Dr Zahid Qamar, Prof Tasneem Pervez, Dr Farooq al-Jahwari

Applied Mechanics and Advanced Materials Research Group (AM)2R

Mechanical and Industrial Engineering Department

Sultan Qaboos University, Muscat, Oman

International Gas Union Research Conference (IGRC-2020), 24-26 February 2020, Muscat, Oman

Engineering is the art of modeling materials we do not wholly

understand, into shapes we cannot precisely analyze, so as to withstand

forces we cannot properly assess, in such a way that the public has no

reason to suspect the extent of our ignorance.

A R Dykes

MOTIVATION AND SIGNIFICANCE

Technology shapes society and society shapes technology.

Robert W. White

In some oil wells, viscosity of the crude is very high (heavy oil)

Steam is injected to increase fluidity and mobility of the oil

Steam injection heats the crude oil in the formation, lowering its viscosity and vaporizing some of the oil to increase its mobility

Polymer Pipes in Oil Drilling

Schematic of steam injection process

Cyclic Steam Stimulation (CSS) is a staged technique using Vertical

Injection and Production

Polymer Pipes in Thermal Wells

Steam injection requires large quantities of water, generally transported from nearby fields

Some oilfields in the Gulf region, especially in Oman

Water-bearing sandstone is highly fragmented

Sand screens are required for sand control

Sand screens made of carbon-steel are not reliable

Quickly undergo serious corrosion

Some oilfields have started to try out non-metallic materials

Such as strengthened polymers (eg hard PVC)

Because of their non-corroding nature

Based on a hit-and-trial approach (in Oman)

Around 15% wells have reported failure due to screen collapse

Petroleum Development Oman (PDO) wanted to obtain more information on strength of these pipes

Experimental testing facility designed and developed at Sultan Qaboos University (SQU)

For integrity assessment of large-diameter hard polymeric pipes

Included design and construction of test setup and required jigs and fixtures

Types of tests

Pipe behavior under tensile loads

Pipe behavior under compressive loads

Collapse behavior

Testing of Polymer Pipes

Scientific understanding of different strength parameters of new pipe materials (high strength PVC)

Prequalification of polymer pipes for appropriate field conditions

Major savings in cost and time for thermally assisted oil wells

Once sufficient confidence is built with PVC concept, this well design can be introduced PDO wide

Major Contributions

Currently PDO drills approximately 30 shallow water supply wells per year

Drill-site water, camp supply, R/O & frac unit supply, water-flood, steam generation

Company-wide switch from carbon steel to PVC for water supply wells

Savings of approximately 150k-200k USED per well

Annual saving of 4.5-6.0 mil USD

Savings of 20-30 mil USD over a 5 year period

Financial Upside

EXPERIMENTAL WORK

To satisfy requirements of international standards

Polymer pipes had to go through a 2-3 month ageing process before mechanical testing

In brine solution matching the salinity of the water field (1.02 sg, equivalent to 35 PPT)

Testing under tensile loads

PVC pipe sections of 1m length and 280 mm dia

Three types of pipe sections

Plain

Built-up (pipe with threaded connection in the middle)

Slotted (screen)

3 samples tested in each case

Testing Scheme and Parameters

The three types of PVC pipes for tension testing: plain, built-up, and slotted

Tension tests conducted on a Dartech universal testing machine (UTM)

Capacity of 2000 kN in tension mode and 4000 kN in compression mode

Loads and deformations (elongation) directly recorded by machine

Pipe dimensions measured before each test (length, outer diameter, thickness)

For more precise and complete data recording, strain gages used

Another advantage: transverse (hoop) strains can also be recorded; measure of anisotropy behavior (Poisson’s ratio)

Machine readings converted into stress-strain data

Stress-Strain Measurements

Expecting that deformations may be large (polymer pipe), very special high-deformation strain gages (TML company) procured

All strain gages hooked up to Dartech machine thru a data logger and computer

Two sets of strain gages (3 axial, 3 hoop) fixed on each test pipe, on top and bottom positions, on opposite sides of the pipe

Marked as TA1 (top axial-1), BH1 (bottom hoop-1), etc

Stress-Strain Measurements

Sets of strain gauges in axial and hoop directions, near top and bottom ends of pipe

SALT WATER AGEING

Soaking to be done for

3 PVC pipes of 7-m length (collapse)

18 sections of 1-m length (tension + compression)

All of 280 mm diameter

Required almost 5000 liters of saline water (roughly 1500 US gallons)

Very difficult and time consuming to

Produce this amount of distilled water

Dissolve enough salt to get required salinity

Soaking in Salt Water

Alternate (unique) solution

SQU desalination research unit near the sea

Seawater can be directly pumped through an underground tunnel and pumping system

Judicious mixing of normal drinking water and high-salinity seawater

Required salt concentration

Soaking in Salt Water

Challenges

Pumping of large amount of seawater (5000 liters)

Transportation from desalination plant to SQU

Special containers to be arranged to keep 18 pipes of 1-m length and 3 pipes of 7-m length under this saline water

Soaking in Salt Water

Transportation and storage of saline water

Immersion of 18 sections of 1-m PVC pipe (plain, built-up, and slotted) in saline water for 2-3 months soaking

DESIGN AND CONSTRUCTION OF EXPERIMENTAL SETUP

The proper study of mankind is the science of design.

Herbert Simon

Tension tests; Dartech universal testing machine

Main challenge; gripping of pipes

Three major issues or design constraints

Pipe dia was too large to go inside the gripping system of the machine

PVC surface was too smooth to provide enough friction for good grip

Too much load could not be applied in the grip region, or the PVC material would fracture here rather than in the main pipe section

Various gripping fixtures were designed, constructed, and tried out

Design and Construction of Tensile Test Setup

Idea based on application used in civil construction industry

Grout: a chemical resin mixed with water that is used for extra high strength reinforcement in concrete structures

A set of inside and outside metallic ring supports on each end of the PVC pipe

Prescribed gap between metal and PVC to pour in the grout mixture

After curing, grout would become a solid mass, exerting a large pressure and creating strong friction between metal and PVC

Hopefully, joint would not shear off under the maximum tensile load needed to fracture the main section of the pipe

Grout could be single-sided or double sided

Single-grout design would be tried out first, being more efficient in terms of time, cost, material, and effort

Grout Design

Design sketches for single-grout and double-grout designs

Exploded view of single-grout design for tensile test gripping fixture

Exploded view of double-grout design for tensile test gripping fixture

Exploded view of extended-length single-grout design for tensile test gripping fixture

Grout design was successful for slotted PVC pipes

Lowest tensile strength

Grout design failed in the case of plain and built-up pipes

Another gripping fixture designed and constructed

Based on the idea of a threaded contact between PVC pipe and inner support ring

Thread Design

Sketch of threaded-contact design for tensile test gripping fixture

Exploded and assembled views of threaded-contact design for

tensile test gripping fixture

TESTING UNDER TENSILE LOADS

Single-grout design used successfully for slotted PVC pipes

Both single and double grout designs failed for plain and built-up pipes

Grout connection sheared and broke off before pipe could fracture

Threaded connection also failed

PVC pipe fractured near the last thread of grip area rather than in the main pipe body

Various stages in preparation of single-grout tensile test fixture

Assembly of tensile fixture components

Pouring of grout in the gap between metal ring and PVC pipe

Grout curing

Assembly after grouting, etc.

Tension Testing

Single-grout tensile test setup; different stages of preparation and assembly

Testing of plain PVC pipe using single-grout fixture; grout assembly failed before pipe could fracture

Testing of plain PVC pipe using double-grout fixture; grout assembly

failed before pipe could fracture

Components of threaded-contact tension fixture, and different stages of

initial assembly

Various stages of final assembly of threaded-contact tension fixture

Testing of plain PVC pipe using threaded-contact fixture; pipe failed near last thread of grip area rather than in the main pipe body

RESULTS AND DISCUSSION

The system records force (kN), elongation/displacement (mm), and micro-strain data for each test

Later; data converted into plots of stress vs strain

Using average values of pipe dimensions measured before each test

Length, outer diameter, thickness

Pipe properties extracted from these graphs

Fracture stress

Fracture strain (ductility)

Elastic modulus

Poisson’s ratio or anisotropy (ratio of hoop strain to axial strain)

For elasticity value (Young’s modulus)

Curve fitting done for roughly 10% (initial linear portion) of graph

Tensile Test Results

Testing of slotted PVC pipe using single-grout fixture; pipe fractured in the middle section or near the end

Stress-strain plots for slotted pipe, without

aging; top and bottom positions; pipe-1 and

pipe-2

Stress-strain plots for slotted pipe, after

aging; top and bottom positions; pipe-1 and

pipe-2

Summary of Test Results

Good to observe that average values of stiffness of slotted PVC pipes are almost the same before and after aging

Value of elastic modulus determined by considering only linear portion of stress-strain curve, and fitting a straight line through these data points

Polymer pipes (not metallic); no portion of stress-strain diagram is really linear

So this straight-line fit is rather approximate

Discussion of Results

Effect of salt-water soaking on anisotropy behavior is not significant

Poisson’s ratio for slotted pipe (before and after ageing) remains in the 0.43-0.44 range

Decrease in pipe diameter (deformation in hoop direction) would be roughly 43-44% of increase in pipe length (deformation in the axial direction) under tensile loading

Discussion of Results

Surprisingly, tensile strength (maximum stress that slotted PVC pipes can withstand before tensile failure) after aging is almost 46% higher

This is of course good news for the field engineers

Discussion of Results

Acknowledgements

Authors acknowledge the support of Sultan Qaboos University (SQU) and Petroleum Development Oman (PDO) for this work.

Science never solves a problem without creating ten more.

Bernard Shaw

Many a thank!