensile strengths. The studies of Soni and Saini (2014) and Moinul

and Saiful (2010) also reported similar outcome with PFA dosages in cementitious system for

28 days and attributed the outcome to slow rate of chemical reactions due to the properties of

the PFA material.

5.5 Water absorption

OC systems are expected to resist and prevent the migration of formation fluid for durability

and long term existence of the oil well. The study of water absorption can provide

understanding of the porous or permeable nature of a hardened cementitious system. This

section presents the results and analysis of the water absorption (WA) of the various OC

systems at different days. The water absorption results include those of the Class G OC

systems prepared with different water to cement ratio and the OC system modified with MA

(MK and PFA). The results are compared to the water absorption of the OC0.44 system at 24

hours and various days up to 28 days.

5.5.1 Water absorption of Class G OC systemThe water absorptions (WA) of the class G OC systems formulated with different water to

cement ratios are presented in this section. The purpose of varying the w/c ratio was to

identify the effect of mix water content on the water absorption of OC system. Figures 5.19

and 5.20 illustrate the percentage of water absorbed by the various class G OC systems at 24

hours and at various days up to 28 days. The water absorption of the OC systems were

compared to that of the OC0.44 system formulated with the specified w/c ratio for class G

cement to evaluate their performance.



OC0.36 OC0.40 OC0.44 OC0.48 OC0.520

3

6

9

12

OC systems

Wat

er ab

sorp

tion (

%)

Figure 5.19-WA of OC system at various w/c ratios at 24 hours

0 7 14 21 280

3

6

9

12

OC0.36OC0.40OC0.44

Soaking period (days)

Wat

er ab

sorp

tion (

%)

Figure 5.20 -WA of OC system at various w/c ratios for various days



Figure 5.19 shows the water absorption of the various OC systems prepared with different

w/c ratios compared to the OC0.44 system at 24 hours. From the results, the percentage of

water absorbed at 24 hours (1 day) by the various class G OC systems was found to increase

with an increase of mix water ratio for the system. The results indicate that the higher the mix

water ratio for the OC system, the higher the percentage of water absorbed for the range of

w/c ratios studied. Similar finding were reported by Castro et al (2011) for ordinary Portland

cementitious system in terms of trend. They also found that cementitious system designed

with very high volume of water to cement ratio absorbed more water. Hearn et al (1994)

noted that the ingress of water into a cementitious system is due to capillary suction which is

linked to the pore spaces in the system and the connection of the pores to each other. Thus,

the water absorption outcome of the various class G OC system of different w/c ratio at 24

hours can be attributed to the volume and connection of pore spaces in each OC system

affected by hydration rate due to the mix water ratio.

Figure 5.20 shows the percentage of water absorbed at different days up to 28 days by the

various OC systems. From the results, it can be seen that the percentage of water absorbed by

the various OC systems decreased with curing days. The maximum percentage of water

absorbed for each system was recorded at the initial day and the lowest percentage of water

absorbed by each system was recorded at 28 days. It was also observed amongst the various

days that the highest decrease in percentage of water absorbed from a previous day occurred

at 7 days from 1 day (24 hours). A decrease of approximately 4% in water abosrbed at 7 days

from 1 day was observed to have occurred for the OC0.36, OC0.40 and OC0.44 systems

while the OC0.48 and OC0.52 systems had decrease of above 5% in water absorbed at 7 days

from 1 day. The decrease recorded for 14 days and 28 days from the previous test day were

found to be lesser than the decrease observed at 7 days from one day for the various systems.

The results of the water absorption of the various systems agree with the findings of Kim et al

(2014). Kim et al (2014) from their study found absorption to increase with the increase of

w/c ratio from 0.45 to 0.65 and also found the absorption to decrease with curing days. They

attributed the outcome to the quantity of hydration products and volume of pores in the

systems at the various days. Thus, the decrease in percentage of water absorbed by each class

G OC system with curing days can be attributed to the volume of pores available in the

systems for the various days as hydration of the system progresses.



5.5.2 Water absorption of modified OC system

The water absorption (WA) of modified class G OC systems using MK or PFA material are

presented in this section. A fixed w/c ratio of 0.44 was used for the various systems to enable

proper identification of the effect of MK or PFA material to the WA of the OC system. The

WA results of the modified OC systems were compared to the water absorption of the

OC0.44 system to evaluate their performance. Figure 5.21 presents the percentage of WA at

24 hours for the OC-MK and OC-PFA systems while Figures 5.22 and 5.23 illustrate the

percentage of WA over curing days by the OC-MK and OC-PFA systems accordingly.


3

6

9

12

OC...

OC-MA systems

Wat

er ab

sorp

tion

(%)

Figure 5.21 -WA of OC-MA system at 0.44 w/c ratio at 24 hours



0 7 14 21 280

3

6

9

12OC0.44OC10MKOC20MK


Wat

er ab

sorp

tion (

%)

Figure 5.22-WA of OC-MK systems at 0.44 w/c ratio at various days

0 7 14 21 280

3

6

9

12OC0.44OC10PFAOC20PFA


Wat

er ab

sorp

tion (

%)

Figure 5.23-WA of OC-PFA systems at 0.44 w/c ratio at various days



Figure 5.21 shows the percentage of water absorbed by the OC-MK system compared to the

OC0.44 system at 24 hours (1 day). From the results, the percentage of water absorbed by the

OC-MK system was found to vary with an increase of MK dosage. The OC10MK, OC20MK

and OC30MK systems were observed to absorb less percentage of water compared to the

OC0.44 system at 24 hours while the OC40MK and OC50MK systems were found to absorb

more water than the OC0.44 system at 24 hours. The modified OC system with dosage of

30% MK material was found to have the least percentage of water absorbed amongst the

various systems at 24 hours.

In Figure 5.22, the effect of MK material on the WA of OC system for a period of 28 days is

presented. From the results, the percentage of water absorbed by the OC-MK systems from

one day (24 hours) up to 28 days can be found to generally decrease with increase in curing

days. The OC10MK, OC20MK and OC30MK systems were observed to absorb less water

than the OC0.44 system at the various days while the OC40MK and OC50MK systems

absorbed more water than the OC0.44 system. It was also found that the highest decrease in

percentage of water absorbed from a previous day occurred at 7 days from one day compared

to the decrease observed for other days. The trend of WA by the OC-MK systems compared

to OC0.44 system through the curing days were found to have similar pattern as recorded at

24 hours. The OC30MK system was found to have the least percentage of water absorbed at

one day and 7 days, while at 14 days and 28 days, the percentage of water absorbed by the

OC30MK system was similar to that of the OC20MK system. The trend of the OC30MK and

OC20MK system can be attributed to the dosage of the MK in the system. The WA of the

OC10MK system, though lower than that of the OC0.44 system through the curing days was

found to decrease in a similar trend to that of the OC0.44 system. The performance of the

OC10MK can be ascribed to the higher proportion of class G cement material to the MK

material for the system. As such, the influence of the cement was more dominant to affect the

WA performance of the OC10MK system. The OC40MK and OC50MK system were found

to have similar WA trend which remained higher than the WA of the OC0.44 system for the

various days. The outcome observed for the OC40MK and OC50MK system can be ascribed

to the dosage of the MK material in the system at the adopted w/c ratio for the study. The

OC40MK system and especially the OC50MK system were observed to be in granular form

with lots of voids.



In the case of the OC-PFA system, Figure 5.21 illustrates the effect of PFA material on the

WA of OC system at 24 hours. From the results, the water absorption of the OC-PFA systems

was found to produce a non linear outcome with an increase of PFA dosage in the OC

system. The OC10PFA and OC20PFA system were observed to absorb less water at 24 hours

than the OC0.44 system while the OC system modified with 30%, 40% and 50% PFA

dosages were found to absorb more water than the OC0.44 system. The percentage of water

absorbed by the various OC-PFA systems can be attributed to the dosage of PFA for the

system. The low water absorbed by the OC system modified with 10% and 20% PFA can be

ascribed to the reduction of the pore volume by the finer particles of PFA at the dosages and

the higher rate of hydration with higher content of class G cement for the systems compared

to the other OC-PFA systems. The hydration rate of the system can be observed from the

setting time which shows the OC system modified with lesser PFA content to have quicker

setting time due to the influence of the cement content and properties.

Figure 5.22 shows the long term effect of PFA material on the water absorption of the OC

system from one day up to 28 days. From the results, the various OC-PFA systems and the

OC0.44 system were found to absorb less water with an increase in curing days. However,

the percentage of water absorbed by the OC system incorporating 30%, 40% and 50% PFA

material were found to be higher than that of the OC0.44 system for the investigated duration.

The WA of the OC system modified with 10% and 20% PFA were found to be lower than

that of the OC0.44 system for the various days up to 28 days. It was also found that the

percentage of water absorbed by the OC10PFA and OC20PFA system for the various days

were similar apart from 7 days. The water absorption results of the OC10PFA and OC20PFA

system indicate a refinement of the pores by the PFA particles at the dosages and the rate of

chemical reaction of the system from the high presence of class G cement contributed to

reduce the absorption of water. The WA outcome of the OC-PFA systems can found to be

similar to observation made by Gingos and Mohammed (2011) for ordinary Portland cement

system. Gingos and Mohammed (2011) found cement mortar incorporating 20% PFA

material to absorb the least percentage of water amongst cement mortars designed with

partial replacement of cement using 10%, 20% and 30% PFA material at 0.5 w/c ratio for 28

days.



5.6 Shrinkage

This section presents the results and analysis of the shrinkage of the various OC systems at

different days. The shrinkage results include those of the Class G OC systems prepared with

different water to cement ratios and the OC system modified with MA (MK and PFA). The

results of the various OC systems are compared to the shrinkage of the OC0.44 system

observed at the various days up to 28 days.

5.6.1 Shrinkage of Class G OC systemThe shrinkage of the class G OC system prepared with different water to cement ratios (w/c)

is presented in this section. The purpose of varying the water to cement ratio of the OC

systems was to identify the effect of mix water ratio to the shrinkage of OC system. Figure

5.23 depicts the shrinkage of the various OC systems at the various days up to 28 days. The

shrinkage of the OC systems were compared to the OC0.44 system formulated with the

specified w/c ratio for class G oil well cement type to evaluate their performance.



0 7 14 21 28-1

-0.8

-0.6

-0.4

-0.2

0

OC0.36OC0.40OC0.44


Shrin

kage

(%)

Figure 5.23-Shrinkage of OC system at various w/c ratios at various days

Form Figure 5.23, the shrinkage values of the class G OC systems designed with different

w/c ratios from the themthird day (3rd day) were observed to increase with curing days. The

shrinkage value is the percentage of change in length of the prism bars of each OC systems.

The shrinkage of the various OC systems was found to differ with w/c ratio and curing days.

At 3rd day, the OC system prepared with a lower mix water ratio than the OC0.44 system

were found to record higher shrinkage values than the OC system prepared with higher mix

water ratio than the OC0.44 system. The OC0.36 system and the OC0.40 system along with

the OC0.44 system were observed to shrink more than the OC0.48 and OC0.52 system. But

from 7 days, the shrinkage of the various OC systems were found to differ from the result

obtained at the initial day (day 1). The OC system prepared with a lower mix water ratio

including the OC0.44 system were found to have lesser shrinkage than the OC systems

designed with higher mix water ratio. The OC0.36 system was observed to have the least

shrinkage values from 7 days up to 28 days, while the OC0.52 system was observed to have

the highest shrinkage values from 7 days up to 28 days. The outcome can be ascribed to the

w/c ratio influencing the rate of hydration for the various systems and the volume of pore



spaces available in the system. A similar observation was recorded by Idiart (2009) for

ordinary Portland cement system. In that study, it was found that cementitious systems

designed with low w/c ratio had higher shrinkage values at initial curing days compared to

cementitious system designed with a high w/c ratio. The outcome was attributed to the rate of

hydration as a result of the w/c ratio of the various cementitious systems (Idiart 2009).

Furthermore, Mokarema et al (2005) added from their study, that the rate of shrinkage is

associated to the volume of pores available in the cementitious system for the system to

shrink into.

5.6.2 Shrinkage of modified OC systemThe shrinkage of the modified OC system using MK or PFA are presented in this section. A

fixed w/c ratio of 0.44 was used for the various systems to enable proper identification of the

effect of MK or PFA to the shrinkage of the OC system. The shrinkage results of the

modified OC systems were compared to the shrinkage of the OC0.44 system to evaluate their

performance. Figures 5.24 and 5.25 depicts the shrinkage of the OC-MK and OC-PFA

system respectively at the various days up to 28 days



0 7 14 21 28-1

-0.8

-0.6

-0.4

-0.2

0

OC0.44OC10MKOC20MKOC30MK


Shrin

kage

(%)

Figure 5.24-Shrinkage of OC-MK system at 0.44 w/c ratio at various days

0 7 14 21 28-1

-0.8

-0.6

-0.4

-0.2

0OC0.44OC10PFAOC20PFAOC30PFA


Shirn

kage

(%)

Figure 5.25-Shrinkage of OC-PFA system at 0.44 w/c ratio at various days



From Figure 5.24, the shrinkage of the various OC-MK systems was observed to increase

with the curing days. However, from the 3rd day up to 28 days, the OC system modified with

10%, 20% and 30% MK were observed to have lesser shrinkage than the OC0.44 system,

while the OC system modified with 40% and 50% MK recorded higher shrinkage than the

OC0.44 system. The shrinkage outcome can be attributed to the dosage of MK in the system

at the w/c ratio influencing the rate of hydration and the volume of pores in the system. Also,

the shrinkage of the OC-MK system was found to be non-linear with the order of MK dosage.

The OC system modified with 30% MK dosage was observed to have the least shrinkage

among the various OC-MK systems, while the OC system modified with 10% and 20% MK

dosages recorded shrinkage values below that of the OC0.44 system but above the OC30MK

system. For the OC system modified with 40% and 50% MK dosages, the shrinkage values of

both OC-MK system were similar and above that of the OC0.44 system and other OC-MK

systems.

The shrinking rates of the various OC-MK systems were also observed to differ with curing

days (Figure 5.24). From the result at the 3rd day, the shrinkage of the various OC-MK

systems was observed to be similar to that of the OC0.44 system. But, with curing days,

commencing from 7 days, the shrinkage of the various systems differed with the OC system

incorporating 10%, 20% and 30% MK resulting in shrinkage values slightly below that of the

OC0.44 system, while the OC40MK and OC50MK system showed shrinkage values slightly

above that of the OC0.44 system. At 14 days, an increase was observed in the shrinkage of

the OC40MK and OC50MK system compared to the OC0.44 system. The shrinkage of the

OC40MK and OC50MK system compared to the OC0.44 system at 14 days were found to

have increased above the shrinkage difference recorded at 7 days from that of the OC0.44

system. The OC system modified with 10%, 20% and 30% MK were observed to have

similar shrinkage difference to the OC0.44 system at 14 days as observed in 7 days. At 28

days, the shrinkage of the various OC-MK systems were found to increase slightly from the

previous day compared to the difference amongst other days. The outcome can be ascribed to

a slower rate of hydration at the stage compared to the previous days. The hydration of

cementitious system is reported to be higher at the early days while lower at later days

(Cemex 2015, Michaux et al 1989). More so, the shrinkage of cementitious system is



associated to its pore volume. Mokarema (2005) associated the shrinkage of cementitious

system to the volume of pores in the system as it hydrates. According to Sabins and Sutton

(1991), the pores in a system accounts for 97.5 to 99% of the total shrinkage of a system.

Thus, the pore volume of the OC-MK systems which knowledge can be acquired from the

water absorption results of the systems contributed to the percentage of shrinkage of each

system as it provided the void for the systems to shrink into as they hydrate.

Figure 5.25 shows the shrinkage of the OC-PFA system compared to the OC0.44 system.

From the result shown, it was found that the shrinkage of the various OC-PFA systems

increased with curing days like that of the OC0.44 system. Also, the shrinkage values of the

OC-PFA system were found to differ from the increasing order of the PFA dosage across the

curing days. From the results of the 3rd day, the shrinkage of the various OC-PFA system

were found to be similar and below the shrinkage value recorded for the OC0.44 system.

However at 7 days, the shrinkage of the OC system modified with 10% and 20% PFA

recorded higher shrinkage than other OC-PFA systems while still below the shrinkage values

of the OC0.44 system, but at 14 days, the shrinkage trend of the various OC-PFA systems

differed. The OC system incorporating 50% PFA was found to record the highest shrinkage

value compared to various OC-PFA systems at 14 days, although, the shrinkage of the OC-

PFA systems remained below that of the OC0.44 system. At 28 days, the OC50PFA system

was found to record a shrinkage value above the OC0.44 system and other OC-PFA system

while the OC40PFA system had similar shrinkage as the OC0.44 system. The OC system

modified with 10%, 20% and 30% PFA were found to remain below the shrinkage of the

OC0.44 system with the OC20PFA system recording the lowest shrinkage compared to the

OC0.44 system. The shrinkage of the OC-PFA systems can be attributed to the pore volume

of the systems, as higher pores indicate more space for a system to shrink into. Also, the

shirnkage was influenced by the slow rate of hydration of the system as a result of the

presence and dosage of PFA material. The outcome is supported by findings made by Backe

(1998), Keck and Riggs (1997) and Zhou (2012.). Keck and Riggs (1997) from their study

established that using PFA which has retarding effect slows the hydration of the system to

shrink while Zhou (2012) reported that using PFA in cement reduces the pore spaces in the

system due to the formation of hydrate products and its smaller particle sizes thereby

reducing the pore volume for the system to shrink into.



5.7 Hydrate products

Hydrate products of cementitious systems are chemical compounds produced from the

reaction of cement chemicals in the presence of water (Thomas, 2015). This section presents

the weight loss at temperature range of 50oC to 250oC for calcium aluminate and silicate

hydrates (C-A-S-H) and 400oC to 550oC for calcium hydroxide Ca(OH)2 hydrate product

contents occurring in the systems. The results of the effect of mix water ratio and the effect of

MA (MK and PFA) on the formation of hydrate products in OC system are detailed in this

section as obtained from the thermogravimetric analysis.

5.7.1 Hydrate products of class G OC systemThe quantities of calcium aluminate silicate hydrates (C-A-S-H) and calcium hydroxide

Ca(OH)2 product determined through the weight loss of the class G OC systems prepared

with different w/c ratios and compared to the OC0.44 system are presented in Figure 5.26 and

5.27 respectively. The purpose of varying the water to cement ratio (w/c) was to identify the

effect of water on the hydrate product of OC system.

0 7 14 21 280

4

8

12

16

20

24

OC0.36

OC0.40

Hydration period (days)

Weig

ht lo

ss (%

)

Figure 5.26-Weight loss of OC system of different w/c ratios from 50oC-250oC



0 7 14 21 280

1

2

3

4

5

6

7

8

OC0.36OC0.40OC0.44


Weig

ht lo

ss (%

)

Figure 5.27-Weight loss of OC system of different w/c ratios from 400oC -550oC

From Figure 5.26, the percentage of weight loss of the various OC systems prepared with

different w/c ratio as a result of the C-A-S-H content were observed to vary as they increased

with curing days. The results show that the weight loss percentage of the OC systems did not

conform to the order of increasing mix water ratio with curing days. From the initial day, the

OC system prepared with the lowest mix water ratio (OC0.36) was found to have the highest

weight loss compared to the other OC systems with the OC 0.52 system observed to have the

lowest weight loss. But, with curing days as observed at the 7th day, the OC0.52 system

recorded the highest weight loss amongst the OC systems with the OC0.36 systems recording

the lowest weight loss. At 14 days, the percentage of weight loss of the various OC systems

was observed to be similar to the outcome observed at 7 days and in same pattern. The OC

systems were found to record an increase in weight loss but not significant compared to

results at 7 days from the initial day. However, at 28 days, a significant increase in weight

loss was observed from the outcomes recorded at 14 days for all the OC system in same

pattern as observed from 7 days. The results show that C-A-S-H increases with curing age for

the various OC system.



Figures 5.27 shows the weight loss of the various OC system prepared with different w/c

ratios indicating the quantity of Ca(OH)2 in the systems. From the results, the OC systems

designed with lower w/c ratios were observed to have slightly higher weight losses than the

OC systems designed with higher w/c ratio. At 7 days, a slight increase in weight loss was

observed for all the OC systems. It was also found that the OC systems with the lesser weight

loss at the initial day recorded a similar weight loss with other systems at the 7 th day. At 14

days, the weight loss of the various OC systems were found to differ with the OC systems

prepared with a higher mix water ratio having higher weight loss. At 28 days, the weight loss

of the various OC systems decreased slightly but retained the same order as observed at 14

days. The reduction of the Ca(OH)2 at 28 days can be attributed to the high formation of C-A-

S-H product and the formation of other products or occurrence of carbonation which results

in calcite formation from the Ca(OH)2. A similar outcome was recorded by Vedalakshmi et al

(2003) regarding the content of Ca(OH)2 product. Vedalakshmi et al (2003) reported a

reduction occurrence with the Ca(OH)2 as C-A-S-H product increased with curing days. Ding

et al (2013) and Zhang and Bachu (2011), added that carbonation process also consumes the

content of Ca(OH)2 to produce calcite in the hydration of cementitious system. Kutchko

(2007) noted that the occurrence of carbonation in a system is associated with the presence of

CO2 in the surrounding. The impact of carbonation on the content of Ca(OH)2 in the

hydration of cementitious system is well documented (Ding et al, 2013; Kutchko 2007,

Concrete experts international 2006; Zhang and Bachu 2011; Hernandez G et al, 2008). Thus,

the outcome indicate that the formation of other products such as calcite altered the

increasing trend of the Ca(OH)2 content at 28 days.

From the results (Figures 5.26 and 5.27), the contents of the hydrate products (CASH and

Ca(OH)2) in the various OC system can be ascribed to the water to cement (w/c) ratio used

for formulation of the systems, thus, affecting the rate of hydration of the various system. The

results showed that OC systems designed with a low mix water ratio had higher weight losses

at the early days indicating more hydrate product which is due to higher hydration as result of

low mix water. At later days, the OC systems formulated with high mix water were found to

have more hydrate products which can be attributed to available moisture. The results agree

with findings of Junior et al (2012) and Vedalakshmi et al (2003) on the impact of water on

hydrate products of cementitious system. Junior et al (2012) found from their study including



ordinary Portland cement system designed with w/c ratio of 0.35, 0.45 and 0.55 that the

systems with higher w/c ratio had higher CASH and Ca(OH)2 for the curing period due to

available moisture resulting from mix water that enabled more reaction. Similar outcome was

reported by Vedalakshmi et al (2003) for w/c ratios of 0.42, 0.54 and 0.67 with curing

duration.

5.7.2 Hydrate products of modified OC system

The weight loss indicating the quantities of hydrate products in the modified class G OC

system using MK or PFA at a fixed w/c ratio of 0.44 are presented in the section. Figures

5.28 and 5.29 respectively depict the weight losses of the OC-MK and OC-PFA systems

between 50oC to 250oC indicating the quantity of calcium aluminate silicate hydrate products

(C-A-S-H) at various days, while Figure 5.30 and 5.31 depict the weight losses of OC-MK

and OC-PFA within the temperature range of 400oC to 550oC indicating the quantity of

Ca(OH)2 (calcium hydroxide) in the systems at various days.

0 7 14 21 280

4

8

12

16

20

24

OC0.44OC10MKOC20MK


Weig

ht lo

ss (%

)

Figure 5.28-Weight loss of OC-MK systems at 0.44 w/c ratio from 50oC to 250oC



0 7 14 21 280

4

8

12

16

20

24

OC0.44OC10PFAOC20PFA

Hydration duration (days)

Weig

ht lo

ss (%

)

Figure 5.29 -Weight loss of OC-PFA systems at 0.44 w/c ratio from 50oC to 250oC

0 7 14 21 280

1

2

3

4

5

6

7

8

OC0.44OC10MKOC20MK

Hydration period(days)

Weig

ht lo

ss (%

)

Figure 5.30 -Weight loss of OC-MK systems at 0.44 w/c ratio from 400oC-550oC



0 7 14 21 280

1

2

3

4

5

6

7

8

OC0.44OC10PFAOC20PFA


Weig

ht lo

ss (%

)

Figure 5.31 -Weight loss of OC-PFA systems at 0.44 w/c ratio from 400oC -550oC

From Figure 5.28, the results of the weight loss of the OC-MK systems due to the presence of

the C-A-S-H content were found to increase with an increase of MK dosage in the OC

system. The weight losses of the various OC-MK systems were also found to be higher than

that of the OC0.44 system at the various days. From the initial day, the weight loss of the

various OC-MK systems were found to be similar to that of the OC0.44 system but differed

from 7 days. At 28 days, a large increase in weight loss was observed for the various OC-MK

systems especially the OC50MK system. From the outcome, it can be concluded that the

dosage of MK in the OC system is responsible for the increase in weight loss, thereby the

high formation of C-A-S-H content accordingly in the systems. This can be attributed to the

properties of the MK material.

Figure 5.29 presents the weight loss as a result of the C-A-S-H content of the OC-PFA

systems compared to the weight loss of the OC0.44 system. The outcome was found to differ

from results of the OC-MK systems which showed higher C-A-S-H content from 7 days for

all the OC-MK system compared to that of the OC0.44 system apart from the initial day



result which was similar. In the case of the OC-PFA systems, the OC0.44 system was

observed to have a higher weight loss than the OC-PFA systems at the initial day. The result

revealed that the higher the dosage of PFA in the system, the lower the weight loss at the

initial day. The outcome remained similar at 7 days with the OC0.44 system recording higher

weight loss than the OC-PFA systems. However, at 14 days, the weight loss trend was

observed to change. The OC10PFA system was found to have a higher weight loss than the

OC0.44 system, while the other OC-PFA systems had lower weight loss than the OC0.44

system. The results revealed an increasing influence of the PFA was beginning to take place

as was confirmed further from the results at 28 days. The results at 28 days showed the

OC10PFA and OC20PFA system to have a higher weight loss than the OC0.44 system. The

result also shows the weight loss of the OC20PFA system to be similar to that of the

OC10PFA system at 28 days. The outcome of the OC-PFA system weight loss with curing

days can be ascribed to rate of hydration resulting from the PFA properties and dosage with

cement properties at the w/c ratio. This can be supported by outcomes observed in the setting

times of the OC-PFA systems which took longer time to set compared to the OC0.44 system.

From Figures 5.30, the results of the content of Ca(OH)2 (calcium hydroxide) in the OC-MK

system can be seen. The calcium hydroxide content is estimated from the weight loss of the

OC-MK system between the temperature ranges of 400oC to 550oC. The results in Figure

5.30 show the weight loss of the various OC-MK systems to be lower than that of the OC0.44

system for the various days. The weight loss of the OC0.44 system (2.91%) was found to be

slightly higher than that of the OC systems modified with 10% and 20% MK while

significantly higher than the OC systems modified with 30%, 40% and 50% MK dosage. The

weight loss observed for the OC10MK and OC20MK system at initial day can be attributed

to the dosage of the MK properties to class G cement properties for the system. At 7 days, the

weight loss of the various OC-MK systems was found to increase from the weight loss

observed at initial day but remained lower than that of the OC0.44 system. At 14 days,

similar result was found for the OC-MK compared to the OC0.44 system. The weight loss of

the various OC-MK systems were observed to be lower than that of the OC0.44 system while

increasing slightly from the result found in 7 days. At 28 days, the weight loss of the various

OC-MK systems was found to remain lower than that of the OC0.44 system with the weight

loss of the OC10MK system being the highest while that of the OC50MK system was the



lowest amongst the OC-MK systems. The result of the OC-MK system compared to that of

the OC0.44 system agree with findings made by El-Diadamony et al (2016) with ordinary

Portland cement system incorporating MK dosages. They found from their study that MK

dosages caused a decrease in the Ca(OH)2 of the cement-MK system and ascribed the result

to a pozzolanic reaction. In this regards, the outcome of the OC-MK systems can be

attributed to the bonding of the chemicals composed by the MK material with that of the

cement material to form C-A-S-H products thereby decreasing the content of Ca(OH)2 while

the C-A-S-H increased. However, the weight loss of the OC-MK systems were observed to

differ from the increasing trend observed at previous days at 28 days. The weight loss of the

OC system modified with 10%, 20% and 30% MK dosage was found to be similar with the

previous day while the weight loss of OC40MK and OC50MK system decreased slightly.

The change in trend with the Ca(OH)2 product for the OC-MK systems indicate the high

formation of the C-A-S-H and other product such as calcite in the hydration process altered

the trend of Ca(OH)2 formation. Eleni et al (2014) noted that the formation of calcite in a

system resulting from carbonation which use Ca(OH)2 in the process is more likely to occur

in systems with higher pore volume due to intrusion of CO2 from the surrounding. From the

water absorption result which is related to the pore volume of the systems, the OC40MK and

OC50MK system where found to absorb the highest percentage of water indicating higher

volume of pores in their system. However, the weight of the various OC-MK systems

remained lower than that of the OC0.44 system which was also found to decrease slightly at

28 days.

Figure 5.31 shows the weight loss of the OC-PFA systems compared to the OC0.44 system at

the temperature range of 400oC to 550oC to estimate Ca(OH)2 content in the system. The

results show the weight loss of the OC-PFA systems to be vary with curing days compared to

than that of the OC0.44 system. From the initial day, the OC systems incorporating 10%,

20% and 30% PFA dosages were found to have a similar weight loss as the OC0.44 system

while the OC systems modified with 40% and 50% PFA dosages showed a lower weight loss

compared to the OC0.44 system. At 7 days, the trend was found to differ with OC systems

incorporating 10% and 20% PFA material observed to have a similar weight loss as the

OC0.44 system whilst the other OC-PFA systems showed lower weight loss compared to the

OC0.44 system. At 14 days, the weight loss of all the OC-PFA systems were found to be



lower than that of the OC0.44 system and a similar result was found at 28 days with the

OC10PFA system having the highest weight loss while the OC50PFA system had the lowest

weight loss. The low Ca(OH)2 content observed for the OC-PFA dosages compared to the

OC0.44 system can be attributed to bonding of the PFA chemicals with that of cement

thereby reducing the Ca(OH)2 content below that of the OC0.44. Similar finding was made

by Shafiq and Nuruddin (2010) using PFA to partially replace cement for cementitious

system. Shafiq and Nuruddin (2010) found that the Ca(OH)2 content of the system decreased

with the presence of PFA dosages and the occurrence was attributed to a pozzolanic reaction

occurrence in the cement and PFA system. However at 28 days, a slight decrease from the

increase in weight loss observed from the initial day to 14 days was observed which differed

with the outcome in the study of Shafiq and Nuruddin (2010). The occurrence suggests that,

as with other systems, the increase in formation of C-A-S-H and other products such as

calcite altered the increasing content of Ca(OH)2 product in the OC-PFA systems. The

influence of calcite in reducing the content of Ca(OH)2 resulting from carbonation occurrence

in cementitious system is well documented (Ding et al, 2013; Kutchko 2007, Concrete

experts international 2006; Zhang and Bachu 2011; Hernandez G et al, 2008).

5.8 Performance evaluation and selection of OC system

OC systems are pumped down several thousands of meters into a drilled (bored) hole to seal

the space between the formation and the casing string. However, several factors affect the

performance of the OC system requiring certain criteria to be satisfied before its application

for cementing an oil well. Thus, the investigated performance of the various OC systems was

evaluated against the performance of the OC0.44 system and the recommended performance

standards by API. This is to ensure that only the OC systems that are satisfactory are selected

for further modification using chemical admixture and investigated for setting time and

strength performance at elevated temperatures.

The study of setting time and strength performance of an OC system at an elevated

temperature can reveal the placement and stability provision capability of an OC system

under certain well condition such as high temperature and will be beneficial for successful

cementing of an oil well. Tables 5.1, 5.2 and 5.3 presents the investigated performance of the

various OC systems compared to the API requirements and the performance of the OC0.44



system for selection and further study of the setting time and strength performance of an OC

system at elevated temperature. The systems that satisfied the density, setting time and

strength performance requirements, flowed when investigated for rheology and had similar or

less absorption of water and shrinkage than the OC0.44 system were accepted as the good

() systems and selected for further study, while systems that did not satisfy some of the

requirements or show similar or less absorption of water and shrinkage than OC0.44 system

were accepted as bad (×) and not selected for further study.

Table 5.1-Performance of the OC systems of various w/c ratio vs. OC0.44 system

Criteria

Class G OC System of various w/c ratio

OC 0.36 OC 0.40 OC 0.44 OC 0.48 OC 0.52

Density =1.773 to 1.965 kg/m3)

× ×

Rheology

Setting time @ 23°C (300-480minutes)

× ×

Compressive strength @ 24 hours (3.4 MPa)

× ×

Tensile Strength @ 24 hours (0.34MPa)

× ×

Water absorption vs. OC0.44 @ 24 hours

× ×

Shrinkage vs. OC 0.44 @ 28days

× ×

Hydrate product



Table 5.2-Performance of the OC-MK systems at fixed w/c ratio of 0.44 compared to the OC0.44 system

Criteria

OC System modified with Metakaolin (MK) material

OC 0.44 OC10MK OC20MK OC30MK OC40MK OC50MK


× × ×

Rheology × × ×

Setting time @ 23°C (300-480 minutes)

× × ×


×


×


× ×


× ×

Hydrate product



Table 5.3-Performance of the OC-PFA system at fixed w/c ratio of 0.44 compared to the OC0.44 system

Criteria

OC System modified with Pulverised fuel ash (PFA) material

OC0.44 OC10PFA OC20PFA OC30PFA OC40PFA OC50PFA


× ×

Rheology

Setting time @ 23°C (300-480 minutes)

× × ×


× ×


× ×


× × ×


×

Hydrate product



Table 5.1 shows the performance of class G OC system formulated with different water to

cement ratio compared to the API requirement and OC0.44 system. From the performance of

the various class G OC system (Table 5.1), it was found that the OC system designed with a

mix water ratio above that used for the OC0.44 system did not satisfy most of the

performance criteria. For the case of the OC0.36 and OC0.40 system formulated with mix

water lower than the w/c of 0.44 used for the OC0.44 system, the OC0.36 was found not to

satisfy the density requirement while the OC0.40 system was found to satisfy the various

requirements. In general, the evaluation outcome indicates that the higher or lower the water

to cement ratio for OC system from the standard w/c of 0.44 for class G OC system, the

likely hood that the OC system will not satisfy the criteria.

Table 5.2, shows the performance of the OC-MK systems compared to the standard API

requirement and the OC0.44 system performance. The outcome of the comparison shows that

only the OC system modified with 10% and 20% MK material satisfied the performance

requirements. The outcome can be attributed to the chemical and physical properties of the

MK material that is different from that of the class G cement and the dosage of the MK

material and adopted w/c ratio for the design of the OC-MK systems. It was found that

beyond the dosage of 20% MK for the OC system, the capability of the OC-MK system to

satisfy the requirements altered. The OC30MK system was found not to satisfy the density

criteria and setting time requirement for OC system, while the OC40MK and OC50MK

system were found not to meet most of the requirements.

For the OC-PFA systems, only the OC system modified with 10% and 20% PFA dosages

satisfied the performance requirements, while the OC system incorporating 30%, 40% and

50% PFA material were found not to satisfy some of the requirements (Table 5.3). The

outcome can be attributed to the chemical and physical properties and effect of the PFA

dosage to the OC system with a contributory effect of the w/c ratio adopted for the

formulation of the systems. As such, only the OC system modified with 10% and 20% PFA

were selected from the category as performance is critical in oil well cementing.

5.9 The effect of temperature on the setting time and strength performance of an OC systemN.N Amucheazi (2018) University of South Wales 123


The investigation in this section deals with the setting time and compressive strength at 24

hours of OC systems under selected temperatures of 23oC, 42oC and 60oC that exist in the oil

well. Establishing the setting time and strength performance of an OC system at 24 hours is

essential in oil well construction. This is to enable proper placement of the system in the oil

well and continuation of drilling after the system has achieved sufficient strength to offer

stability to the casing in the oil well. Thus, this section presents the results of the setting time

and compressive strength at 24 hours of the selected OC systems at different temperatures.

The selected OC systems are the OC0.44 system, OC10MK, OC20MK, OC10PFA and

OC20PFA system. The OC0.40 system which is one of the systems that satisfied all of the

criteria was not selected for this aspect of the study. This is because; the focus with the w/c

ratio was to gain a general knowledge of mix water influence on systems. Moreover, the

reduction of cement content is encouraged due to environmental concerns (Shariar 2011,

Christodoulou 2000). As such, only the setting time and compressive strength results of the

selected modified OC systems were examined and compared to that of the OC0.44 system to

understand their different performance under elevated temperatures. The outcome was used

to develop specific models to predict the performance of the OC system incorporating MK or

PFA material. Models can simplify the design process of OC systems which can be beneficial

in reducing the time, number of tests and cost in designing cementitious system for varying

conditions such as temperature in oil wells.

5.9.1 Impact of temperature on the setting time of OC systems

The impact of temperature on the setting time of the various OC systems as presented in

Figure 5.32 shows the setting time (ST) of the OC0.44, OC-MK and OC-PFA systems to

decrease with an increase in temperature. The outcome indicates that an increase in

temperature accelerates the transformation of the OC system from fluid state to stiffened

state. The occurrence can be attributed to an increase of hydration from the hot condition

introduced by elevated temperature.



OC0.44 OC10MK OC20MK OC10PFA OC20PFA0

120

240

360

480

600

720

23°C42°C60°C

OC-SCM system

Setti

ng ti

me (

minu

tes)

Figure 5.32 -Impact of temperature on the ST of OC system modified with 10% and 20% MA dosages

The results (Figure 5.32) also showed that the effect of temperature (23oC, 42oC and 60oC) on

the setting time of the various OC systems differed with the material composition of the

systems. The outcome can be ascribed to the response of the properties of the various

materials (cement, MK and PFA) to temperature thereby influencing the rate at which the

chemicals of the cement and MA react in the system. From the results, the impact of

temperature which decreased the setting time of the various systems was found to be more

obvious with the OC-MK systems compared to the OC0.44 system and the OC-PFA systems.

It was also found that the dosage of the particular MAs recorded different setting times with

exposure to the temperatures. The result show that the impact of temperature in decreasing

the setting time of the OC-MK was more obvious with the OC system modified with 20%

MK material which can be seen to record the shortest setting time at the various temperatures.

The outcome can be attributed to the combined effect of the MK properties with the dosage

and the increase of temperature.

For the case of the OC-PFA system (Figure 5.32), the setting time of the OC-PFA system

compared to the OC0.44 system was observed to take an opposite trend to the setting of the



OC-MK system in comparison with the OC0.44 system. It was observed that the increase in

temperature resulted in a decrease in the setting time of the OC-PFA systems however, the

OC-PFA systems recorded higher setting times than the OC0.44 system. It was also found

that the higher the dosage of PFA material in the system, the higher the setting time of the

OC-PFA system at the different temperature measured with that of the OC0.44 system. The

occurrence can be ascribed to the retarding effect from the properties of the PFA material on

the setting time of OC system found with OC-PFA systems at 23oC.

5.9.2 Impact of temperature on compressive strength of OC system

The impact of temperature (23oC, 42oC and 60oC) on the compressive strength of the various

OC systems can be seen in Figure 5.33.

OC0.44 OC10MK OC20MK OC10PFA OC20PFA0

5

10

15

20

25

23°C42°C60°C

OC-SCM systems

Com

pres

sive s

treng

th (M

Pa)

Figure 5.33- Impact of temperature on the CS of OC system modified with 10% and 20% MA dosages at 24 hours



It can be observed that the increase in temperature resulted in an increase in the compressive

strength for the various OC systems. This is due to a higher rate of chemical bonding with the

systems. However, the compressive strength performance of the OC-MK and OC-PFA

systems compared to the OC0.44 system with an increase in temperature were observed to

vary. From Figure 5.33, it can be seen that the compressive strength of the OC10MK and

OC20MK system at 23oC and 42oC were found to be above that of the OC0.44 system but at

60oC, the compressive strength of the OC-MK system was found to be less than that of the

OC0.44 system. Amongst the OC-MK systems, the OC20MK system was found to have the

lowest strength at 60oC.

For the OC-PFA systems (Figure 5.33), the compressive strength of the OC-PFA system was

observed to increase with an increase in temperature although lower than the strength of the

OC0.44 system. Amongst the OC-PFA systems, the OC10PFA system was observed to have

a higher compressive strength compared to the OC20PFA system at the various temperatures.

It was also observed in the compressive strength performance of the OC-PFA system, unlike

the OC-MK system, increased with temperature.

5.10 Effect of chemical admixture on the setting time and strength performance of OC systemDue to the oil well conditions especially the temperature which can alter the performance of

an OC system, various types of chemical admixtures are used to modify the behaviour of the

OC system for adequate performance in the oil well. Thus, investigation of the impact of

chemical admixture on the performance of an OC system is necessary. This is to determine

the suitable admixture dosage to obtain the desired behaviour to enable adequate performance

of the OC system to be achieved under the specific conditions in the oil well. However, this is

not an easy task as it takes lots of time and experiments with several trials which increases

the cost of cementing the oil well. As such, the present study aimed to simplify the process by

investigating the impact of the chemical admixture such as ADVA flow 340- super plasticizer

on the setting time and compressive strength of the selected OC systems (that satisfied the

performance criteria) under different temperatures. The outcome was then modelled to

simplify the selection of desired chemical admixture dosage for the design of an OC system



for sufficient setting time and strength performance for adequate placement and stability

provision in the oil well. The results of the setting time and compressive strength of the

selected OC systems using super plasticizer chemical admixture are presented in this section.

5.10.1 Influence of chemical admixture dosage on setting time of OC systemThe setting time results of the various OC systems modified with Super plasticizer (SP)

admixture at different temperatures are presented in this section. The setting time results of

the modified OC systems were compared to that of the OC0.44 system to evaluate the impact

of the SP dosages on the setting time of the OC systems. Figures 5.34, 5.35 and 5.36

respectively depict the impact of SP on the setting times of OC0.44, OC-MK and OC-PFA

systems at various temperatures (23oC, 42oC and 60oC).

0 0.2 0.4 0.600000000000001 0.8 10

240

480

720

960

1200OC0.44OC10MKOC20MKOC10PFAOC20PFA

SP dosages (%)

Setti

ng ti

me (

minu

tes)

Figure 5.34 –Setting times of OC0.44, OC-MK and OC-PFA systems incorporating SP admixture dosages at 23oC



0 0.2 0.4 0.600000000000001 0.8 10

240

480

720

960

1200

OC0.44OC10MKOC20MKOC10PFAOC20PFA

SP dosages (%)

Setti

ng ti

me (

minu

tes)

Figure 5.35- Setting times of OC0.44, OC-MK and OC-PFA systems incorporating SP admixture dosages at 42oC

0 0.2 0.4 0.600000000000001 0.8 10

240

480

720

960

1200


SP dosages (%)

Setti

ng ti

me (

minu

tes)

Figure 5.36 –Setting times of OC0.44, OC-MK and OC-PFA systems incorporating SP admixture dosages at 60oC



From the results (Figure 5.34 - 5.36), the impact of the SP admixture on the setting of the

modified OC system was found to differ with type and dosage of the MA and temperature.

For the effect of the SP admixture on the OC-MK systems, it was found from the results that

the higher the dosage of MK in the system, the higher the dosage of the SP admixture

required to extend the setting time of the system (Figures 5.34-5.36). Comparing the OC-MK

system to the OC0.44 system, the OC-MK systems were found to require a higher SP dosage

to extend thier setting period. Particular dosages of the SP admixture used to extend the

setting time of the OC0.44 system for adequate placement in the oil well were found to be

insufficient on the OC-MK systems to record similar setting times as the OC0.44 system.

This is due to the properties of the MK influencing the transformation of the OC system to

solid at shorter time. The OC system modified with 20% MK was found to require a higher

SP admixture dosage to extend its setting time. More so, the temperature was found to

influence the dosage of the SP admixture required to extend the setting time of the OC-MK

system. The higher the temperature the system is exposed to, the higher the dosages of the SP

admixture needed to extend the setting time of the control system and the OC-MK system.

For the OC-PFA systems presented in Figures 5.34, 5.35 and 5.36, the opposite pattern was

found compared to the OC-MK system. The setting time of the OC-PFA systems were found

to increase with additions of the SP admixture dosage much more than that of the OC-MK

system. It was also found that the higher the dosage of the PFA material in the system, the

lower the dosage of SP admixture needed to achieve a higher setting time unlike the case of

the OC-MK system. This can be attributed to the influence of the PFA on the setting time of

the OC system as found earlier without addition of SP admixture. Comparing the OC-PFA

systems to the OC0.44 system with the different SP admixture dosages, the setting times of

the OC-PFA systems were observed to remain higher than that of the OC0.44 system at the

various temperatures. Nonetheless, it was observed that the increase of temperature resulted

in a higher dosage of the SP admixture needed to increase the setting time of the OC-PFA

systems.



5.10.2 Influence of chemical admixture on the compressive strength of OC system

The 24 hours compressive strength results of the various OC system modified with Super

plasticizer (SP) chemical admixture at different temperatures are presented in this section.

The compressive strength results of the modified OC systems are compared to that of the

OC0.44 system to evaluate the impact of the SP admixture dosages on the strength

performance of the OC systems at 24 hours. Figures 5.37, 5.38 and 5.39 respectively depict

the impact of the SP admixture on the compressive strength performance of OC0.44, OC-MK

and OC-PFA systems at various temperatures (23oC, 42oC and 60oC) in 24 hours.

0 0.2 0.4 0.600000000000001 0.8 10

8

16

24

32

40


SP dosages (%)

Com

pres

sive s

treng

th (M

Pa)

Figure 5.37 –Compressive strength at 24 hours of OC0.44, OC-MK and OC-PFA systems incorporating SP admixture dosages at 23oC



0 0.2 0.4 0.600000000000001 0.8 10

8

16

24

32

40


SP dosages (%)

Com

pres

sive s

treng

th (M

Pa)

Figure 5.38-Compressive strength at 24 hours of OC0.44, OC-MK and OC-PFA systems incorporating SP admixture dosages at 42oC

0 0.2 0.4 0.600000000000001 0.8 10

8

16

24

32

40OC0.44OC10MKOC20MKOC10PFAOC20PFA

SP dosages (%)

Com

pres

sive s

treng

th (M

Pa)

Figure 5.39-Compressive strength at 24 hours of OC0.44, OC-MK and OC-PFA systems incorporating SP admixture dosages at 60oC



From the results (Figure 5.37-5.39), the 24 hours compressive strength of the various OC

systems differed with the addition of the SP admixture dosages at the different temperature

degree. Nonetheless, the type and dosage of MA in the OC system influenced the strength

outcomes with the dosages of the SP admixture incorporated at the various temperature

degrees.

Comparing the impact of the SP admixture on the compressive strength of the OC-MK

systems to that of the control OC0.44 system, the various systems (OC0.44 and OC-MK

system) were found to increase in strength to a certain dosage of the SP admixture before

recording lower strengths at 23oC. The OC0.44 system and the OC10MK system were found

to record an optimum strength with the SP admixture dosage of 0.4%, while the OC20MK

system recorded optimum strength at the SP admixture dosage of 0.6% for the various

dosages of the SP admixture at 24 hours. The occurrence can be ascribed to the retardation

effect of the SP admixture to the OC system as found with the setting time thereby adjusting

the hydration rate of the system which slowed the strength gain of the systems within 24

hours. However, at higher temperatures, the effect of the SP admixture dosages was found to

initiate an increase to the strength performance of the various systems accordingly. However,

the OC-MK system recorded lower strengths compared to the OC0.44 system at 60oC which

can be ascribed to the effect of high temperature on the strength performance of the OC-MK

system as observed without presence of the SP admixture in the OC-MK system (Figure

5.39). The OC20MK system was found to have the lowest strength among the OC-MK

system and the OC0.44system at 60oC.

For the compressive strength of the OC-PFA systems modified with the SP admixture as

shown in Figures 5.37, 5.38 and 5.39, a different trend was observed with the dosages of the

PFA in the system compared to the OC-MK systems at 23oC. With increase of the SP

admixture dosage in the system, the OC-PFA systems were found to have a lower strength

compared to the initial strength recorded without the presence of the SP admixture dosage at

24 hours. The OC system modified with 20% PFA material was found to have the lowest

strength at 24 hours with an increase of the SP admixture dosage compared to the OC0.44

system and the OC10PFA system. The finding can be associated to the low rate of hydration

of the OC-PFA system as found with the hydration products results (Figure 5.29 and 5.30)



which was also complimented by the retardation effect of the SP admixture. Thus, the gain in

strength was slowed over time resulting in lower strengths found with the OC-PFA system at

24 hours. But at elevated temperature, the compressive strengths of the OC-PFA systems at

24 hours were found to increase with increase in the SP admixture dosage. The highest

strengths for the OC-PFA systems at 24 hours were found to occur at 60oC with the presence

of the highest dosage of the SP admxiture in system. However, the strength gains of the OC-

PFA systems were still below that of the OC0.44 system even as they increased at higher

temperature and the presence of the SP admixture.


Chapter 6-Model and Analysis

CHAPTER 6: MODELLINGThe present study pursued the development of a model to simplify the design process of an

OC system with a focus on the setting time and strength performance of the OC system at 24

hours using a multiple regression approach (MRA) with the aid of Design Expert software.

The development of the model involved fitting the outcome of the OC system as a result of

the impact of adopted the selected MAs dosages, SP admixture dosages and temperatures

(input parameters) to a mathematical equation to predict the setting time and compressive

strength performance (output variable). The validations of the developed models were carried

out using the analysis of variance (ANOVA) generated by the statistical package of the

adopted software (design expert software). The ANOVA provided the significance of the

model terms with the SD and R2 which were used to evaluate the confidence and accuracy of

the model to predict the output values (setting times and strength performance).

6.1 Model validation Tables 6.1 and 6.2 present the Prob>F value and coefficient of the various input parameters

while Tables 6.3 and 6.4 present the adopted input parameters and their interactions for the

model based on their significance to predict the setting time and compressive strength of the

OC system. The Prob>F value and coefficient provide a comparison of the effect of the input

parameter(s) to the output variable. The values of Prob>F less than 0.05 indicate the model

term has a significant impact. From the generated ANOVA, the value of Prob>F for some of

the input parameters coefficient were found to be above 0.05 (Table 6.1 and 6.2). This

indicates that such parameters do not have a significant effect on the output variables. Thus,

the significant input parameters were selected for the model (Tables 6.3 and 6.4). The F value

used to verify the merit of a model also showed that the developed models are satisfactory

with Prob>F below 0.05 (Table 6.5). The model F value presented by the ANOVA implied

that the models are significant and there is only a 0.01% chance that the model F value could

occur due to noise.

Furthermore, the values of the coefficient which indicate the degree of influence an input

parameter or the interaction of input parameter(s) have on the output value were found to

vary (Tables 6.3 and 6.4). The higher the coefficient of an input parameter in the model, the



greater the influence it has on the outcome of the output variable. Also, where the coefficient

of a single or interacting parameters is negative, it indicates that the parameter(s) will have a

reducing effect on the output variable value as the parameter(s) increases while other

parameter(s) are fixed. Likewise, where the coefficient of a single or interacting parameter(s)

is positive, the parameter(s) tends to have an increasing effect on the output variable value if

other parameters are constant.

From the analysis in Tables 6.3 and 6.4, the coefficient of the single input parameter of the

MK and PFA material differed in the setting time and compressive strength analysis which

can be explained from their effect to the OC system. Thus, the single MK parameter has a

negative coefficient for the setting time and positive coefficient for compressive strength

while the opposite was found with the PFA parameter which had a positive coefficient for the

setting time and negative coefficient for the compressive strength performance. The

coefficient of the SP admixture was found to also differ for the setting time and compressive

strength performance. The SP admixture dosage was found to extend (increase) the setting

time of the OC systems and therefore has a positive coefficient for the setting time whilst for

the compressive strength it was found to have a negative coefficient due to its retardation

effect to strength gain. The coefficient of temperature was calculated to have a negative

coefficient for the setting time and compressive strength performance which is due to the

decreasing effect of temperature on the setting time of the OC systems and the adverse

impact of temperature to the compressive strength of the OC systems. Table 6.6 shows the

final quadratic equations of the models to predict the output variables including only the

significant terms.



Table 6.1-The ANOVA of the Prob>F value of the setting time of the OC-MA system

Model parameter(s) OC-MK Setting time OC-PFA Setting time

Coefficient Prob.>F Coefficient Prob.>F

Intercept 833.1 - 803.3 -

T -20.48 <0.0001 -21.7 <0.0001

SP +395.4 <0.0001 +575.7 <0.0001

MA -8.92 <0.0001 +9.48 <0.0001

T2 +0.12 <0.0001 +0.16 <0.0001

SP2 +46.27 0.01 +49.05 0.037

MA2 -0.05 0.16 -0.01 0.78

T.SP -3.98 <0.0001 -8.24 <0.0001

T.MA +0.17 <0.0001 -0.17 <0.0001

SP.MA -5.18 <0.0001 +7.3 <0.0001

Table 6.2-The ANOVA of the Prob>F value of the compressive strength of the OC-MA system

Model parameter(s) OC-MK compressive strength OC-PFA compressive strength


Intercept 8.04 - 17.29 -

T -0.04 <0.0001 -0.536 <0.0001

SP -14.86 <0.0001 -18.08 <0.0001

MA +0.33 0.07 -0.11 <0.0001

T2 +3.8 0.004 +9.757 <0.0001

SP2 +2.09 0.31 +6 0.0003

MA2 +5.41 0.22 +0.01 0.0005

T.SP +0.5 <0.0001 +0.49 <0.0001

T.MA -0.01 <0.0001 -0.008 <0.0001

SP.MA +0.04 0.51 -0.13 0.02



Table 6.3-The ANOVA of the significant model terms for the setting time of the OC-MA system

Model parameter(s) OC-MK Setting time OC-PFA Setting time


Intercept 834.84 - 803.82 -

T -20.48 <0.0001 -21.75 <0.0001

SP +395.49 <0.0001 +575.7 <0.0001

MA -9.95 <0.0001 +9.22 <0.0001

T2 +0.126 <0.0001 +0.167 <0.0001

SP2 +46.27 0.01 +49 <0.035

MA2 - - - -

T.SP -3.98 <0.0001 -8.249 <0.0001

T.MA +0.17 <0.0001 -0.179 <0.0001

SP.MA -5.18 <0.0001 +7.3 <0.0001

Table 6.4-The ANOVA of the significant model terms for the compressive strength of the OC-MA systems

Model parameter(s) OC-MK compressive strength OC-PFA Compressive strength


Intercept 7.34 - 17.29 -

T -0.04 <0.0001 -0.536 <0.0001

SP -12.28 <0.0001 -18.08 <0.0001

MA +0.46 0.07 -0.11 <0.0001

T2 +0.0038 0.004 +0.009 <0.0001

SP2 - - +6 0.0003

MA2 - - +0.01 0.0005

T.SP +0.5 <0.0001 +0.49 <0.0001

T.MA -0.012 <0.0001 -0.008 <0.0001

SP.MA - - -0.13 0.02



Table 6.5-ANOVA for significance of model

System Output variables F value Prob.>F

OC-MK Setting time 1237.59 < 0.0001

Compressive

strength

213.7 < 0.0001

OC-PFA Setting time 1557.2 < 0.0001

Compressive

strength

312 < 0.0001

Table 6.6-Generated quadratic models for the prediction of the OC-MK and OC-PFA systems Setting time and compressive strength

OC-MK

ST=834.84 – 20.48(T) + 395.49 (SP) – 9.95(MK) + 0.126(T2) +

46.27(SP2) – 3.98 (T.SP) + 0.17(T.MK) – 5.18(SP.MK)Eqn-6.1

CS=7.34 – 0.04(T) – 12.28(SP) + 0.46(MK) + 0.003(T2) +

0.5(T.SP) – 0.012(T.MK)Eqn-6.2

OC-PFA

ST=803.82 – 21.75(T) + 575.7(SP) + 9.22(PFA) + 0.167(T2) +

49(SP2) – 8.249(T.SP) – 0.179(T.PFA) + 7.3(SP.PFA) Eqn-6.3

CS=

17.29 – 0.536(T) – 18.08(SP) – 0.11(PFA) + 0.009(T2)

+6(SP2) + 0.01(PFA2) + 0.49(T.SP) – 0.008(T.PFA) –

0.13(SP.PFA)

Eqn-6.4

6.2 Model Confidence Table 6.7 presents the maximum, minimum and mean values of the output variables of the

OC-MK and OC-PFA system as well as the standard deviation (SD) and coefficient of

variance (COV) values to account for the confidence of the model in predicting the output

variables. The percentage (R2) of the output variables the models can predict was also

provided by the software. From Figures 6.1-6.4, the least R2 amongst all the developed

models was established to be 0.97 which indicates that 97% of the output variable can be



replicated or predicted by the model. Other models were found to have R2 values above 0.97

or 97% indicating higher percentage of the output variable can be predicted or replicated by

the models.

Table 6.7-Performance of the Model in predicting the output variables of the systemsSystem Maximum

actual value

Minimum

actual value

Mean SD COV

(%)

OC-MK ST 784 60 308 12.84 4.2

CS 38 8 16 1.52 8.9

OC-PFA ST 1080 80 414 16.59 4

CS 38 4 14 1.12 8

SD= standard deviation, COV=coefficient of variation given by SD/Mean * 100

The AAD% of the output variables was also generated using equation 4.7 to determine the

deviation in predicting the setting time and compressive strength performance of the various

OC systems by the models for acceptability. The calculated AAD% for the prediction of the

setting time and compressive strength at 24 hours for the OC system modifed with MK and

the SP admixture dosage for the various temperatures was 4.42% and 7.63% respectively. For

the OC system modified with PFA and the SP admixture dosages for the various

temperatures, the AAD% for the prediction by the models was calaculated to be 4.01% for

the setting time and 6.81% for the compressive strength. The relationship between the

predicted and actual values of the setting times and compressive strength at 24 hours of the

various systems are presented in Figure 6.1 -6.4. They show the deviation from the best line

of fit for the entire output variables of the setting times and compressive strength

performance of the OC system incorporating MK and PFA material and modified with

dosages of the SP admixture at different temperatures. The location of the data points from

the best line of fit can be seen to differ for the various OC systems and the particular output

variable (i.e setting time or compressive strength of the OC-MK or OC-PFA system) due to

the outcome from the tests.



Actual Setting time

Pre

dict

ed S

ettin

g tim

ePredicted vs. Actual

60.00

241.00

422.00

603.00

784.00

60.00 241.00 422.00 603.00 784.00

Figure 6.1-The R2 and AAD of the setting time values (in minutes) of the OC-MK systems

Actual Compressiv e strength

Pre

dict

ed C

ompr

essi

ve s

treng

th Predicted vs. Actual

7.60

15.20

22.80

30.40

38.00

7.60 15.20 22.80 30.40 38.00

Figure 6.2- The R2 and AAD of the compressive strength values (in MPa) of the OC-MK systems


R2= 0.99AAD=4.42%

R2= 0.97AAD=7.63%


Actual Setting time

Pre

dict

ed S

ettin

g tim

ePredicted vs. Actual

70.73

323.05

575.36

827.68

1080.00

70.73 323.05 575.36 827.68 1080.00

Figure 6.3- The R2 and AAD of the setting time values (in minutes) of the OC-PFA systems

Actual Compressiv e strength

Pre

dict

ed C

ompr

essi

ve s

treng

th Predicted vs. Actual

4.00

12.50

21.00

29.50

38.00

4.00 12.50 21.00 29.50 38.00

Figure 6.4- The R2 and AAD of the compressive strength values (in MPa) of the OC-PFA systems


R2= 0.99AAD=4.01%

R2= 0.98AAD=6.81%


6.3 Model accuracyWith the significant input parameters that make up the models and the provided AAD%, the

accuracy of each model to predict the output variable was examined. The developed models,

as presented in Table 6.6, were used to predict the setting time and compressive strength of

the OC systems and the accuracy of each model was evaluated using the determined AAD by

comparing the predicted trend of the setting times and compressive strength performance

values to the trend of the actual measured values obtained with two randomly selected

samples (Figures 6.5 -6.6). The outcome showed that the ST and CS models for the systems

were capable of replicating the trend and predicting the setting time and compressive strength

of the OC systems modified with MK and PFA and the SP admixture dosages across the

different temperatures with acceptable deviation.

20 25 30 35 40 45 50 55 60 650

100

200

300

400

500

600

448

235

118

445

242

126

Actual STPredicted ST

Temperature oC

Setti

ng ti

me (

Minu

tes)

Figure 6.5-Predicted vs. Actual setting time of OC20MK system incorporating 0.6% SP dosage



20 40 60 800

100

200

300

400

500

600634.5

331

155

647

330

148

Actual STPredicted ST

Temperature oC

Setti

ng ti

me (

Minu

tes)

Figure 6.6-Predicted vs Actual setting time of OC10PFA system incorporating 0.4% SP dosage

20 40 60 800

5

10

15

20

25

11.5

16.5

23.5

13

17.6

22Actual CS

Predicted CS

Temperature oC

Com

pres

ive st

reng

th (M

pa)

Figure 6.7- Predicted vs Actual compressive strength at 24 hours of OC20MK system incorporating 0.6% SP dosage



20 40 60 800

5

10

15

20

25

5.8

9.7

17.5

6.5

9.9

17

Actual CS

Predicted CS

Temperature oC

Com

pres

sive s

teng

th (M

pa)

Figure 6.8- Predicted vs actual compressive strength at 24 hours of OC10PFA system incorporating 0.4% SP dosage

To further determine the prediction accuracy of the models to simplify the design of systems

for oil well cementing, the developed models where used to predict the setting time (ST) and

compressive strength (CS) of systems developed in other studies. However, class G systems

incorporating admixtures used in the current study could not be found in the open literature as

this study is one of the recent attempt of using such admixtures to modify class G based OC

system. Thus, two ordinary Portland cement (OPC) based system, each incorporating

different source of MK developed by Mitrovic and Nikolic (2014) and a neat class G OC

system developed by Broni et al (2015) were adopted to further determine the prediction

accuracy of the models.

Figure 6.9 show the developed model prediction of the setting time of the various OPC-MK

systems (Actual ST1 and Actual ST2) developed by Mitrovic and Nikolic (2014) under 23oC,

while Figures 6.10 show the model prediction of the compressive strength of a neat class G

OC system developed by Broni et al (2015) at different temperatures. Studies of OPC based

system bearing MK at higher temperature were not found in the open literature as OPC based

systems are mainly used for above ground cement works in atmospheric temperature.



Cement Cement-MK10 Cement-MK200

120

240

360

480

600

720

Actual ST1Actual ST2Predicted ST

System content

Setti

ng ti

me(

minu

tes)

Figure 6.9- Prediction of the setting time of OPC based systems incorporating MK of different properties developed by Broni et al (2015) using model in the current study

31°C 65°C0

5

10

15

20

25

30

Actual CS1Predicted CS

Temperature C

Com

pres

sive s

treng

th (M

Pa)

Figure 6.10- Prediction of the 24 hours compressive strength of the class G OC system developed by Broni et al (2015) using models in the current study



From the predicted values versus actual values of the systems in Figures 6.9 and 6.10, the

predicted values where observed to deviate from the actual values, but with similar trend to

the ST and CS of the systems developed by Mitrovic and Nikolic (2014) and Broni (2012).

The shortfall of the developed models to accurately predict the ST and CS of the systems

developed by the other researchers can be attributed to the difference in properties of the

materials from those used in the study and for the development of the models. In addition, the

w/c ratios, presence of other materials in the systems, test approach and curing method used

in those studies could have impacted the outcome differently from the results obtained in the

current study and the development of the models. As such, this can be regarded as the limit of

the developed models in predicting the ST and CS of OC systems, not only if the systems are

developed with same materials, dosages and exposed to similar conditions investigated. The

limitation of the developed models in the current study can also be supported by conclusion

made by Shariar (2011) in a study of models for OC systems performance. Shariar (2011)

concluded from findings of the study that models should only be valid for their specific tested

parameters and materials used, as difference in test approach, properties of materials, other

type of materials and their combinations, can exhibit different characteristics. Shariar (2011)

further cautioned that materials or admixtures, even when sourced from the same category,

but from different source, could behave differently and thus require separate investigation for

a system and model development. Thus, the prediction capabilities of the developed models

in the current study are only valid for systems consisting of the type and properties of

materials and range of parameters investigated in the current study. However with further

experiments, the developed models can be extended beyond their limits of material types and

proportions, and the range of test parameters used.

Furthermore, a response surface of the setting time and compressive strength of the various

systems as a function of the temperature (T) and SP admixture dosages were generated and

are presented in Figures 6.11 -6.18 displaying the setting time and compressive strength

values as a result of the effect of the input parameters of the model (T, SP and MA)

investigated in the study.



0

170

340

510

680 S

T (

min

utes

)

23.00

32.25

41.50

50.75

60.00 0.00 0.25

0.50 0.75

1.00

A: Temp

B: SP

Figure 6.11 –Response surface of the setting time of the OC system incorporating 10% MK

0

170

340

510

680

ST

(m

inut

es)

23.00

32.25

41.50

50.75

60.00 0.00 0.25

0.50 0.75

1.00

A: Temp

B: SP

Figure 6.12– Response surface of the setting time of the OC system incorporating 20% MK



0

9.5

19

28.5

38

CS

(M

Pa)

23.00

32.25

41.50

50.75

60.00

0.00

0.25

0.50

0.75

1.00

A: Temp B: SP

Figure 6.13 – Response surface of the 24 hours compressive strength of the OC system incorporating 10% MK

0

9.5

19

28.5

38

CS

(M

Pa)

23.00

32.25

41.50

50.75

60.00

0.00

0.25

0.50

0.75

1.00

A: Temp B: SP

Figure 6.14 – Response surface of the 24 hours compressive strength of the OC system incorporating 20% MK



0

275

550

825

1100 S

T (

min

utes

)

23.00

32.25

41.50

50.75

60.00 0.00 0.25

0.50 0.75

1.00 A: Temp

B: SP

Figure 6.15-Response surface of the setting time of the OC system incorporating 10% PFA

0

275

550

825

1100

ST

(m

inut

es)

23.00

32.25

41.50

50.75

60.00 0.00 0.25

0.50 0.75

1.00 A: Temp

B: SP

Figure 6.16 – Response surface of the setting time of the OC system incorporating 20% PFA



0

8

16

24

32

CS

(M

Pa)

23.00

32.25

41.50

50.75

60.00

0.00

0.25

0.50

0.75

1.00

A: Temp B: SP

Figure 6.17-Response surface of the 24 hours compressive strength of the OC system incorporating 10% PFA

0

8

16

24

32

CS

(M

Pa)

23.00

32.25

41.50

50.75

60.00

0.00

0.25

0.50

0.75

1.00

A: Temp B: SP

Figure 6.18 – Response surface of the 24 hours compressive strength of the OC system incorporating 20% PFA



From the response surfaces, the trend of the effect of the input parameters on the output

variables can be seen to replicate the actual effects of the input parameters on the setting time

and compressive strength of the OC systems as observed in the tests. For instance, from

Figures 6.11 -6.14, the effects of temperature and SP admixture dosages on the setting time

and 24 hours compressive strength performance of the OC10MK and OC20MK systems are

properly illustrated. The setting times of the OC-MK system were found to decrease with an

increase in temperature whilst the increase of SP admixture dosages increased the setting

time. In the case of the compressive strength at 24 hours, the effect of the elevated

temperature and the SP admixture dosages which resulted in an increase of compressive

strength performance for the OC-MK systems is represented in the response surface. Also,

the higher strength observed for the OC10MK system than the OC20MK system at 60oC was

accounted for in the response surfaces (Figure 6.13 and 6.14).

For the OC-PFA systems, Figures 6.15 -6.18 illustrate the effect of temperature and SP

admixture dosages on the setting time and 24 hours compressive strength performance of the

OC system modified with 10% and 20% PFA. As illustrated in Figures 6.15 and 6.16, the

setting time of the OC-PFA system is found to decrease with an increase in temperature. On

the other hand, the effect of the SP admixture is shown to increase the setting time of the OC-

PFA with an increase of SP dosage as obtained in the tests. The effect of the SP admixture in

increasing the setting time of the OC-PFA systems was found to be more obvious with the

OC20PFA system from the tests. This was because the presence of PFA in the system also

increased the setting time of the system. Similarly, the response surface showed higher

setting time values for OC20PFA system compared to the OC10PFA system if outputs are

examined for the same temperature in the surfaces. The effect of temperature and SP

admixture dosages on the 24 hours compressive strength of the OC-PFA system is also

properly illustrated in the response surface presented in Figure 6.17 and 6.18. The increase in

temperature was found to increase the compressive strength performance of the OC-PFA

system at 24 hours, while the dosages of the SP dosages offered lower strength to the OC-

PFA system at 23oC.

The influence of the combined effect of temperature and SP admixture in the model is also

properly illustrated in the response surface. For instance, at a temperature of 60oC with no SP



admixture in the systems, the setting times of the OC system modified with MK or PFA

dosages were found to have the lowest outcome values. But, with presence of SP admixture

dosage up to 1% for the various OC systems at 23oC, the setting times of the various systems

were observed to be the highest illustrating the combined effect of the input parameters (MA

and SP) to the output variable as observed with the tests (Figures 6.11, 6.12, 6.15, 6.16).

Thus, the response surface properly illustrates the effect of the input parameter(s) on the

output variables showing the potential prediction with the model if the parameters within the

range tested are fed into the model equations.

Furthermore, the contours in the response surface show the model can generate a specific

outcome value at different temperatures with respect to the dosage of SP admixture applied

for the modification of the OC systems incorporating MK or PFA materials. Figures 6.19 -

6.26 present the contours illustrating specific setting time values and specific 24 hours

compressive strength values that can be obtained with the OC systems incorporating MK or

PFA material at different temperatures with the addition of different SP admixture dosages in

the system.

ST (minutes)

A: Temp

B: S

P

23.00 32.25 41.50 50.75 60.00

0.00

0.25

0.50

0.75

1.00

166.593

267.121367.648

468.176

568.703

Figure 6.19 –Same setting time of OC10MK system occurring at different temperatures with different dosages of SP admixture



ST (minutes)

A: Temp

B: S

P

23.00 32.25 41.50 50.75 60.00

0.00

0.25

0.50

0.75

1.00

166.593267.121

367.648

468.176

Figure 6.20 – Same setting time of OC20MK system occurring at different temperatures with different dosages of SP admixture

CS (MPa)

A: Temp

B: S

P

23.00 32.25 41.50 50.75 60.00

0.00

0.25

0.50

0.75

1.00

13.4307

17.4429

21.4552

25.4674

29.4796

Figure 6.21 – Same compressive strength of OC10MK system at 24 hours occurring at different temperatures with different dosages of SP admixture



CS (MPa)

A: Temp

B: S

P

23.00 32.25 41.50 50.75 60.00

0.00

0.25

0.50

0.75

1.00

13.4307

17.4429

21.4552

25.4674

Figure 6.22– Same compressive strength of OC20MK system at 24 hours occurring at different temperatures with different dosages of SP admixture

ST (minutes)

A: Temp

B: S

P

23.00 32.25 41.50 50.75 60.00

0.00

0.25

0.50

0.75

1.00

230.38

374.518

518.656

662.794

806.932

Figure 6.23 – Same setting time of OC10PFA system occurring at different temperatures with different dosages of SP admixture



ST (minutes)

A: Temp

B: S

P

23.00 32.25 41.50 50.75 60.00

0.00

0.25

0.50

0.75

1.00

230.38

374.518518.656

662.794

806.932

Figure 6.24 – Same setting time of OC20PFA system occurring at different temperatures with different dosages of SP admixture

CS (MPa)

A: Temp

B: S

P

23.00 32.25 41.50 50.75 60.00

0.00

0.25

0.50

0.75

1.00

9.6539113.9238

18.1937

22.4637

26.7336

Figure 6.25 – Same compressive strength of OC10PFA system occurring at different temperatures with different dosages of SP admixture



CS (MPa)

A: Temp

B: S

P

23.00 32.25 41.50 50.75 60.00

0.00

0.25

0.50

0.75

1.00

9.65391

13.9238

18.1937

22.4637

Figure 6.26 – Same compressive strength of OC20PFA system occurring at different temperatures with different dosages of SP admixture

From Figures 6.19 to 6.26 presenting the contours, it can be seen that a specific setting time

can be achieved at a different temperature depending on the dosage of the SP admixture used

in the system. For instance, Figure 6.19 shows that the setting time of 166.5 minutes for the

OC10MK system at 42oC can also be obtained at 47oC and 54oC by incorporating the SP

admixture dosages of 0.25% and 0.50% respectively in the system. Figures 6.20 shows that a

setting time of 367.6 minutes can be achieved with the SP admixture dosages of 0.50%,

0.75% and 1% for OC20MK system at 27.5oC, 32.25oC and 37.5oC accordingly.


Chapter 7-Discussion

CHAPTER 7: DISCUSSIONThis chapter presents the explanation to the different performances obtained for the OC

systems with different w/c ratio, type and dosage of MA (MK and PFA), super plasticizer

chemical admixture (SP) and temperature degree. More so, the practical implication

emanating from the research in respect to applying the various admixtures or materials used

in the current research work for oil well cementing is provided in this section.

7.1 Variations in density of the various OC system

In the current study, there were variations in the density of the various formulated OC

systems; the class G OC system of different water to cement ratio and the OC system

modified with different dosages of specific type of MA (MK or PFA). The various OC

systems were observed to vary in density with the ratio of mix water to cement and the

dosages of the MK or PFA to cement at the fixed w/c ratio of 0.44.

For the class G OC system, the density variations can be attributed to the specific gravity of

the class G cement material and the density of water for the OC system. The density of water

is lower than that of cement material. As such, the variation in density of the various class G

OC systems is a result of the densities of the materials. Hence, increasing the ratio of water to

cement for the OC system directly indicates the replacement of the higher specific gravity of

cement material with the density of water resulting in a lower density for the class G OC

systems. It was also observed that the decrease in density of the OC system with increase in

water ratio was not linear. This indicates that a sedimentation occurrence as noted by Yang

(2015) altered the expected linear trend.

In the case of OC system modified with MK or PFA material, there were variations in the

density of the OC system with the dosages of the MK or PFA material to that of the neat class

G cement material at the fixed w/c ratio (w/c 0.44). The density of the OC system was lower

with the increase of MK and PFA dosages replacing the class G cement for the system. The

variation in the density of the OC system suggests that the specific gravity of particular

mineral admixture influenced the density of the OC-MA systems. The specific gravity of the

MK and PFA are lower than that of the class G cement. Hence, the presence of either MA in



the system partially replaced the specific gravity of the class G cement material resulting in

lower density for the OC system at the fixed w/c ratio. In this case, an increase in dosage of

either MK or PFA material in the system with reduction of the content of the cement material

further decreases the density of the system while the mix water ratio is fixed.

7.2 Variations in rheological properties of the various OC system

The rheological properties of cementitious systems are noted to be influenced by several

factors which include the properties of the cement particles, the hydrate phases of the

cementitous system, the water to cement ratio adopted for the design of the cementitious

system, the surrounding condition, etc (Vidick et al, 1987). The results of the rheological

properties of the various OC systems investigated were observed to vary with water to

cement ratio and the dosage of the MA type used in modifying the OC system at fixed water

to cement ratio. Thus, the variation of the rheological properties of the various OC systems is

a result of the water to cement ratio for the class G OC systems and the physical and chemical

properties of the cement and MAs used for the formulation of the OC-MA systems at fixed

w/c ratio.

The Class G OC systems with different water to cement ratios were found to decrease in yield

stress and plastic viscosity with an increase of mix water content for the OC system

formulation. According to Reddy et al (2014) and Suryakanta (2014), the nature of

lubrication is the most vital factor that affects the flow behaviour of cementitious system and

water is found to have a lubricating effect that reduces inter-surface friction among particles

of a cementitious system. Thus, the increase of water ratio in designing the Class G OC

system increased the lubrication of the system and enhanced flow. More so, the water to

cement ratio is found to have an influence on the rate of hydration of a cementitious system

to transform into a solid substance capable of resisting flow. According Junior et al (2012)

the increase in mix water for cementitious system decreases the system temperature which

decelerates the thermal activation of the hydration reaction. From the setting time of the class

G OC system which is related to the rate of hydration of the systems to solidify, the present

study found that an increase in the ratio of mix water to cement increases the setting time of

the class G OC system. This outcome (setting time) indicates that the lower yield stress and

plastic viscosity values observed for the class G OC system designed with higher water to



cement ratio is the additional influence of the increase in ratio of mix water to cement

affecting the rate of hydration of the system to harden at longer time, thereby, allowing

increased mobility of the system due to higher fluidity.

In the case of the OC system modified with MA, different outcomes were observed with the

type and dosage of MA for the system. According to Barnes (1989), the rheology of

cementitious system can be influenced by the shape of the cementitious particles used for

developing the system. The physical (particle) properties of the MAs used for modifying the

performance of the OC system are found to differ from that of the class G cement as recorded

by Christodoulou (2000) (Table 3.2). Also, the MK and PFA material are recorded to possess

different particle characteristics to each other (Table 3.2). The MK particles are recorded to

have a specific surface area of 12000 m2/kg higher than that of the class G cement of 385

m2/kg while the specific surface area of the PFA particles are recorded to vary between 300

to 600 m2/kg compared to that of the class G cement material. Additionally, it is recorded by

Shariar (2011) and Bapat (2012) that cement and MK particles possess irregular shapes while

that of the PFA particles are spherical which offers the minimum inter-surface contact area

among the various types of shapes. According to Shariar (2011), cementitious systems with

spherical particles tend to have lower resistance to motion due to ability of the spherical

particles to roll. This explains the reason for the decrease in yield stress and plastic viscosity

recorded for the OC-PFA systems. With an increase in the PFA dosage, the spherical shaped

PFA particles presenting a lower particle to particle surface contact area decreased the

friction between the cementitious particles and improved the mobility of the cement particles

in the system as spheres can roll. In the case of the OC-MK systems, the results of the

rheological properties (Yield stress and plastic viscosity) of the system were observed to be

higher with an increase of MK dosage compared to the OC0.44 system. The outcome imply

that the high specific surface area of the MK particles which are irregular increased the

particle to particle inter-surface contacts thereby resisting motion to flow due to friction

between the MK and cement particles that make the system.

Also, like the effect of the physical properties of cement particles on rheology of

cementitious system, the chemical properties of the cementitious materials can influence the

rheology of systems. As noted by Vidick et al (1987), the hydrate phases of cementitious



systems, essentially the C3A products, influence the rheological properties of cementitious

systems. The bonding of Ca(OH)2 and Al2O3 to form C3A in a cementitious system librates

heat which can cause loss of moisture in cementitious system thereby affecting the fluidity of

the system to flow (Cemex, 2015). The MK and PFA material are found to have high

contents of Al2O3 as recorded by Christodolou (2000) which bonds with Ca(OH)2 produced

from cement in water to form C3A in pozzolanic reaction. However, the different particle

properties and other chemical compositions of the MK and PFA material influenced the

formation of the C3A differently in the OC system. The high specific surface area of the MK

particles increases the particle density (packing) of the system, enabling high inter-surface

contacts amongst the chemical bearing particles of the cement and MK material; thereby,

resulting in high bonding of chemicals which include Ca(OH)2 and Al2O3 for C3A. Thus, the

increase of C3A in the system, made possible by the presence of the MK chemical and

particles properties in the system caused the liberation of high heat and loss of moisture

resulting in rigidity of the system. This can explain the higher yield stress and plastic

viscosity values observed with an increase of MK dosages in the system indicating greater

resistance to flow at the fixed w/c ratio of 0.44. It was also observed that the dosage of 30%,

40% and 50% MK material for the OC system resulted in very stiff systems that could not be

measured by the test equipment.

In the case of the OC-PFA systems, the low specific surface area presented by the PFA

particles due to their spherical shape decreased the inter-surface contact between the chemical

bearing particles of the systems, thereby, decreasing the rate of chemical bonding at a time to

form C3A compared to class G OC system or the OC-MK systems. In addition, the presence

of un-burnt carbon which can be found in PFA material is noted to have a decreasing effect

on the alkalinity (pH level) of cementitious system thereby affecting the formation of C3A

(Thomas, 2007) and sulphate also found in PFA has a contributory effect in retarding the

formation of C3A. With the presence of sulphate, the formation of C3A is altered to form

ettringite containing calcium and sulphate ions which creates a diffusion barrier around the

C3A (Portland Cement Association, 2001). This proceeds to control the formation of C3A

allowing the system to have a dormant period in its hydration (Luke, 2002, Jiabiao and

Hongfang 2010). Thus, the presence of PFA material in the OC system initiated the following

effects with its particle shape and surface area and its sulphate composition which retard the



formation of C3A in cementitious system, thereby, allowing greater mobility of the OC

system and decreasing the yield stress and plastic viscosity values of the system with an

increase in PFA dosage.

7.3 Variations in setting times of the various OC system

From the laboratory test results of the setting time of the various OC systems, it was shown

that the setting time of the various systems varied with water to cement ratio and specific type

and dosage of MA used for modifying the OC system. The variations of the setting time of

the various OC systems indicate that the chemical and physical properties of the cementitious

solids and the water to cement ratio for the OC system design influenced the setting of the

systems. From several studies (Nonat and Mutin, 1992; Brook et al, 2000; Struble 2011), the

setting of cement based system is postulated to result from two fundamental steps: firstly, the

coagulation establishing the contacts between the cementitious particles and secondly, the

formation of various hydrate products in the contact zone making the coagulation structure

rigid. According to Hu et al (2014), among the various hydrate products, the tri-calcium

aluminates (C3A) and tri-calcium silicate (C3S) hydrate products whose hydrations are rapid

mainly influence the setting of the cementitious system including the water to cement ratio

which influence the space or contact between the particles of the cement material and alter

the chemical bonding of the system. More so, the particle properties of a cement material

contribute to the rate of chemical bonding to form hydrate products in the system (Bapat,

2012, Shariar 2011).

Different setting times were obtained for the Class G OC systems with different w/c ratio.

The setting time of the class G OC systems was found to increase with an increase in the mix

water ratio for the system. The difference in the setting time of the various class G OC

systems can be explained as a result of decreased particle density of the system initiated by

higher mix water to cement ratio. The decreased density means that a higher amount of

coagulation (hydrate product) is required for the OC system designed with higher water to

cement ratio to establish a bond between the cementitious particles and become solid. On the

other hand, the OC systems designed with lower water to cement ratio had a higher particle

density as the particles are closely packed. The high particle density enables greater particle

to particle inter-surface contacts thereby improving the bonding of the vital chemicals (CaO,



Al2O3 and SiO3) in the presence of water to produce the essential hydrate products in the

system. This made the OC system solidify in less time compared to the system designed with

higher mix water ratios. It is also reported by Hu et al(2014) that a lower water to cement

ratio leads to higher heat generation and the high heat contributes in accelerating the rate of

hydration which makes the system set faster compared to system designed with higher water

to cement ratio. This explains the reason behind the short setting times observed for the

OC0.36 system and the long setting times observed for the OC 0.52 system compared to the

OC0.44 system and the other class G OC systems.

For the setting times of the class G OC system modified with a MA at a fixed water to

cement ratio, the setting times of the systems varied with an increase in MK and PFA dosage.

The increase in MK dosages for the OC system resulted in shorter setting times whilst the

increase in PFA dosages resulted in longer setting times of the OC system. The different and

varying setting time of the OC-MAs imply that their chemical and physical properties at the

water to cement ratio (w/c 0.44) influenced their setting times. The setting of a cementitious

system is mainly influenced by the formation of C3A and C3S hydrates and the shape and size

of the cementitious particles (Brook et al, 2000; Struble 2011; Bapat, 2012). In cement and

MA blends, a pozzolanic reaction takes place to form C3A and C3S made possible from the

high amount of CaO content in the cement and the SiO3 and Al2O3 from the MA in the

presence of water. The C3A and C3S affects the early hardening of the cementitious system,

however, the C3A librates high heat which can lead to rapid (undersired) stiffening of the

system dependning on the concentration or if not controlled (Klien and Dana 1985, Ropp,

2013, Cemex 2015). The MK and PFA material were recorded to have high contents of SiO3

and Al2O3 and different particle properties as recorded by Christodoulou (2000) compared to

the properties of the class G cement material (Tables 3.1).

Thus, incorporating the dosages of the MK material enhanced the particle density of the OC

system and increased the inter-surface contact between the chemical bearing particles of the

cementitious materials. In this regards, chemical reactions needed to form essential hydrates

increased with the presence of the MK material leading to transformation of the OC system

from fluid state to solid state in a shorter time. A range of products is formed in the reaction

of the cement and MK system which includes calcium silicate hydrates (C-S-H), gehlenite



hydrates (Ca2Al[AlSiO7]) and other calcium aluminate hydrate (C-A-H) ( De silva and

Glasser, 1993). The composition of the C-S-H includes the C3S and C2S products but the C3S

mainly contributes to the transformation of the system due to its rapid formation in the

hydration process (Michaux et al, 1989). Gehlenite hydrate is a well defined crystalline phase

and can be of variable composition. It can incorporate relatively large amounts of sodium

(NA+) and potassium (K+) which displace Ca2+ in the lattice, and this can affect the pore

solution of the system (Jones, 2002). The C-A-H hydrates, essentially the C3A compounds,

react rapidly, liberating high heat during formation which shortens the solidification period of

the cementitious system. According to Michaux et al (1989), the C3A is the reason gypsum is

added to Portland cement to control its rapid reactivity and prevent undesired rapid

solidifications. However, the high content of Al2O3 in the MK would have increased the

presence of C3A to gypsum proportion thereby enabling the OC-MK systems to set in shorter

times than the OC0.44 system. Thus, these effects of the C3S and C3A in the system which is

also enhanced by the high inter-surface contacts of the cementitious particles made possible

by the physical properties of the MK material, explains the shorter setting times observed for

the OC system with the increasing dosage of the MK material for its modification such as the

OC50MK system.

For the case of the PFA material used in modifying the OC system, the setting outcome of the

OC-PFA can be ascribed to the spherical shape and specific surface area of the PFA particles.

In this regards, the increase in PFA dosage reduced the inter-surface contact area of the

chemical bearing particle of the OC system. This in turn decreased the amount of chemical

bonding taking place to form a sufficient quantity of essential hydrate products to transform

the OC-PFA systems to a stiffened state compared to the OC0.44 system. Also, the setting of

a cementitious system is noted by Thomas (2007) to be retarded by un-burnt carbon that is

present in the PFA materials. According to Thomas (2007), the presence of un-burnt carbon

from the PFA material has a decreasing effect on the alkalinity (pH level) of cementitous

systems. The content of un-burnt carbon can be determined from the LOI of a material and

was recorded to be higher in the PFA material compared to that of cement (Table 3.1).

Hence, the increase of PFA material means an increase in the amount of un-burnt carbon

present in the OC system, thereby, decreasing the rate of hydration due to decrease in pH

level of the system by the carbon presented by the PFA material.



Furthermore, PFA retards the hydration of C3A due to the additional sulphate from the PFA

in the system which bonds to form a protective layer around the C3A consisting of calcium

and sulphate ions (Luke, 2002). This occurrence proceeds to control the formation of C3A

resulting in an extended dormant period in its hydration which elongates the transformation

of the system from fluid to solid (Jiabiao and Hongfang 2010). It is also reported by Barnes

and Bensted, (2002) that the occurrence of soluble calcium and sulphate ions as surface

deposits on the PFA particles delays the nucleation and re-crystallization of C-S-H which

also plays a role in the setting of systems. The effect of retardation can also be higher

depending on the calcium and sulphate content and dosage of PFA in a cementitious system

(Luke, 2002). Thus, considering the sulphate content, the LOI which indirectly provides the

content of carbon in the PFA material and the low calcium content of the PFA material

adopted for the study, the increase of the PFA dosage initiated the following effect leading to

increased setting times. In this regards, increase in dosage of PFA altered the reactivity of the

system and resulted in much longer time of setting for the OC system.

7.4 Variations in strength performance of the various OC system

Like variations in the density, rheology and setting times of the various OC systems, there

were variations in the strength (compressive and tensile strength) performance of the class G

OC system of different water to cement ratio and the class OC system modified with different

type of MA (MK and PFA) and their dosages with curing days. The strength performance of

the various OC systems agrees with the report of Buntoro and Rubiandini (2000) that the

strength of a hardened cementitious system depends on the properties of the cement material,

water to cement ratio, admixture dosage and curing duration while Hussain et al (1994),

Hafez et al (2008) and Hu et al (2014) added that the strength of a cementitious system is

largely dependent on the formation of the hydrate products in the system with curing days.

The strength performance of the class G OC system designed with different w/c ratio was

found to differ from each other. The strength of the OC systems designed with lower mix

water ratios were found to be higher than the OC systems prepared with higher mix water

ratios. The reasons for the variation in strength of the system can be attributed to the ratio of

mix water to cement which has an influence on the formation of hydrate products and the

volume of pores in the hardened OC systems to be occupied by the hydrate products. This



agrees with findings and reports made by Dahab and Omar (1989), Bell (1996), Hussain et al

(1994), Hu et al (2014) and Kim et al (2014) on factors influencing strength performance of a

cement system. It is reported by Bell (1996) that the quantity of water available for chemical

reactions to form hydrate products in cementitious systems influences the strength

performance of the cementitious system. Bell (1996), postulated that a lower ratio of mix

water for cementitious system means that the water is rapidly used in the hydration process to

achieve strength over a shorter period than systems with higher mix water ratio. More so,

Bell (1996) reported that if the ratio of mix water is well in excess of the consistency limit,

then a low strength is produced even though the system may continue to increase in strength

with curing days. According to Kim et al (2014) the ratio of mix water for cementitious

system influences the porosity of the system which has an effect on the strength of a system.

The effect of mix water on porosity occurs by alterations to the particle density of a

cementitious system by the ratio of the mix water above or below adequate requirement for

the system design. Bentz et al, (2001) added that higher pore volume as result of higher mix

water to cement content can adversely affect the strength performance of cementitious

system.

From the current study, the findings of the hydrate product formation for the class G OC

system of different water to cement ratio revealed that the OC system designed with the

lower mix water to cement ratio had a higher percentage of calcium aluminate silicate hydrate

(C-A-S-H) product at the initial day, while with an increase in curing days the systems

designed with the higher mix water to cement ratios had higher C-A-S-H products. This

implies that the class G OC systems prepared with lower mix water ratios had faster

formation of essential hydrate products for strength compared to the class G OC systems

prepared with higher mix water ratios which agrees with the report of Bell (1996). Also, from

the results of the water absorption study which can be related to pore volume of cementitious

systems, the OC systems formulated with a higher mix water to cement ratio were found to

absorb a higher percentage of water at the various curing days. In this regards, the higher

amount of hydrate products formed in the class G OC system designed with a higher water to

cement ratio at later days was not sufficient to fill the high pore volumes of the systems or

densify the systems to produce strengths as the OC systems designed with a lower mix water

ratio and lesser pore volume.



In the case of the OC system modified with a different type and dosages of MA, the reason

for the strength variation of the systems at the adopted w/c ratio for their design suggest that

the rate of hydration due to the properties of the MK and PFA material influenced the

outcomes. As reported by Hafez et al (2008), in a cement and MA system, a pozzolanic

reaction takes place to form a C-A-S-H product essential for the strength of systems.

However, the formation of the C-A-S-H product can be rapid or slow based on the chemical

and physical properties of the cementitious materials (CAC, 2008; Hafez et al, 2008).

According to PCA (1997), while C-A-H plays a role in the setting phase of a system, the C-S-

H products mainly influence the strength of the system. The C-A-H products, particularly the

C3A may contribute to strength but have a rapid reaction at the beginning of hydration with

an exothermic reaction which can cause loss of moisture and the undesired solidification of a

system if not controlled (PCA, 1997). Thus, modifying the OC system with the MK material

enhanced the inter-surface contact area of the chemical bearing particles in the system due to

the high specific surface area of the MK particles. This resulted in bonding of chemicals in

the OC-MK system to form the essential hydrate product at a higher rate for strength

compared to the OC0.44 system with lesser specific surface area particles of the class G

cement. In the case of the OC-PFA system, the formation of the hydrate product for strength

differed due to the specific surface area of the PFA particle compared to that of the class G

cement. The presence of PFA material reduced the inter-surface contact area amongst the

particles of the system, resulting in a lower rate of chemical bonding to form hydrate product

for strength.

The study of the hydrate products shows the content of the C-A-S-H products to be higher in

the OC-MK system with an increase in MK dosages at the various days compared to the

OC0.44 system. On the other hand, C-A-S-H products of the OC-PFA systems are found to

be lower with an increase of PFA dosages at the various days. The exothermic reaction as a

result of the high Al2O3 and SiO3 in the MK bonding with Ca(OH)2 with high particle inter-

surface area contacts presented by the MK particles in the system enabled faster and higher

formations of C-A-S-H in the OC-MK systems compared to the OC0.44 system and the OC-

PFA systems. However, a high formation of the C3A product librates heat which can affect

the performance of a cementitious system requiring adequate gypsum to control its effect for

proper hydration of the cementitious system (Srinivan, 2014). The formation of C3A with



gypsum for its control can contribute to the strength of a cementitious system. This explains

the improved strengths obtained for the system modified with dosages of 10%, 20% and 30%

MK compared to the OC0.44 system. But, increasing the dosage of the MK with its high

Al2O3 content and high specific surface particles in the system would have resulted in a high

presence of C3A products. This created an imbalance of the available gypsum to control the

effect of C3A, as such, affecting the hydration of the OC40MK and OC50MK system to gain

strengths comparable to the OC0.44 system and other OC-MK systems. The OC40MK and

OC50MK systems were observed to be in granular state and with large pores that adversely

affected the strength performance of the systems due to the loss of moisture caused by the

formation of C3A. The granular state and high pore occurrence can be ascribed to the adopted

w/c ratio (0.44) not being sufficient for adequate plasticity of both systems. As such, the

formation of C3S and C2S products to fill the large pores and render improved strengths than

the OC0.44 system with curing days was insufficient. In the case of the OC system modified

with 10%, 20% and 30% MK material, the availability of moisture combined with the high

particle inter-surface contact area initiated by the MK dosages increased the rate of C3S and

C2S formation and enabled adequate densification that improved the strength of the systems

above that of the OC0.44 system with curing age.

For the strength variation of the OC-PFA systems, the variation would have resulted from the

specific surface area of the PFA particles in the system. Although the PFA possessed high

contents of Al2O3 and SiO3 similar to that of the MK material, the specific surface area of the

PFA particles reduced the inter-surface contact area amongst the chemical bearing particles in

the system. This decreased the rate of chemical bonding to form essential hydrate product to

fill the pores of the various OC-PFA system compared to the rate of hydration of the OC0.44

system for higher strength. More so, the influence of PFA in a cementitious system is

attributed to the proportion of calcium content among other composition such as carbon.

Hydration to form the C-S-H products for strength is retarded by un-burnt carbon that is

present in the PFA material. According to Thomas (2007), un-burnt carbon decreases the

alkalinity of cementitious system. As such, the increase of PFA material increases the

presence of un-burnt carbon in the OC system which affects the pH level and initiates a lower

rate of reaction for strength gain of the OC system. The LOI of the PFA material indicates the

content of un-burnt carbon in the PFA material adopted for the study. These factors including



the particle properties and carbon content in PFA explain the low and gradual strength

development obtained for the OC-PFA system with increase of PFA presence in the system

for the duration examined.

7.5 Variations in water absorption of the various OC system

Water absorption in a cementitious system is characterised by the volume of pore spaces in

the hardened cementitious system (Hearn et al, 1994). The pore network and the chemical

activities occurring in the pore spaces of the cementitious system are also essential

parameters that influence the percentage of the water absorbed by a hardened cementitious

system. Additionally, water absorption in hardened cementitious system is influenced by time

(Hooton et al, 1993). The curing days or time contributes positively to the water absorption in

cement based systems. With an increase in curing days, there is a corresponding increase in

the amount of chemical activity in the cementitious system; thereby more hydrate products

are formed. The hydrate products that are formed crystallise and occupy the pore spaces

which resist the ingress of water. When cementitious products of C-A-H and C-S-H

crystallise, the bond between particles is increased and the internal pores are subsequently

closed. Also, when there is more interaction between the particles, the pore spaces are

reduced and the movement of fluid through the system is highly decreased or prevented.

The water absorption of the various OC systems in this study involved the class G OC

systems designed with different water to cement ratio and the OC systems modified with

various dosages of MK and PFA materials at fixed w/c ratio. A similar curing regime was

adopted for the investigation of the percentage of water intake by the various OC systems for

the curing period. From the results of the water absorption investigation, different trends were

exhibited by the class G OC with different water to cement ratio and the OC systems

modified with various MK and PFA dosages.

For the Class G OC systems with different w/c ratio, the outcomes indicate that the particle

density of the cementitious particles and the rate of chemical activity occurring in the OC

system with the w/c ratio adopted for the design of each system influenced the percentage of

water absorbed. From the hydration product results, the hydrate product content of the



various class G OC systems showed that systems designed with a lower mix water ratio had a

higher content of C-A-S-H products compared to systems designed with a higher mix water

at the initial day. The content of C-A-S-H products at the initial day indicates that the low

ratio of mix water to cement enabled improved packing of the cementitious particles and

benefitted the reactivity of the chemicals to form a high content of C-A-S-H products. Thus,

the improved packing of the system reduced the volume of pores compared to systems with

higher water content and also the high content of C-A-S-H products contributed in reducing

the volume of pores spaces in the class G OC system designed with a low mix water ratio by

occupying the pores.

In the case of OC systems designed with a high mix water ratio, the outcome indicates that

the high mix water adversely affected the density of the OC system resulting in a high

volume of pores in the system. Castro et al (2011) noted that the increase in ratio of mix

water to cement increases the porosity of cementitious systems. However, with curing days,

the percentage of water absorbed by the various systems was found to decrease which can be

ascribed to the formation of hydrate product and density of the systems. Although, the OC

system designed with a higher mix water to cement ratio was found to have a higher

percentage of hydrate products (C-A-S-H and Ca(OH)2) with curing days, the quantity of

hydrate products formed was insufficient to reduce the high pore volume of the system

compared to the small volume of pores initiated by the enhanced packing of cementitious

particles in the OC system designed with low mix water. This occurrence explains the reason

for the water absorption variation recorded for the class G OC system of different water to

cement ratio.

Variation in the percentage of water absorbed was also observed for the OC systems modified

with MK and PFA dosages. The percentage of water absorbed with time by the OC-MK

systems and the OC-PFA systems indicate that the impact of the MAs particles on the particle

density of the systems and the chemical activity occurring in the pores of the systems

influenced the water absorption outcome of the OC-MA systems. Hooton (1986) noted that

the process by which admixtures reduce ingress of fluid in cement-based systems is through

the adjustment of pore spaces of the system. Ayub (2013) also identified that the formation of

hydrate products in the Cement-MA systems contributes to the reduction of the pore volume



of the system. The percentage of water absorbed by the modified OC system using MK or

PFA material was observed to decrease with curing days, however, the increase in dosage of

MK or PFA varied in the percentage of water absorbed compared to the OC0.44 system.

The OC systems modified with 10%, 20% and 30% MK dosages were found to absorb less

water than the OC0.44 system while the OC systems modified with 40% and 50% MK

dosages absorbed more water than the OC0.44 system. The variation is due to the rate of

hydration of the system with the content of Al2O3 and SiO3 in the MK which bonds with the

Ca(OH)2 occurring from cement and water to form C3A and C3S product. The formation of

C3A in high quantity can cause loss of moisture and adversely affect the formulation and

structure of the system. In this regards, the adopted w/c ratio for the OC-MK system was

insufficient to manage the reactivity of the high C3A occurring in the OC40MK and

OC50MK system. As such, the plasticity of both systems was affected resulting in system

with a high number of pores which allowed high ingress of fluid. The OC40MK and

OC50MK systems were observed to be in granular form after preparation with large visible

pores. In the case of the OC systems modified with 10%, 20% and 30% MK dosage, the low

water absorption observed for the system is related to the reduction of pore spaces in the

systems by the improved particle density of the system made possible by the high specific

surface of the MK particles and the high formation of hydrate product above that of the

OC0.44 system.

In terms of the OC-PFA system, the OC10PFA and OC20PFA systems were found to absorb

less water than the OC0.44 system while other OC-PFA systems had higher water absorption

than the OC0.44 system. This is due to the particle properties of the PFA material and the rate

of chemical activities to form sufficient hydrate products to occupy and decrease the volume

of pores to prevent high absorption of water by the systems. The outcome with the OC

system modified with 10% and 20% PFA material is because of the effect of the high

presence of cement in combination with the PFA dosages which enabled higher formation of

hydrate products in both systems and the adjustment of the particle density of the system by

some of the PFA particles with high specific surfaces in the 10% and 20% PFA dosages.



7.6 Variations in shrinkage of the various OC system

The shrinkage values of the various OC systems revealed varying decreases in length for the

systems with curing days. According to Backe et al (1999), a cementitious system shrinks

over time as it hydrates and the extent of shrinkage depends on the properties of the cement

material and the volume of pores in the cementitious system including the surrounding

conditions. Zhang et al (2010) added that the volume of hydrated cement based system is

smaller than its initial prepared volume because of the molecules of the mix water and

cement becoming arranged into a more compact configuration in the hydration process. Thus,

the rate of chemical reactions and the content of hydrate products in the pores of a

cementitious system influence the shrinkage of the cementitious system over time.

The shrinkage results of the various OC systems showed that the shrinkage values recorded

for the various OC systems increased with curing days. The shrinkage trends were also found

to be inconsistent with an increase of mix water ratio or dosage of MA (MK or PFA material)

in the system at the fixed w/c ratio. For the OC system with different water to cement ratios, a

change was found in the shrinkage trend at the 7th day with the systems designed with higher

mix water shrinking more than the systems formulated with a lower mix water. The

occurrence indicates the rate of hydration and pore volume in the systems influenced the

shrinkage. It is noted that a rapid initial increase in shrinkage takes place after the setting of

the cementitious system caused by continuous chemical reactions which include the reaction

of Al2O3 with Ca(OH)2 which is dominant in the early stages of hydration (Sant et al, 2007).

The OC systems formulated with lower mix water ratio were found to set earlier than the OC

systems formulated with higher mix water ratios. This implies, that the rapid initial increase

in shrinkage began occurring in the OC system formulated with a lower mix water ratio

resulting in a higher shrinkage than the OC system formulated with a higher mix water ratio

at the early days. However at the 7th day, the shrinkage trend changed which can be ascribed

to the pore volume of the systems with continuous hydration. It is established that

cementitious systems shrink into the available pore spaces in their hardened state (Mokarema,

2005). As such, the OC systems designed with higher mix water ratios and possessing higher

pore spaces as identified from the “water absorption results” shrank most with further curing

days as hydration progressed.



In the case of the OC system modified with MK and PFA material, the outcomes indicate that

the rate of hydration and the properties of the MAs influenced the shrinkage of the system

over time at the fixed w/c ratio. Mokarema (2005) associated the reduction of cement based

system shrinkage to the particle properties of the admixture which enhanced the density of

the system. Backe (1998) added that retarders can reduce the shrinkage of OC system through

decreasing the rate of hydration.

The OC systems incorporating 10%, 20% and 30% dosages of MK material were found to

have shrinkage values below that of the OC0.44 system at the various curing days. This

outcome indicates that the improved particle density of the system by the MK particles which

also enhanced the chemical activity in the system caused the low shrinkage observed with the

systems compared to the OC0.44 system. The improved particle density reduced the volume

of initial pores in the system while the increase in chemical activity implies high formation of

the hydrate products which occupy the pore spaces of the system over time. This reduced the

volume of existing pores in the systems with curing days; thereby decreasing the pore spaces

available for the system to shrink into compared to the pore volume of the OC0.44 system.

However, with an increase in dosage of MK material up to 40% and 50% for the system, the

presence of C3A increased in the system with the high content of Al2O3 in the MK bonding

with Ca(OH)2 from the cement and water and increasing the heat of hydration. The effect of

the high heat as a result of the exothermic reaction in the formation of C3A proceeded to loss

of moisture leading to poor plasticity for the systems modified with 40% and 50% MK

dosages. This resulted in large number of pores in both systems to shrink into which

continued as hydration proceeded over the curing period. The pore volume of the OC system

modified with various dosages of the MK material can be appreciated from the results of the

water absorption indicating the percentage of water intake as a consequence of pore volume

available in the systems.

For the OC-PFA systems, the OC systems modified with various dosages of PFA material

showed slight variation in shrinkage with curing days. This outcome indicates that the rate of

hydration due to the properties of the PFA material influenced the shrinkage of the OC-PFA

system. The particles of the PFA material used in the current study is recorded to vary

between 300 to 600m2/kg compared to that of the class G cement of 385m2/kg. Moreso, it is



recorded by Shariar (2011) that the shape of cement particles are irregular while that of PFA

are spherical which offers the minimum inter-surface contact area amongst the various types

of shapes. Thus, the presence of the PFA material in the OC system adjusted the pore size of

the OC system, while the PFA particles with a lower specific surface area than that of the

cement decreased the rate of chemical reactions due to reduction of particle surface contacts.

The decreased rate of chemical reactions implies that the OC-PFA systems hydrated slowly

to shrink into the available pores in the respective systems. The shrinkage results showed that

at the early age, the systems modified with the lower dosage of PFA material had shrinkage

values greater than the systems modified with higher dosage of PFA material. This is due to

the presence of higher class G cement content to low PFA dosage for such systems. However

with curing days, the solids proportion would have enabled the OC system modified with

lower PFA dosages to form hydrate products to fill the adjusted pore spaces and reduce the

shrinkage rate in later days than systems incorporating high PFA dosage. The shrinkage of

the various OC-PFA systems in later days showed that the OC system modified with 30%,

40% and 50% PFA material had higher shrinkage values than the OC systems modified with

10% and 20% PFA material. This occurrence indicates that the higher pore volumes of the

OC system modified with 30%, 40% and 50% PFA dosages and the improved rate of

chemical activities with an increase in curing age influenced their shrinkage above the

systems modified with 10% and 20% PFA dosages.

7.7 Variations of hydrate product content of the various OC system

The results from the laboratory tests revealed that the hydration product content of the

various OC systems at various age differed with the w/c ratio and the type and dosage of MA

in the system. It is noted that when neat cement is mixed with water for cementitious system,

bonding of chemicals occurs resulting in several products in the hydration process.

Essentially the calcium aluminate hydrate and calcium silicate hydrates noted as C-A-S-H

products, and calcium hydroxide Ca(OH)2 products are formed. In the partial replacement of

cement with MA diluted in water for cementitious system, it is reported that the quantity of



the C-A-S-H products increases (Gopalakrishnan et al, 2001). Hafez et al (2008), noted that

in cement and MA systems, a dissolution of the alumina and silica from the MA takes place

to form hydrate products through increased solubility as a result of the pH condition that

prevails in the cement-MA system. This enables the alumina (Al2O3) and silica (SiO3) from

the MA to bond with the calcium hydroxyl ions Ca(OH)2 to form additional C-A-S-H

products. As such, the reasons for the varying content of hydrate products observed for the

various OC systems with curing age can be ascribed to the rate of chemical reaction

influenced by the mix water to cement ratio and the properties of the MAs used to modify the

OC system.

As shown in the hydrate product results (Figure 5.26 and 5.27), the C-A-S-H and Ca(OH)2

products in the OC systems designed with different water to cement ratios differed with

curing age. This occurrence indicates that the particle density of the OC systems as a result of

change in water to cement ratio influenced the rate of chemical bonding with curing age to

form hydrate products. The case of the C-A-S-H products showed that the OC systems

designed with lower mix water ratios formed more hydrate at the initial day but with an

increase in curing age, the rate of hydrate product formation increased in the OC systems

designed with higher mix water ratio. This outcome resulted from the increase of mix water

to cement ratio which decreased the particle density of the OC system. Thus, the decreased

density reduced the interaction of the chemical bearing particles of the cement, thereby

decreasing the rate of C-A-S-H products formation in the OC systems designed with higher

mix water ratio at the early age. This can be supported with the setting time outcome that

showed the OC system designed with lower mix water ratios to set in a shorter time likely

due to higher chemical activities in such systems at the stage. But with curing age, the OC

systems designed with higher mix water ratios were found to have more C-A-S-H products is

due to increased content of the Ca(OH)2 product which bonds with Al2O3 and SiO3.

According to Junior (2012), the formation and content of Ca(OH)2 in cement system depends

on the available free water and curing age. The case of the Ca(OH)2 product formation in the

various OC systems showed that the OC systems designed with lower mix water ratios had

higher Ca(OH)2 formation at the early age while at later age, OC systems designed with

higher mix water ratios had higher Ca(OH)2 product. This occurrence indicates that the high

content of cement for the OC system designed with a low water ratio enabled higher



formation of Ca(OH)2 with available free water used for the preparation of system at the early

day. However, with an increase in curing days, the continuous hydration consumed the free

water quicker in the systems designed with a higher cement content. As such, higher moisture

made available by higher water content for the OC systems designed with higher mix water

ratio enabled the formation of more Ca(OH)2 product than the OC systems designed with low

mix water ratios with an increase in curing days.

In the case of the OC systems modified with MA, the reason for the varying hydrate products

content with curing age can be ascribed to the properties and dosages of the specific type of

MA for the system. As shown in Table 3.1 the chemical compositions of the MK and PFA

materials differ from that of the class G cement material. The MK material is found to have a

high content of SiO3 and Al2O3 while the PFA material contains a high content of SiO3

similar to that of the MK material but a lesser content of Al2O3 compared to the MK material.

Compared to the class G cement material, both MAs have higher contents of SiO3 and Al2O3

but a much lower content of CaO to the class G cement. In addition, the particle properties of

the MAs are different as the MK particle are found to have a higher specific surface area than

the particle of class G cement, while the specific surface area of the PFA particle varies

between 300 to 600 m2/kg compared to that of the class G cement particles. PFA particles are

spherical in shape and spheres amongst the different shapes is established to offer the

minimum surface to surface contact area amongst shapes (Bapat, 2012).

As such, mixed with water, the high specific surface area of the MK particles would have

enhanced the particle density of the OC systems and increased the inter-surface contact area

amongst the chemical bearing particles, thereby, enabling a high rate of chemical bonding to

take place at a time in the OC-MK system. In the case of PFA presence in the OC system, the

specific surface area of the particles due to its surface charateristics would have presented a

smaller inter-surface contact area amongst the chemical bearing particles of the system. This

in turn reduced the amount of chemical bonding at a time compared to the rate of chemical

bonding in the control system and the OC-MK systems.

Furthermore, the high pozzolanic reaction which librates a lot of heat during the formation of

C3A enhanced by the MK particles in the system would have contributed to an increase in the

hydrate products content in the OC-MK systems. According to Antinos et al (2006) and



Bernal et al (2010), the acceleration of a pozzolanic reaction is also influenced by an

exothermic reaction and results in an increased formation of alumina-silicates products. Thus,

the high C3A formation in the OC-MK system with an increase of MK dosage may have

caused a faster dissolution of other oxides especially silica from the MK to form more C-S-H

products compared to the PFA with its different particle characteristics and lower Al2O3

content than MK. The results of the hydrate products showed that, the higher the dosage of

MK in the OC systems with an increase in curing days, the higher the content of C-A-S-H

became, while the Ca(OH)2 content was found to be very low which can be attributed to the

consumption of Ca(OH)2 in pozzolanic reaction as it bonds with Al2O3 and SiO3 from the MK

to form C-A-H and C-S-H products.

For the OC-PFA system, the hydrate products results showed that the increase of PFA dosage

in the system resulted in a lower C-A-S-H product formation than the OC 0.44 in the early

days. This may be attributed to the low rate of chemical reaction emanating from the reduced

inter-particle surface contacts initiated by the spherical shape of PFA particles. However,

with an increase in curing days, the OC-PFA systems modified with 10%, 20% and 30% PFA

dosages recorded higher C-A-S-H products compared to that of the control system. This

outcome may be associated to contribution of C-A-S-H and the high content of Ca(OH)2

occurring from the proportion of cement in the system. The Al2O3 and SiO3 from the PFA

bonds with the Ca(OH)2 to form C-A-S-H products. The use of Ca(OH)2 by the PFA to form

C-A-S-H products in cement-PFA systems in turn reduces the presence of Ca(OH)2 in the

system. This may explain the high content of C-A-S-H observed for the OC-PFA systems

modified with lower PFA dosage while containing higher contents of Ca(OH)2 for the

examined period due to the proportion of cement in each system.

7.8 Variations of OC system performance with temperature and chemical admixture

The rate of cementitious system hydration is dependent on temperature. If there is an increase

in the temperature, the rate of reaction of chemicals in the cementitious system also increases.

The mechanism by which temperature affects the rate of chemical reaction can be explained

from the vibration of chemical molecules (Cox, 2016). Clark (2002) added that particles tend



to move faster with an increase in temperature and consequently, more particles collide

within a given time period, speeding up the rate of reaction. According to Barnett (2006),

when a cementitious system is exposed to a higher temperature, it tends to solidify earlier

than when exposed to a lower temperature. However, this does not necessary mean that the

performance of the cementitious system such as its compressive strength will be better at a

higher temperature. Elkhadiri et al (2009) noted that the physical form of a hardened

cementitious system at an elevated temperature can be poorly structured with a high volume

of pores due to the chemical reactions occurring at a faster rate. This can explain the earlier

setting times and variation of strength performance at 24 hours obtained for the selected OC

system at elevated temperature.

For the setting time of the OC-MK system compared to that of the OC0.44 system at an

elevated temperature, the setting time of the OC-MK systems were found to set earlier than

that of the OC0.44 system which can be ascribed to the effect of the higher rate of chemical

activity in the OC-MK system combined with the increase in hydration heat by the elevated

temperature. The setting time of the OC systems at 23oC showed the OC-MK system to set

earlier than the OC0.44 system which is due to the chemical and particle surface area of the

MK enhancing the rate of chemical activity in the system and essentially the C3A formation

which librates high heat in the hydration process. Thus, with an increase in temperature, the

rate of chemical activity in the OC-MK system was increased compared to that of the OC0.44

system. The impact means, a higher rate of chemical vibration resulting to collision and

bonding of chemicals to transform the fluid OC-MK systems to solid state in less time than

the OC0.44 system. This can also explain the setting difference observed for the OC-MK

systems at the elevated temperature as the OC20MK system was found to set earlier than the

OC10MK system.

In the case of the setting time of the OC-PFA systems compared to the OC0.44 system at

elevated temperatures, the setting time of the OC-PFA systems was found to decrease but

remained higher than that of the OC0.44 system at the various temperatures. The outcome

can be explained from the retarding effect presented by the PFA properties to the setting of



the system as observed at 23oC. Narmluk and Nawa (2014) explained that the duration of the

various hydration phases of cementitious system differs with the properties of incorporated

admixture type. They added that the dormant hydration phase of a cementitious system

incorporating PFA is found to be higher than that of neat cement system (Narmuk and Nawa,

2014). As such, although the OC-PFA system decreased in setting time with an increase in

temperature, the dormant hydration period of the OC-PFA system which is higher than that of

the OC0.44 system would have delayed the impact of the temperature on the reaction of the

chemicals. Thus, with presence and increase of PFA dosage in the OC system, the effect of

temperature was further delayed resulting in the higher setting times observed with the

OC20PFA system compared to the OC10PFA and other OC systems.

For the strength performance of the selected OC systems at 24 hours, the effect of elevated

temperature was observed to increase the strength of the various OC systems apart from the

OC20MK system which recorded a similar strength outcome at 42oC and 60oC. The strength

performance of a cementitious system is known to be influenced by the C-S-H products

formed in the system (Jennings and Tennis 1994; Hussain 1994; Shasi 2002). However, at

elevated temperatures, the formation of the C-S-H products responsible for the strength of a

cementitious system is adversely affected. According to Bentur et al (1979), the

polymerisation of the C-S-H products rises with an increase in the hydration temperature

which can in-turn affect the strength of cementitious system. Elkhadiri et al (2008) reported

that a high temperature initiates high rates of chemical reaction at the initial stage of

hydration which results in a non-uniform distribution of the hydrate product across the micro-

structure of the cementitious system. In this case, the cementitious system is not properly

densified with the C-S-H product resulting in a high volume of pore spaces which leads to a

reduction in the strength of the cementitious system. This explains the strength difference

found with the OC-MK system compared to the OC0.44 system at higher temperatures

especially with the OC20MK system at 60oC. The OC-MK systems were observed to set

earlier than the control system which indicates a higher rate of chemical activity. Thus, with

an increase of temperature to 60oC, the formation of C-S-H and other hydrate products would

have been affected, leading to a lower strength than the OC0.44 system. On the other hand,



although the OC-PFA systems had a lower strength than the OC0.44 system, the OC-PFA

systems showed an increase in strength with an increase in temperature which is likely due to

a balance of the retardation effect of PFA to the OC system by the higher temperatures.

Furthermore, the addition of the super plasticizer chemical admixture (SP) to optimize the

performance of the selected OC system was found to alter the setting time and strength

performance of the various systems. The setting times of the various OC systems were

observed to increase with an increase of SP admixture dosages at the various temperatures. In

the case of the compressive strength performance at 24 hours, the increase in the SP

admixture dosage increased the strength of the systems at elevated temperature while at 23oC,

the strength of the systems varied with the increase in the SP admixture dosage. The various

outcomes (setting time and strength) can be explained from the mechanism by which the SP

admixture influences the properties of cementitious system which is through de-flocculation

of clumped cement particles.

According to Paiya et al (2012), when cement is mixed with water, the particles of the cement

material from their individual state tend to flocculate into clumps. These clumps entrap some

of the mix water used for the formulation of the cementitious system and prevent proper

fluidity in the system. The addition of agents such as super plasticizer tends to break down

the flocs which releases the entrapped water and improves fluidity of the system for further

hydration (Paiva et al, 2012). According to Yamada et al (1998), incorporating the SP

admixture in a cementitious system disperses the cement particles in the system with

polymers of the SP adsorbing around the particles with negative charges. This repels the

cement particles from each other due to the particles bearing similar electrostatic charge, and

the extent of repulsion depends on the thickness of the polymer around the respective cement

particles (Alsadey, 2012, Yamada et al 1998). In this regards, the presence and increase of the

SP admixture dosage in the various OC system increased the setting times of the systems by

deflocculating the clumped particles. This released more water to lubricate the system

thereby extending the transformation period of the various OC systems from fluid state to

solid. The lubrication of the system as a result of the released water also results in more

moisture being available for hydration which would have benefitted the system with

increased strengths, especially at higher temperatures. For the variation in strength



performance of the OC systems at 23oC with the presence of the SP admixture, the

occurrence can be explained from the retardation or elongation effect on the transformation

of the system with the higher dosages of the SP admixture. The effect of the SP dosage on the

performance of the various systems can also be attributed to the properties of the type of

material(s) for the OC system.

7.9 Variations in the developed models for the OC systems

The model equations developed for the prediction of the setting time and compressive

strength performance of the OC system in the current study were found to differ. The

difference in the model equations is due to the performance values for the systems

introducted to the software. It was found that the prediction of setting time and strength

performance values of the OC systems by the model equations increased or decreased with

the magnitude of each input parameter value (e.g Temperature or SP dosage) inserted in the

appropriate model equation. The models were also found to have negative and positive

coefficients for the different input parameters. The reason for the positive and negative

coefficients can be explained from the effect of the input parameter(s) on the setting time and

strength performance of the OC system and to enable the models to replicate or predict the

output values accordingly.

The setting time of the OC systems was found to be affected differently by the various

parameters. The presence of MK and temperature are found to decrease the setting time of the

OC system while the presence of PFA and SP admixture are found to increase the setting

time of the OC system compared to the OC0.44 system. Thus, a negative coefficient was

generated for the MK and temperature parameters while the PFA and SP admixture had

positive coefficient for the setting time prediction by the model equations.

For the strength performance at 24 hours, the presence of MK was found to increase the

strength of the OC system while temperature caused a reduction in the strength of the OC-

MK systems compared to that of the OC0.44 system. In the case of the OC-PFA systems, the

presence of PFA resulted in the system having lower strength than the OC0.44 system and

did not achieve strength above that of the OC0.44 system at higher temperature. With SP



admixtures, the effect to strength of the OC system differed with at various temperature.

Lower dosages of the SP admixture were found to improve the strength of the OC systems at

23oC while higher dosages decreased the strength of the system at 23oC. At elevated

temperatures, the dosages of the SP admixture were observed not to maintain the OC-MK

system strength above that of the OC0.44 system and it was also observed not to improve the

strength of the OC-PFA system above that of the OC0.44 system. As such, a positive

coefficient was generated for the input parameter of the MK for improving the strength of the

OC system, while negative coefficient was generated for the input parameters of PFA, SP

admixture and temperature to replicate or predict the strength performance of the OC system

at 24 hours by the model.

The AAD% calculated for the models were also identified to differ which is due to the

deviations in predicting the different setting time and compressive strength at 24 hours of the

OC system by the model. The AAD% represents the avareage of the sum of the occurring

deviation with predicting the various individual output variables by the model. Similarly, the

difference with the response surfaces is due to the different output variables with respect to

the temperatures and the dosages of SP admixture.

Furthermore, it was observed that the predictive models could not accurately predict the

setting time and compressive strength of systems developed by other researchers which

include Mitrovic and Nikolic (2014) and Broni (2012). The shortfall of of the predictive

models developed in the current study to predict the ST and CS of the other systems can be

attributed to the difference in properties of the materials. Moreso, the preparation approach

including the w/c ratio, other materials present, test and curing method adopted for the system

in those studies could have influenced the outcome. This is in agreement with findings

established by Shariar (2011) in a study models for OC system which concluded that the

difference in the materials and preparation approach of systems alter the performance

outcome of systems



7.10 Practical implication

The outcomes obtained in the current research have revealed the possibility of modifying the

performance of OC systems by adjusting the water to cement ratio, partially replacing cement

with MA (MK or PFA) and incorporating chemical admixture (SP) to the system to achieve

adequate performance at high temperature degrees in oil wells. However, only the

modification of the class G OC system with the dosages of MK or PFA up to 20% at w/c of

0.44 offered satisfactory performance based on the API criteria. The other systems were

found to either satisfy some of the requirement while falling short in other requirements

compared to the standard class G OC system (OC 0.44) or not meeting any of the API

criteria.

The impact of the satisfactory dosages (10% and 20%) of MK or PFA in the modification of

the OC system performance presents significant practical implications for cementing of oil

wells. For instance, regarding the density of oil well cementitious system, the practical

implication of modifying OC system with MK or PFA materials reveals that both MAs

decrease the density of OC system. This can be beneficial in cases where the pressure

presented by annular fluids in the oil well is lower than the density of the standard class G

OC system. Thus, the additions of MK or PFA material can be used to achieve a balance

between the density of the OC system and the annular fluids to avoid loss of the system to the

geological formation.

In the case of setting of OC system, the modification of the OC systems using MK resulted in

a quicker setting time than that of the standard class G OC system, while the use of PFA and

SP admixture extended the setting times of the OC system. In practice, it is generally

accepted that the waiting on cement (WOC) to set duration should range between 5 to 8 hours

for surface casing zone and 12 to 24 hours for high temperature zones in the oil well. As

such, the MK material can be used to decrease the waiting on cement (WOC) time as

necessary and especially in low temperature cases to save operation costs from the delay of

WOC to set. The PFA and SP admixture can be beneficial for the proper placement of the OC

system in the oil well by delaying the solidification of the system as they delay the setting

time especially in high temperature formation zones. Furthermore, the research reveals the



practical implications to the strength, rheology, pore spaces and shrinkage of OC system

modified with MK or PFA dosages up to 20% in the system.

The use of MK in modifying the OC system performance shows that MK dosages up to 20%

can be used to achieve a high strength in a reduced time compared to the standard class G OC

system. It also shows that MK material can be used to reduce the pore spaces and shrinkage

of the class G OC system to prevent migration of formation fluids that can comprise the

integrity of the system and existence of the oil well. In the aspect of rheology in placing a

system in the well, the use of MK dosages up to 20% for the OC system was able to flow

indicating it can be placed. For the case of using PFA to modify the performance of the class

G OC system, the PFA dosages up to 20% shows practical benefits in reducing the pore

spaces and shrinkage of the class G OC system while offering satisfactory strength to

stabilise the casing strings in the oil well. The practical benefit of the PFA material to the

performance of the class G OC system also extends to the placement of the OC system in the

oil well as it enhances the rheological properties for flow of the OC system.

The implication of using the MK and PFA for OC system modification also extends to the

environment. The exploits of MA in cement technology is accepted as an approach to safe

guard the environment as it reduces the amount of CO2 emission and energy consumption

associated with cement prodcution. Thus, the use of MK and PFA material in OC systems

presents significant environmental benefits to oil well cement technology for the construction

of wells including other civil engineering aspects. The satisfactory dosages of 10% and 20%

MK and PFA material with beneficial effects to the performance of OC system entails a

reduction in the amount of class G cement used for OC system, in turn, a decrease of the

adverse impact of cement to the environment. The production of Portland cements consumes

high energy to generate temperature of about 1500oC to fuse various chemical elements into

cement clinker while emitting one tonne of CO2 into the atmosphere for each tonne of cement

produced. With the partial replacement of the class G cement using MK and PFA material for

OC system, the amount of energy that is used and CO2 released in production of Portland

cement material is reduced. According to Torgal and Jalali, (2011) and Srinivan et al (2014) a

tonne of MK material obtained from kaolin clay requires a temperature between 600oC to

800oC which consumes less energy with lower CO2 emission of 0.07 tonne than that of



cement production which requires a temperature of 1500oC generated by 5000MJ of energy

with an emission of one (1) tonne of CO2 for a tonne of cement. The PFA material being a

by-product of coal fired electricity generating plants does not require additional processing

and emits no CO2 (Sear, 2004). As such, the MK and PFA material are good candidates that

can offer beneficial implications to the performance and environmental aspects of oil well

cement technology for cementing of oil wells.

Furthermore, the outcome of this research study is practically beneficial to industries,

organisations, regulatory institutions and all bodies involved in the promotion and use of MA

in cement technology. The study is also of practical importance to engineers or workers that

are new to the field of cement technology and its application. The performance of the

cementitious system modified with the MK and PFA material used in this study can serve as

a guide for modification of other classes of Portland cement. It will also be beneficial for

introducing similar or other types of MAs to achieve sustainable cement technologies in the

quest of protecting the environment. This in turn presents socio-economic implications as it

expands the demand and production of sustainable cementitious materials including natural

minerals, by-products or recycled materials for cement technology.

The design/experimental process for the OC systems and models for the setting time and

strength performance of the OC system will also be of good relevance to the modification of

cement based systems. The design process and model for the OC system performance

presents a method to simplify the modification of cementitious system to achieve the desired

performance and sustainable systems for cementing purposes. Practically, the implication is a

cutback in the number of experimental tests, costs and time in designing a system to meet

desired performance for purpose guided with performance prediction by models.


Chapter 8-Conclusion

CHAPTER 8: CONCLUSION AND RECOMMENDATIONThe present study aimed to modify and model the performance of oil well cementitious system

for a range of temperature conditions in oil wells using MA and chemical admixture. This

was to achieve desired performance and simplify the design process for OC systems. The

current chapter presents the conclusion from the findings of the study and recommendations

for further improvement of oil well cement system.

8.1 Conclusion

The findings from the current study reveal that the selected mineral and chemical admixture

are good candidates including the developed mathematical model for modifying OC system

for desired performance in oil well engineering. Also, the use of the selected mineral

admixtures (MK and PFA) is deemed a sustainable approach to reduce the adverse impact of

cement on the environment. The following conclusions are therefore drawn from the findings

of this research;

The research demonstrates that the performance (density, rheology, setting, strength,

water absorption, shrinkage and hydration) of an OC system is influenced by the mix

water to cement ratio, the temperature of the surrounding environment which the OC

system is exposed to and the chemical and physical properties of the materials

(cement material, mineral admixture and chemical admixture) used for the system;

hence the difference observed with the performance of the various OC system. The

properties of the MK and PFA materials are recorded to be different from that of the

class G cement material, therefore using the MK and PFA to partially replace the

class G cement material adjusted the properties of the OC system and altered the

performance of the system. The effect of mix water content and the surrounding

condition also alters the performance of OC system. The increase in mix water

content and surrounding temperature meant that chemical activity occurred at

different rate and influenced the different end performance for the various systems.



The research also revealed that the type and dosage of mineral admixture (MK or

PFA) offers significant effect on the performance of the OC system. At low dosages

(10% and 20%) with the adopted w/c ratio for the systems, the presence of the MK

and PFA initiated similar or improved performance to the OC system compared to

that of the standard class G OC system, but beyond a certain threshold, the dosage of

the mineral admixture depending on the type (MK or PFA) characterised by its

properties degraded the performance of the OC system below requirements or

performance of the standard OC system. The MK material, up to a dosage of 20%,

was found to be effective in satisfying the density and setting time requirements for

OC system, allowed the system to flow with the recorded rheological properties while

also improving the strength, reducing the water absorption and decreasing the

shrinkage of the system. The PFA material, up to the dosage of 20%, was found to be

effective in meeting the requirements for density, setting time and strength

requirements of OC system, while enhancing the flow of the system by decreasing its

rheological properties, reducing the water absorption and decreasing the shrinkage of

the OC system.

The research showed that the addition of a superplasticizer chemical admixture of

0.2% to 1% by weight of mix water alters the performance of the OC system. The use

of the superplasticiezer chemical admixture to modify the performance of the OC

system revealed that the intensity of the chemical admixture to improve the

performance of the OC system depends on the type and dosage of the mineral

admixture incorporated in the design of the OC system as well as the temperature of

the surrounding.

The research also revealed that a model can used to simplify the design of an OC

system. A model was developed to predict the performance of the system to simplify

the design process. From the findings, the study showed that the output produced by

the model is related to the historical data entered as the input parameters. The test

results compared to the predicted performance of the systems by the model showed



the model output values were within acceptable deviation from values obtained for the

systems in the tests.

Thus it can be concluded from the findings of this current study that the performance of OC

system is dependent on the chemical and physical properties of the cement, type and dosage

of mineral and/or chemical admixture incorporated, the water to cement ratio and the

condition of the surrounding which the OC system is exposed to. Finally, the success of the

research concludes that the modification and modelling of OC system can be achieved using

mineral and chemical admixture and a mathematical modelling approach. In this regards, the

research has revealed and contributed a new knowledge on how MK, PFA and superplasticer

admixture along with models can be used to pursue desired performance for a class G cement

based system to cement oil wells with temperatures ranging from 23oC to 60oC.

8.2 Recommendations for future work

The present study has assessed a range of parameters in the act of designing an OC system

for the cementing of oil wells. The parameters which include a range of w/c ratio, MK, PFA

and SP admixture dosages to modify the performance of OC system only applies to the

temperatures investigated in the study. Thus, there is still a range of work required in the

scope of OC system for cementing of oil wells. Based on the present study, the following

recommendations are hereby proffered for future work;

The use of different w/c ratios and types of cement, mineral admixtures and chemical

admixtures in formulating OC system can exhibit different performance. As such, an

investigation of the influence of different of w/c ratios and types of admixtures on the

performance of OC system is encouraged.

It would be pragmatic to establish the impact of the combination of MK and PFA

dosages on the performance of class G OC system in fluid and solid state including

other types of cements and with different range of w/c ratio and other types of mineral

and chemical admixtures as such was not pursued in this research due to time and

resources.



In the current research work, the investigation of the effect of the oil well condition on

the performance of OC system was limited to temperature. Therefore, an investigation

into the modification of OC system using MK, PFA with/or other admixtures to

ascertain the impacts of a different well condition or combination of oil well

conditions on systems is necessary.

Further research should be carried out to investigate the impact of MK, PFA, or other

types of materials on OC system perfoamnce not investigated in the present research

such as fluid loss, durability etc.

In depth environmental assessment and economic benefits of mineral admixtures that

can be used in developing OC systems should be attempted. This will encourage the

users and manufacturers of such materials and also increase confidence of

entrepreneurs to investment in the technology.

In developing the model in this study, a limited size of data was used, it is therefore

necessary to explore other data ranges to extend the prediction capability of the

model. The empirical equations and response surface of the models are limited to the

performance of the type of cement, MA and chemical admixture used in this study

including the range of temperatures. In this case, further experimental work will be

needed to improve the predictive models and extend the limits beyond the type of

cement, admixtures and their dosages and temperatures examined in the present study.


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Appendix

Appendix

Appendix 1- Density results of the various oil well cementitious systems in kg/m3.

Class G OC systems Density (kg/m3)

OC0.36 2020

OC0.40 1960

OC0.44 1900

OC0.48 1820

OC0.52 1750

OC-MK systems Density (kg/m3)

OC10MK 1850

OC20MK 1790

OC30MK 1720

OC40MK 1680

OC50MK 1650

OC-PFA systems Density (kg/m3)

OC10PFA 1880

OC20PFA 1850

OC30PFA 1810

OC40PFA 1760

OC50PFA 1710


Appendix

Appendix 2- Rheological properties (Yield stress and plastic viscosity) results of the various oil well cementitious systems.

Glass G OC systems Yield stress (Pa) Plastic viscosity (Pa.s)

OC0.36 17.4 0.12

OC0.40 14.5 0.09

OC0.44 12.2 0.075

OC0.48 10.5 0.06

OC0.52 7.3 0.03

OC-MK systems Yield stress (Pa) Plastic viscosity (Pa.s)

OC10MK 24 0.16OC20MK 33 0.17OC30MK - -OC40MK - -OC50MK - -

OC-PFA systems Yield stress (Pa) Plastic viscosity (Pa.s)

OC10PFA 11.8 0.07OC20PFA 11 0.06OC30PFA 10.6 0.05OC40PFA 10.4 0.05OC50PFA 10.4 0.05


Appendix

Appendix 3- Setting time results of the various oil well cementitious systems at 23oC.

Class G OC systems Setting time (Minutes)

OC0.36 325

OC0.40 370

OC0.44 425

OC0.48 495

OC0.52 597

OC-MK systems Setting time (Minutes)

OC10MK 382

OC20MK 327

OC30MK 222

OC40MK 98

OC50MK 25

OC-PFA systems Setting time (Minutes)

OC10PFA 438

OC20PFA 479

OC30PFA 531

OC40PFA 590

OC50PFA 653


Appendix

Appendix 4- Unconfined compressive strength results of the various oil well cementitious systems in MPa at 23oC.

Class G OC systems 1 day(24 hours) 7 days 14 days 28 days

OC0.36 16 59 67 75

OC0.40 13 45 56 64

OC0.44 9 27 44 52

OC0.48 2 18 31 37

OC0.52 0 14 19 22

OC-MK systems 1 day(24 hours) 7 days 14 days 28 days

OC10MK 10 28 49 57

OC20MK 10 31 53 61

OC30MK 11 38 56 66

OC40MK 8 26 40 45

OC50MK 3 17 25 28

OC-PFA systems 1 day(24 hours) 7 days 14 days 28 days

OC10PFA 8 24 40 48

OC20PFA 7.5 24 38 46

OC30PFA 4 21 33 40

OC40PFA 1 15 24 30

OC50PFA 0 12 19 25


Appendix

Appendix 5- Unconfined tensile strength results of the various oil well cementitious systems.

Class G OC systems 1 day(24 hours) 7 days 14 days 28 days

OC0.36 1.1 4.8 5.3 5.7

OC0.40 0.9 3.67 4.3 4.8

OC0.44 0.76 1.92 3.1 3.6

OC0.48 0 1.1 1.9 2.2

OC0.52 0 0.7 1 1.3

OC-MK systems 1 day(24 hours) 7 days 14 days 28 days

OC10MK 0.78 1.98 3.41 3.8

OC20MK 0.81 2.4 4.1 4.3

OC30MK 0.94 2.9 4.5 4.8

OC40MK 0.68 1.87 2.6 2.89

OC50MK 0.1 0.88 1.16 1.4

OC-PFA systems 1 day(24 hours) 7 days 14 days 28 days

OC10PFA 0.74 1.82 2.9 3.4

OC20PFA 0.67 1.76 2.65 3.1

OC30PFA 0.41 1.3 2.2 2.8

OC40PFA 0 0.97 1.6 1.9

OC50PFA 0 0.68 1.1 1.3


Appendix

Appendix 6- Water absorption results of the various oil well cementitious systems in percentage (%).

Class OC systems 1 day (24 hours) 7 days 14 days 28 days

OC0.36 6.56 3.59 1 0.52

OC0.40 7.14 4.08 1.24 0.57

OC0.44 7.26 4.23 1.44 0.67

OC0.48 8.49 5.2 2.17 1.18

OC0.52 8.74 5.59 2.67 1.21

OC-MK systems 1 day (24 hours) 7 days 14 days 28 days

OC10MK 7.01 4.01 1.19 0.58

OC20MK 6.99 3.88 0.96 0.51

OC30MK 6.88 3.6 0.94 0.52

OC40MK 9.47 6.34 3.44 1.92

OC50MK 11.25 7.65 4.15 2.24

OC-PFA systems 1 day (24 hours) 7 days 14 days 28 days

OC10PFA 7.16 4.15 1.32 0.58

OC20PFA 7.08 3.9 1.2 0.54

OC30PFA 8.51 5.53 1.88 0.81

OC40PFA 9.23 5.88 2.65 0.93

OC50PFA 10.25 6.52 2.97 1.23


Appendix

Appendix 7- Drying shrinkage results of the various oil well cementitious systems in percentage (%).

Class G OC systems 1 day (24 hours) 7 days 14 days 28 days

OC0.36 -0.260 -0.428 -0.540 -0.639

OC0.40 -0.295 -0.453 -0.575 -0.674

OC0.44 -0.295 -0.491 -0.611 -0.695

OC0.48 -0.218 -0.505 -0.670 -0.768

OC0.52 -0.239 -0.551 -0.730 -0.825


OC10MK -0.291 -0.481 -0.596 -0.670

OC20MK -0.284 -0.463 -0.579 -0.660

OC30MK -0.284 -0.456 -0.572 -0.653

OC40MK -0.309 -0.505 -0.649 -0.782

OC50MK -0.316 -0.523 -0.649 -0.775


OC10PFA -0.246 -0.467 -0.582 -0.684

OC20PFA -0.249 -0.449 -0.568 -0.674

OC30PFA -0.239 -0.425 -0.579 -0.677

OC40PFA -0.221 -0.435 -0.582 -0.695

OC50PFA -0.228 -0.446 -0.586 -0.702


Appendix

Appendix 8a- Thermo-gravimetric analysis results of the C-A-S-H products of the various oil well cementitious systems in percentage (%).


OC0.36 2.721 4.021 4.457 17.28

OC0.40 3.318 4.27 4.844 17.56

OC0.44 2.394 4.325 5.526 17.89

OC0.48 2.401 4.8 6.456 18.01

OC0.52 1.83 5.393 6.562 18.26


OC10MK 2.41 6.456 8.31 20

OC20MK 2.452 6.562 8.561 20.3

OC30MK 2.471 6.558 8.968 20.5

OC40MK 2.501 7.27 9.412 21.7

OC50MK 2.331 7.325 9.613 24


OC10PFA 2.371 3.831 6 19

OC20PFA 2.358 3.42 4.76 19

OC30PFA 2.555 2.9 3.32 17.84

OC40PFA 2.209 2.61 2.93 17

OC50PFA 1.7117 2.56 2.87 16


Appendix

Appendix 8b- Thermo-gravimetric analysis results of the CA(OH)2 product of the various modified oil well cementitious systems in percentage (%).


OC0.36 3.153 3.8 3.826 3.4

OC0.40 3.117 3.8 3.853 3.41

OC0.44 2.912 3.836 4.1 3.51

OC0.48 2.901 3.875 4.18 3.54

OC0.52 2.906 3.757 4.21 3.6


OC10MK 2.9 3.287 3.35 3.3

OC20MK 2.859 2.916 2.96 3

OC30MK 2.481 2.6 2.62 2.7

OC40MK 2.078 2.13 2.15 2

OC50MK 1.738 1.78 1.79 1.74


OC10PFA 2.903 3.788 3.9 3.4

OC20PFA 2.899 3.758 3.82 3.1

OC30PFA 2.829 3.3 3.4 2.9

OC40PFA 2.521 2.671 2.68 2.4

OC50PFA 2.253 2.38 2.38 2.3


Appendix

Appendix 9a: Setting time results of the selected oil well cementitious systems at different temperature and SP dosages.

23oC

SP Dosages (%) OC0.44 OC10MK OC20MK OC10PFA OC20PFA

0 425 382 327 438 479

0.2 476 435 352 538 580

0.4 551 498 398 647 698

0.6 622 560 445 750 844

0.8 701 610 475 873 967

1 784 690 520 976 1080

42oC

SP dosages (%) OC0.44 OC10MK OC20MK OC10PFA OC20PFA

0 187 167 130 198 217

0.2 237 200 165 262 290

0.4 294 243 200 330 374

0.6 355 296 242 396 460

0.8 410 345 292 473 550

1 480 398 330 546 630

60oC


0 80 76 60 88 98

0.2 102 96 76 116 134

0.4 142 118 95 148 182

0.6 164 132 126 180 226

0.8 215 168 148 230 283

1 256 200 180 284 330


Appendix

Appendix 9b: Unconfined compressive strength results of the selected oil well cementitious systems in MPa at various temperature and SP dosages at 24 hours (1 day)

23oC


0 9 10 10 8 7.5

0.2 9 10 11 7.6 6.2

0.4 9.4 10.3 12 6.5 5.5

0.6 9 10 13 6.2 5.2

0.8 8.9 9.5 11 6 5

1 8.3 9.2 11 6 4.3

42oC


0 11 13 13 8.6 8

0.2 12 14 14.6 9.4 8

0.4 12.8 15.7 16.9 9.9 8.4

0.6 15 16.8 17.6 12 9

0.8 17.7 18.2 20.3 14.3 9.9

1 19.2 20 22.4 17 12

60oC


0 21 16 13 15.8 12.4

0.2 24 18 15 16.6 13.8

0.4 26 21 19.5 17 15

0.6 30 23.7 22 21 19.7

0.8 35 28.5 27.7 25.2 24

1 38 33 32 30 29


Appendix


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