OTC-25851-MS

Real Time Monitoring of Oil Based Mud, Spacer Fluid and PiezoresistiveSmart Cement to Verify the Oil Well Drilling and Cementing OperationUsing Model Tests

C. Vipulanandan, P. Ramanathan, K. Ali, and B. Basirat, CIGMAT-University of Houston; H. Farzam, Cemex;

W. J. Head, and J. M. Pappas, Research Partnership to Secure Energy for America

Copyright 2015, Offshore Technology Conference

This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 47 May 2015.

This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contentsof the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflectany position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without thewritten consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words;illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract

Well control operations are critical to ensure successful cementing of the oil wells with any losses. Withincreased drilling depths for production of oil and gas there are greater challenges due changes in thenatural geological formations with in situ pressure and temperature conditions. Recent case studies on oilwell failures have clearly identified cementing and drilling mud contamination as some of the issues thatresulted in various types of delays in the cementing operations. For a successful cementing operation, itis critical to monitor the drilling and cementing operation during the installation so that necessaryremediation can be made to minimize the delays and losses of cement. At present there is no technologyavailable to monitor cementing operations without using buried sensors within the cement sheath and alsomonitor the movement of the drilling mud and spacer fluid to determine the changes real time during theinstallation of oil or gas wells.

In this study, small well models were designed, built, and used to demonstrate the concept of real timemonitoring of the flow of smart drilling mud, space fluid and smart cement and hardening of the cementin place. Also, a new method has been developed to measure the electrical resistivity of the materials usingthe two probe method. Using the new concept, it has been proven that resistivity dominates the behaviorof drilling mud and smart cement. LCR meters (measures the inductance (L), capacitance (C) andresistance (R)) were used at 300 kHz frequency to measure the changes in resistance. Several laboratoryscale model tests have been performed using instrumented casing with wires and thermocouples. Whenthe drilling mud was in the model borehole the measured resistance was the highest based on the highresistivity of the drilling mud. Notable reduction in electrical resistance was observed with the flow ofspacer fluid and cement. Change in the resistance of hardened cement has been continuously monitoredup to about 100 days. Also, a method to predict the changes in electrical resistance of the hardeningcement outside the casing (Electrical Resistance Model ERM) with time has been developed. The ERMpredicted the changes in the electrical resistances of the hardening cement outside the cemented casingvery well. In addition, the pressure testing showed the piezoresistive response of the hardened smartcement.

IntroductionAs deepwater exploration and production of oil and gas expands around the world, there are uniquechallenges in well construction beginning at the seafloor. Also, preventing the loss of fluids to theformations and proper well cementing have become critical issues in well construction to ensure wellboreintegrity because of varying downhole conditions (Labibzadeh et al., 2010; Eoff et al., 2009; Ravi et al.,2007; Gill et al., 2005; Fuller et al., 2002). Moreover, the environmental friendliness of the mud andcements is a critical issue that is becoming increasingly important (Durand et al., 1995; Thaemlitz et al.,1999; Dom et al., 2007). Lack of mud and cement returns may compromise the casing support and excesscement returns cause problems with flow and control lines (Ravi et al., 2007; Gill et al., 2005; Fuller etal., 2002). Hence, there is a need for minimizing fluid losses by proper placement and monitoring of thedrilling mud and cementing operation in real time. Unexpected fluid losses could be addressed real-timeby changing the compositions of the drilling mud and/or cement slurry if real-time monitoring becomesavailable.

Successful deepwater cementing requires minimum fluid loss with drilling mud and cement slurry unitweights compatible with the formation (Eoff et al., 2009; Griffith et al., 1997). There are a number ofchallenges associated with installation of casing in deepwater. The challenges include low fracturegradients resulting from young unconsolidated sands and shallow drilling hazards, which may occur fromshallow water flows or hydrate formations, bottom-hole temperature and pressure, rapidly varyinggeological formations, and fluid loss, all because of a lack of real-time monitoring of the operations.Recent case studies in the Gulf of Mexico (GOM) have documented the use of specially formulatedlightweight foamed cement slurries to avoid cement sheath damage caused by shallow-water flows.

Two separate studies were performed on oil and gas well blowouts in the U.S. coastal area, one donebetween the years of 1971 to 1991, and the other study during the period of 1992 to 2006, before theDeepwater Horizon blowout in the Gulf of Mexico in 2010. The two studies clearly identified cementfailures as the major cause for blowouts [Izod et al., 2007]. Cementing failures increased significantlyduring the second period of study when 18 of 39 blowouts were due to cementing problems. Also theDeepwater Horizon blowout in the Gulf of Mexico in 2010, where there was eleven fatalities, was at leastpartially due to cementing issues [Carter et al., 2014]. With some of the reported failures and growinginterest of environmental and economic concerns in the oil and gas industry, integrity of the cement sheathis of major importance. At present there is no technology available to monitor the cementing operation realtime from the time of placement through the entire service life of the borehole. Also there is no reliablemethod to determine the length of the competent cement supporting the casing.

Drilling MudsDrilling mud is used for cuttings removal, suspension of cuttings, release of cuttings at the seafloor,minimizing formation damage, reducing filtration rate, cooling, and lubrication of the drill bit and drillstring, buoyancy support of the drill string and corrosion prevention. Drilling mud consists of two phases,namely a liquid phase (water or oil), which is the base, and a solid phase, which is comprised of clay andadditives. In general, the composition of drilling mud is selected based on the guidelines set by theAmerican Petroleum Institute (API). Drilling mud systems are broadly characterized as water, oil andsynthetic based muds.

Oil based muds (OBM) These are classified as invert emulsion or oil-based muds.

i. Oil-based muds: These are formulated only with oil as the base/liquid phase and are often used ascoring fluids. These systems might pick up water from the formation, but no additional water orbrine is added. Specialized oil-based mud additives include: emulsifiers and wetting agents(commonly fatty acids and amine derivatives), high-molecular weight soaps, surfactants, aminetreated organic materials, organo clays, and lime for alkalinity.

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ii. Invert emulsion muds: They are water-in-oil emulsions generally with calcium chloride brine asthe emulsified phase and oil as the continuous phase. They may contain as much as 50 percentbrine in the liquid phase. Concentration of additives and brine content/salinity are varied to controlrheological, filtration and emulsion stability.

Spacer FluidAchieving successful zonal isolation is an important step in oil or gas well drilling and completionoperations. The spacer must isolate drilling fluid from cement slurry to avoid development of largeinterval of contaminated cement which can lead to severe consequences of gas migration [Sarab et al.,2009; Miranda et al., 2007]. Also, the fluids and solids lost from slurries can cause damage by enteringthe formation and reducing its effective permeability, which in turn will decrease the wells productivity.Hence, to avoid all of these situations, a proper spacer mix should be employed before the cement ispumped.

Oil Well CementWell cementing is performed to provide a protective seal around the casing, prevent lost circulation and/ora blowout, and promote zonal isolation. The API standards suggest the chemical requirements determinedby ASTM procedures and physical requirements, determined in accordance with procedures outlined inAPI RP 10B and ASTM. There are several cement classes, A through H, which can be used to cementoil or gas wells.

Cement slurry flow ability and stability are the major requirements in well cementing. Oil and gas wellcements (OWCs) are usually made from Portland Cement clinker or from blended hydraulic cements.OWCs are classified into grades based upon their Ca3AlnOp (Tricalcium Aluminate C3A) content:Ordinary (O), Moderate Sulphate Resistant (MSR), and/or High Sulphate Resistant (HSR) types. Eachclass is applicable for a certain range of well depth, temperature, pressure, and sulphate environments.OWCs usually have lower C3A contents, are coarsely ground, and may contain friction-reducing additivesand special retarders such as starch and/or sugars in addition to or in place of gypsum.

Cements such as Class G and Class H are considered to be two of the most used cements in OWCapplications. These cements are produced by pulverizing clinker consisting essentially of calcium silicates(CanSimOp), with an addition of calcium sulphate (CaSO4) (John, 1992). Class H cement is produced bya similar process, except that the clinker and gypsum are ground relatively coarser than for a Class Gcement, to provide a cement with a surface area generally in the range 220 300 m2/kg (John, 1992).

When admixtures are added with cement, tensile and flexural properties are modified. Also, admixtureswill have an effect on the rheological, corrosion resistance, shrinkage, thermal conductivity, specific heat,electrical conductivity and absorption (heat and energy) properties of cement (Bao-guo, 2008). Cementslurries are pumped down the casing several thousand feet below ground level and back up the outside ofthe casing (casing annulus) formation; hence, because of the changes in velocities, pressures, andtemperatures and unknown hole sizes, as well as mixing and cement formation interaction questions,determining cement setting time is always challenging.

ObjectiveThe major objective of this work was to demonstrate the monitoring of the installation of oil or gas wellsusing smart oil based drilling mud, smart spacer fluid, and smart cement; also, to investigate monitoringof the curing of the cement sheath outside the casing, to include the piezoresistive response of smartcement to applied pressures.

The specific objectives were as follows:

1. Monitor changes in electrical resistance outside the casing during the installation.2. Monitor and predict changes in electrical resistance of the hardening cement sheath outside thecasing.

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3. Verify the piezoresistive response of the cement sheath.

Theory and ConceptsIt was very critical to firstly identify the sensing properties for the cement and drilling mud that can beused to monitor the performance. After years of studies and based on the current study on well cementsand drilling muds, electrical resistivity () was selected as the sensing property for both cements anddrilling muds. This is unique since in that the same monitoring system can be used to evaluate theperformance of cement and drilling muds. Hence, two parameters (resistivity and change in resistivity)will be used to quantify the sensing properties as follows:

(1)

where:

R electrical resistanceL Linear distance between the electrical resistance measuring points A effective cross sectionalareaK Calibration parameter is determined based on the resistance measurement method

Normalized change in resistivity with the changing conditions can be represented as follows:

(2)

Resistivity of the materials () to changes (composition, curing, stress, fluid loss, and temperature) hasbeen quantified. Correlating the changes, such as composition, curing, stress, cracking, fluid loss, andtemperature, to the resistivity () (Eqn. (1)) and change in resistivity () (Eqn. (2)) will support themonitoring of the material (cement and drilling mud/fluid) behavior.

Impedance Spectroscopy Model (Vipulanandan et al., 2013)

Equivalent Circuit Identification of the most appropriate equivalent circuit to represent the electricalproperties of a material is essential to further understand its properties. In this study, an equivalent circuitto represent smart cement and smart drilling mud was required for better characterization through theanalyses of the IS data.

There were many difficulties associated with choosing a correct equivalent circuit. It was necessary tosomehow link the different elements in the circuit to different regions in the impedance data of thecorresponding sample. Given the difficulties and uncertainties in establishing this link, researchers tendto take a pragmatic approach and adopt a circuit which they believe to be most appropriate from theirknowledge of the expected behavior of the material under study, and demonstrate that the results areconsistant with the circuit used.

In this study, different possible equivalent circuits were analyzed to find an appropriate equivalentcircuit to represent smart cement and drilling mud.

Case 1: General Bulk Material Resistance and Capacitor In the equivalent circuit for Case1, thecontacts were connected in series, and both the contacts and the bulk material were represented using acapacitor and a resistor connected in parallel (Fig. 1).

Figure 1Equivalent Circuit for Case 1

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In the equivalent circuit for Case 1, Rb and Cb are resistance and capacitance of the bulk material,respectively; and Rc and Cc are resistance and capacitance of the contacts, respectively. Both contacts arerepresented with the same resistance (Rc) and capacitance (Cc), as they are identical. Total impedance ofthe equivalent circuit for Case 1 (Z1) can be represented as follows:

(3)

where is the angular frequency of the applied signal. When the frequency of the applied signal is verylow, 0, Z1 Rb 2Rc, and when it is very high, , Z1 0.

Case 2: Special Bulk Material Resistance Only Case 2 is a special case of Case 1 in which thecapacitance of the bulk material (Cb) is assumed to be negligible (Fig. 2). The total impedance of theequivalent circuit for Case 2 (Z2) is as follows:

(4)

When the frequency of the applied signal is very low, 0, Z2 Rb 2Rc, and when it is very high, , Z2 Rb (Fig. 3).

Figure 2Equivalent Circuit for Case 2

Figure 3Comparison of Typical Responses of Equivalent Circuits for Case 1 and Case 2

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Testing of smart cement, smart spacer fluid, and smart drilling mud clearly indicated that Case 2represented their behavior at a frequency of 300 kHz.

Materials and MethodsIn this study, oil based drilling mud spacer fluid resistivity and smart cement were used.

Oil based mud Oilbased mud was prepared by mixing mineral oil, water and cetyltrimethylammoniumbromide (CTAB) surfactant together. The ratio of oil to water was 4:1. Initially, 1 percent of cetyltrim-ethylammonium bromide surfactant was mixed slowly with water, and then oil was added gradually to thesurfactant solution. The density of the oil based mud was 7.2 ppg and the electrical resistivity was 110.m.

Spacer fluid The spacer fluid was prepared by mixing 0.5 percent Guar gum, 0.4 percent bio-Surfactant,and 3 percent KCl. The density of the spacer fluid was about 8.3 ppg (1 g/cc) and the electrical resistivitywas 0.8 .m. Class H cement was used with a water-to-cement ratio 0.38. The cement was modified withan addition of 0.075 percent carbon fiber by total weight of the cement slurry.

Smart cement Class H cement was used with a water-to-cement ratio 0.38. The cement was modifiedwith an addition of 0.075 percent conductive filler by total weight of the cement slurry. The initialresistivity of the cement slurry was 1.06 .m. Typical piezoresistive response after one day of curing ofthe smart cement with water-to-cement ratio of 0.38 used in this study is shown in Fig 4.

Model Laboratory models were designed and built at the University of Houston. The model was built tomonitor the slurry level (drilling mud, spacer fluid, and cement) during the installation and hardening ofthe cement. The observed resistance with time clearly indicated the level of slurry, which could be usedto determine the depth at which the drilling fluid and cement were located. Several models were built andtested separately to demonstrate real-time monitoring.

The model was built using a plexiglass and metal pipe, as shown in the Fig. 5, to simulate the formationand casing. The casing was instrumented with electrical wires to monitor the resistance change. Thedistance between two sensors was 4 to 6 inches, and there were six levels of sensors, as shown in the Fig.

Figure 4Typical Piezoresistivity Behavior of Oil Well Cement with 0.075% CF After One Day of Curing (AC frequency 300 kHz)

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5. Different combinations of the sensors were connected to a 300 Hz LCR device to measure resistancebetween those sensors.

The horizontal electrical wire leads (sensors/monitors) are noted as a, b, c, or d. The vertical wire leadsare marked by the numbers 1 to 6 to differentiate the various levels and simulate well depth. Figure 5shows the different levels of liquid in the simulated well.

Results and DiscussionApproximately 10 L of drilling mud was initially placed at the bottom of the model in the annulus of

the well model via a pressure chamber, as shown in the Fig. 6. The electrical resistances of differentcombinations of sensors were measured at each level to determine the effect of the drilling mud as moremud was added to the annulus. After measuring the resistance for all seven levels, the drilling mud wasgradually replaced by 10 L of spacer fluid by applying pressure of around 20 psi from the pressurechamber. The same procedure was followed for the spacer fluid as was performed with the drilling mudin order to measure the resistance when the spacer fluid reached each level.

Figure 5Laboratory Scale Oil Well Model and Monitoring System

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After measuring the resistance of the spacer for all seven levels, it was replaced by the cement slurry.This too was done by applying pressure of about 20 psi from the pressure chamber, and same procedurewas followed as before to continuously monitor the electrical resistances between the selected wire leads.

Installation Monitoring the drilling fluid: Resistance was measured for different vertical levels in thesmall well model with time, as shown in Figure 7. When there was no fluid in the well (at 0 level of thecasing) the resistance was in the range of 155 to 205 k, which represented the air resistance for theparticular distance monitored at the relative humidity of the laboratory. When the oil based mud level roseto level 1, all of the vertical resistances started to decline to the 35 to 60 k range. This sudden changeclearly showed that oil based mud reached level 1. Similar reductions in electrical resistances wereobserved with horizontal measurements.

Figure 6Experimental Setup of Small Model #2 Where Cement is Displacing Spacer Fluid

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When the oil based mud reached level 2 (as observed through the plexiglass model), the sensors a1-a2(resistance in the vertical direction between level 1 and 2) showed a sudden change in the electricalresistance: its resistance dropped to 14 k from about 36 k. The change occurred because the drillingmud, with a resistivity of 110.m, filled the space the between level 1 and 2. When the mud reached level3 (as observed through the plexiglass model), the resistance between a1-a3 also dropped. The same patternwas also observed for the other sets of readings. This consistent behavior showed that the level of thedrilling fluid can be monitored effectively by measuring the resistance. The electrical resistance changesobserved during the placement of the drilling fluid was very similar to the first model.

Monitoring the spacer fluid: The same procedure that was used to monitor the drilling fluid wasfollowed for the spacer fluid. Figure 8 shows the changes in the resistance readings for the spacer fluid.When the spacer fluid filled the annuli the resistance rapidly declined because of the lower resistivity ofthe spacer fluid.

Figure 7Vertical Resistance Measurements for the Oil Based Drilling Fluid (b) Monitoring the Spacer Fluid

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Monitoring the cement slurry: After totally filling the model with the spacer fluid, it was displaced bythe cement slurry. The same procedure was followed, and Figure 9 shows the resistance readings for thecement slurry as a function of vertical level.

The resistance was in the range of 25 to 35 when there was no cement slurry in the well (at 0 levelof water), representing the presence of the spacer. All of the vertical resistances increased to between 55and 67 when the cement slurry reached level 1. This change clearly showed that the cement slurryreached level 1. The increase occurred because the resistivity of the cement was higher than that of spacerfluid. The cement became contaminated with the spacer fluid in the process of displacing it, and thereforethe resistance values dropped to a range of 30 to 50 between level 1 and 2. Based on the increase in

Figure 8Vertical Resistance Measurements for Spacer Fluid (c) Monitoring the Cement Slurry

Figure 9Vertical Resistance Measurements for Cement Slurry

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the resistance, one can conclude that no contamination occurred at level 1. Also, the dehydration processin the cement caused the resistance to increase over time because its resistivity increased with time as itcured.

Continuous monitoring the fluids used in the model: Figure 10 shows the resistance change with timein the well for different types of the test fluids. Air resistance initially dropped to the oil based drillingmuds resistance, and it was followed by a decline to a resistance representing the spacer fluid and cementslurry.

The variation of resistance with time clearly showed that the level of the slurry can be monitored bymeasuring the resistance with time.

Curing of Cement SheathResistivity of the smart cement: The resistivity of the cement slurry with curing time of up to 100 days

was determined using smart cement slurry samples (in a 2 inch diameter and 4 inch high cylindrical mold)that was used for the small model #3 study. The resistivity increased with curing time under roomtemperature and humidity (Figure 11).

Figure 10Vertical Resistance Measurements in the Oil Well Model

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Predicted (Electrical Resistance Model -ERM) and measured resistance for hardening cement: Usingthe parameters K and the resistivity-time relationship (Fig. 11), the changes in the cement sheathresistance in the small model#3 were predicted using the relationship in Eqn. (1). Figures 12 throughFigure 17 show the variations of the predicted resistance value and also the actual measured values fordifferent wire setup/combinations.

Vertical resistance: We now describe the wire setups, which are used to determine resistance as afunction of curing time.

Wire setup-a: For the wire setup-a, the wire combination a1-a2 showed that the predicted values werelower than the measured values for up to 14 days of curing, but after that time the measured resistance

Figure 11Variation of Smart Cement Resistivity with Curing Time for Samples Cured under Room Conditions (23C and 50% relativehumidity (RH))

Figure 12Comparing the Predicted and Measured Resistance for Wire Setup-a for Wire Combination a1-a2.

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values were in the range of the predicted resistance (Figure 12). This may be because in the small model#2 the cement was dehydrating under pressure and temperature, whereas the resistivity used (Fig. 11) topredict the resistance was cured under room conditions under atmospheric pressure.

For wire setup-a wire combination a1 and a4, the measured values were very close to the predictedvalues (Figure 13). The wire at level a4 was very close to the surface of the small model, which showedvery similar hydration to the test sample (Fig. 11). The measured resistance values matched very well withthe predicted values.

Wire setup-b: For wire setup-b the wire combination b1 and b3 showed that the predicted values werelower for about 30 days of curing (Figure 14). The initial hydration of small model was different from thesamples curing in the 2-inch x 4-inch mold under room condition. The measured values matched very wellwith the predicted values after 1 month since the curing temperatures were very similar.

Figure 13Predicted and Measured Resistance for Wire Setup-a for Wire Combination a1-a4

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The measured values for wire combination b1 and b4 were very close to the predicted values (Figure15). A similar trend was observed with wire a1 and a4 (Fig. 13).

Wire setup-c: For wire setup-c the wire combination c1 and c2 initial predicted values were slightlylower than the measured resistance as observed with either setup-a or setup-b. In this region the cementwas curing at a higher temperature and pressure than mentioned before. The measured resistance after twoweeks of curing matched very well with the predicted resistance (Fig. 16).

Figure 14Predicted and Measured Resistance for Wire Setup-b for Wire Combination b1-b3

Figure 15Predicted and Measured Resistance for Wire Setup-b for Wire Combination b1-b4

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And the measured values for wire combination cl and c4 are within the range of the predicted values(Figure 17), similar to what was observed with setup-a and setup-b.

Pressure TestTo simulate a pressure test, air pressure (Pi was applied inside the casing to load the cement-sheath and

the electrical resistances (Ro in Ohms) that measured between Level 1-2, Level 2-3, Level 3-4, and Level4-5 (Figure 6). These values were monitored while the air pressure was applied inside the casing (Figure18).

Figure 16Predicted and Measured Resistance for Wire Setup-c for Wire Combination c1-c2

Figure 17Predicted and Measured Resistance for Wire Setup-c for Wire Combination c1-c4

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Case 1: Initial Condition (No pressure, Pi 0): The variation of initial resistance is shown in Figure19. The initial resistance was higher at the bottom (level 1-2) due to weight of the cement sheath and waslower at the top level (level 4-5). The electrical resistance increased with depth and varied from 400 to800 ohms. This is partly due to the piezoresistive property of the smart cement, in which the electricalresistance will be higher because of an increase in pressure with depth.

Figure 18Configuration of the Pressure Applied Inside the Steel Casing

Figure 19Variation of Initial Resistance with Depth after 100 Days of Curing

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Case 2: Pi 60 psi: Internal pressure of 60 psi was applied and the resistance changes were measuredafter 26 hours. The change in resistance was normalized with initial resistance R/R(%) is shown inFigure 20. The resistivity change in the smart cement due to the applied pressure was about 0.5 to 0.6percent, indicating the piezoresistivity of the smart cement.

Case 3: Pi 100 psi: Pressure of 100 psi was applied and the resistance changes were measured after26 hours and reported in the form of R/R (%) in Figure 20. The resistivity change in the smart cementdue to the applied pressure of 100 psi was about 2.5 percent, indicating the piezoresitivity of the smartcement.

Case 3: Pi 140 psi: Internal pressure of 140 psi was applied and the resistance changes weremeasured after 26 hours and reported in the form of R/R in Fig. 20. The resistivity cha nge in the smartcement due to the applied pressure was about 6.5 to 7 percent, indicating the piezoresitivity of the smartcement.

Piezoresistive modeling: The stress at every point can be separated into mean stress and deviatoricstress. The change in the deviatoric stress due to an applied pressure (Pi) along the axis of the casing(z-axis) is represented as Szz. Using equilibrium and stress analyses, it can be shown that Szz is directlyproportional to the applied internal pressure, Pi (Eqn 5). Hence, the change in deviatoric stress can berepresented as follows:

(5)

The variation of internal applied pressure with the resistivity of smart cement (z/z) is shown in Fig.21, and the response of the smart cement is shown to be nonlinear.

Figure 20Changes in Resistance with Applied Pressure for Smart Cement after 100 Days of Curing

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p-q model (Vipulanandan et al. (1990))The nonlinear p-q model was developed by Vipulanandan et al. (1990) and was used to predict z/z

variation with the applied pressure. The relationship can be represented as follows:

(6)

The model parameters p and q were 1.2 and 3, respectively. Hence, it is possible to predict the pressurein the casing and also the stress in the cement sheath by measuring the change in resistivity of the smartcement.

ConclusionsBased on the resistivity monitoring and model study following conclusions are advanced.

1. The two-probe method was effective in measuring the bulk resistance of the drilling mud, spacerfluid, and smart cement slurries. Based on the changes in resistance measurements it will bepossible to identify the fluid rise in the well borehole.

2. Using the laboratory model it was possible to demonstrate the real-time monitoring of the wellbore with drilling mud, spacer fluid, and smart cement slurries.

3. Based on the concept developed in this study, it is possible to use the K parameter to predict thechanges in the resistance of drilling and hardening smart cement.

4. Using a nonlinear p-q model the change in electrical resistivity of smart cement was related to theapplied pressure in the casing. The smart cement was very sensitive to the applied pressure.

AcknowledgementsThis study was supported by the Center for Innovative Grouting Materials and Technology (CIGMAT) atthe University of Houston, Texas. Funding for the project (Project No. 10121-4501-01) is providedthrough the Ultra-Deepwater and Unconventional Natural Gas and Other Petroleum Resources Researchand Development Program authorized by the Energy Policy Act of 2005. This programfunded from

Figure 21Model Prediction of Changes in Resistivity with Applied Pressure for Smart Cement after 100 Days of Curing

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lease bonuses and royalties paid by industry to produce oil and gas on federal landsis designed to assessand mitigate risk enhancing the environmental sustainability of oil and gas exploration and productionactivities. RPSEA is under contract with the U.S. Department of Energys National Energy TechnologyLaboratory to administer three areas of research. RPSEA is a 501(c)(3) nonprofit consortium with morethan 180 members, including 24 of the nations premier research universities, five national laboratories,other major research institutions, large and small energy producers and energy consumers. The missionof RPSEA, headquartered in Sugar Land, Texas, is to provide a stewardship role in ensuring the focusedresearch, development and deployment of safe and environmentally responsible technology that caneffectively deliver hydrocarbons from domestic resources to the citizens of the United States. Additionalinformation can be found at www.rpsea.org.

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OTC-25851-MS 21

Real Time Monitoring of Oil Based Mud, Spacer Fluid and Piezoresistive Smart Cement to Verify th ...IntroductionDrilling MudsOil based muds (OBM)

Spacer FluidOil Well Cement

ObjectiveTheory and ConceptsImpedance Spectroscopy Model (Vipulanandan et al., 2013)Equivalent CircuitCase 1: General Bulk Material Resistance and CapacitorCase 2: Special Bulk Material Resistance Only

Materials and MethodsOil based mudSpacer fluidSmart cementModel

Results and DiscussionInstallation

Curing of Cement SheathPressure Test

Conclusions

AcknowledgementsReferences