SPE 112497

Evaluation of Formation Damage/Completion Impairment Following Dynamic Filter-Cake Deposition on Unconsolidated Sand V.G. Constien, SPE, Constien & Associates

Copyright 2008, Society of Petroleum Engineers This paper was prepared for presentation at the 2008 SPE International Symposium and Exhibition on Formation Damage Control held in Lafayette, Louisiana, U.S.A., 13–15 February 2008. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers 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 SPE copyright.

Abstract Evaluation of formation and completion (gravel pack and screen only) flow capacity damage in unconsolidated formation material has proven difficult in the laboratory because typical core flood equipment does not adequately confine the formation material to allow drilling and cleanup fluids to be circulated at representative shear rates and pressures over the formation face. A testing protocol and equipment have been developed which allows dynamic flow and fluid loss across unconsolidated formation material at shear rates, temperatures, differential pressures, and times representative of mud circulation rates in the wellbore. The dynamically prepared filtercakes grow to an equilibrium thickness depending on the shear rate, differential pressure, and temperature and are more representative in composition and mass to wellbore filtercakes so that their damage to formation, gravel packs and / or screens can be better evaluated. The effectiveness of cleanup systems can also be determined by circulating the fluids across the mud filtercakes at typical shear rates, differential pressures and temperatures before allowing static soak conditions. At the end of the test, the screen and formation material or screen, gravel pack and formation material are stressed together simulating a wellbore collapse. The permeability of the formation plus gravel pack and screen or formation and screen only are then measured and compared to the initial values to determine flow capacity impairment. Introduction Openhole completions in poorly consolidated formations require that the drill-in fluid, the sand control system and the cleanup system for removal of the drill-in fluid filtercake all perform together in order to provide acceptable solids control and production rates. Laboratory evaluation techniques for drilling fluid filtercake damage to the flow capacity have generally involved the generation of filtercakes on consolidated formation material or cores in a static fluid loss condition. The fluid loss into the formation and the properties of the filtercake have such a direct bearing on the results of any laboratory test which attempts to evaluate cleanup systems and flow capacity damage. It is important to create the filtercake in a manner which closely resembles the wellbore conditions. This is especially true when optimizing systems for use in completions in soft sands which will have gravel packs or screen only completions. The remaining filtercake in a wellbore can have a severe impact on the flow capacity of a sand control screen or gravel pack and screen.1,2,3 The present work discusses a method for creating dynamically deposited filtercakes on unconsolidated formation material and determining the flow capacity damage to the sand control method and formation resulting from the filtercakes. Conducting dynamic fluid loss and flow capacity to screens using unconsolidated formation material presents several experimental challenges. Several authors have described dynamic fluid loss techniques for depositing filtercakes for fracturing

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and drilling fluids. The majority of work on drilling fluids has involved stirred fluid loss cells4 on ceramic discs or consolidated core material5. The stirred fluid loss cells have generally not been designed to quantify the shear rate being applied to the mud during the test but instead have used a stirring apparatus which changes the rpm of the mixer located in the mud above the porous media. While this is an improvement over static tests, correlation to wellbore conditions is still difficult. Also, these devices do not provide simulation of the formation material and remaining filtercake being produced onto the screen or gravel pack. The literature related to dynamic fluid loss testing of hydraulic fracturing fluids is more extensive and has defined important variables such as the geometry of the cells and flow conditions for studying particle migrations toward porous media 6. Again, these tests were run on consolidated core material but do give insights into cell design and flow entrance effects which are useful when simulating representative flow conditions. Unconsolidated formation material varies in permeability from the low millidarcy range to several darcys resulting in mud filtercakes which form primarily on the surface of the material to some invasion resulting in plugging in pore throats deeper into the formation material. In either case, the thickness and to some extent the composition of the filtercake is controlled by the shear stress exerted by the fluid flowing in the wellbore annulus and the yield stress of the cake7. The cakes grow until the fluid stress is equal to the yield stress of the filtercake. For drilling fluids which are carrying drill solids and particulate fluid loss additives, the filtercakes will contain those particulates which reach the formation surface and are not wiped off by the shear force driving particles tangentially to the wall8, 9

. This may result in different concentrations and sizes of solid particles and also for some of the soluble components of the fluid. Description of the Laboratory Equipment and Test The apparatus used in the present studies utilizes flow through a rectangular geometry which includes a cylindrical section containing a sample of unconsolidated formation material. The cells are designed so that the initial permeability of the formation material can be obtained in a stressed condition between sand control screens. The upper screen is then backed off the formation face leaving the formation face open to the bottom of the slot flow cell. The slot width is identical to the diameter of the formation face and the length of the slot on the entrance and exit to the formation material is long enough so that laminar flow of the mud is obtained across the formation. Differential pressure is maintained across the formation so that the formation and filtercake remain stable during mud and cleanup systems flow. Temperature is controlled by several heating zones which monitor and control different parts of the cells and flow lines. The cells are designed to be attached in series to a mud circulating system which controls the flow rate and pressure drop across the formation face. Table 1 contains the specifications for the device. Figure 1 is a schematic of the associated flow equipment. Figure 2 is a picture of a three cell setup in the dynamic flow loop and Figure 3 is a picture of a single test cell. The equipment was designed for use in a commercial testing environment where the entire unit could be dismantled and thoroughly cleaned between tests and returned to service quickly.

Test Condition Equipment Maximum Temperature (oF) 300 Differential Pressure (psi) 800 Shear Rate Across the Formation Face (s-1) 300 Maximum # of test Cells 4 Formation Pack length (in) 0.5 – 6.0

Table 1: Test Equipment Set-up The cells are designed so that screen only or screen and gravel pack tests can be run simulating an open annulus or wellbore collapse environment. The screen samples are mounted with appropriate shroud and drain layer components if so desired. The initial permeability of the screen is obtained before the test begins and the final screen permeability at the conclusion of the test. The formation material is prepared in similar fashion as would be done for a gravel pack or screen only qualifying test.10 The material is split to the desired sample size and the particle size determined for each sample before loading into the cell. The length of the pack can be adjusted to up to several inches: however, it has been found that generally about one inch in length is sufficient for most permeability ranges. This length is particularly useful because the amount of formation material available for testing is usually limited. The formation material is often prepared as concentrated slurry in brine prior to loading into the cell to ensure that all of the particles are water-wet and to aid in compaction. The formation material is then loaded into the test cell where uniaxial compression stress is applied to the formation. The confinement stress is at least as much as the differential pressure to be applied during the DIF leakoff portion of the test. The cell is heated to the test temperature and then flow initiated in the production direction and the initial permeability determined. At this point the upper confining screen maybe be backed out

SPE 112497 3

from the surface of the formation material and removed from the cell. The slot above the formation material is left open to simulate a wellbore annulus. The test cells containing the unconsolidated formation material are connected to a circulating loop and drill-in fluid (DIF) loaded into the flow lines and cell. The DIF is circulated at the rate to achieve the desired shear rate at the surface of the formation material based on the wellbore data. Fluid loss lines remain closed until the mud has reached the desired test temperature and the differential pressure. These conditions are all determined based on the wellbore conditions for drilling the production interval. A typical set of test conditions is shown in Table 2. Appendix A gives more information on the test procedure. Before the testing can begin, there are several steps which must be completed to determine the conditions for the tests. The first step in the process is to calculate the annular flow shear rate conditions of the mud in the actual wellbore. The shear conditions are then matched in the slot flow geometry above the formation material by selecting the gap width and the pump rate. Temperature control is also critical and may change between circulating and static conditions. The differential pressure across the formation material is determined from the equivalent circulating densities during mud flow and the differential pressure during static conditions. Finally, the dynamic fluid loss and static fluid loss time are selected. The resulting test schedule is then programmed into the test equipment. At the completion of the mud fluid loss, cleanup flushes and soaks may be run before the screen or gravel pack is placed into the test cells and stressed into the formation material. Flow in the production direction is then initiated and the final permeability of the formation and screen or screen and gravel pack is determined. The screen is then removed and the final permeability of the screen is determined. The resulting data is used to compare flow capacity for different mud and cleanup systems. Equally important to getting the flow, differential pressure and temperature conditions to simulate the wellbore environment is the mud sample itself. The filtercake properties will also be very dependent on the composition of the mud sample and particularly the concentration and particle size of solid fluid loss additives and the drill solids from the wellbore. It is very helpful to have representative mud samples which have been analyzed for composition including drill solids content and particle size distribution. If laboratory prepared mud samples are to be used in the testing, representative drill solids should be added to the mud at the concentrations expected during drilling.

Test Parameters Conditions

Well Conditions: Hole Size ID Nominal (inches) Drill Pipe OD (inches) Mud Circulation Rate (gpm)

9.0 5.5 500

Wall Shear Rate in the Annulus (sec-1

) 166

DP for Mud Fluid Loss (psi) 500

Mud Temperature (oF)

Circulation Static

120 120

Dynamic Fluid Loss Time (hr) 4

Static Fluid Loss Time (hr) 4

Drill-in Fluid Water-based Drilling Fluid

Drill Solids (ppb ) 15 ppb shale + 35 ppb sand

Core Saturation Fluid 3% KCl

Fluid for Flowback 3% KCl

Table 2: Example of Test Conditions

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Figure 1: Schematic of the Dynamic Fluid Loss and Cleanup Equipment Figure 2: Three Cell Dynamic Fluid Loss Test Equipment Figure 3: Dynamic Fluid Loss Cell Dynamic and Static Fluid Loss on Unconsolidated Formations. Figure 4 illustrates the effect of dynamic fluid loss for a water-based DIF on three unconsolidated formations. The test parameters are shown in Table 2. The formation material consisted of three different particle size distributions which were representative of the sizes expected in the wellbore. A water-based DIF was tested to evaluate how effective the fluid loss control additive system would be at limiting losses on the different formation sands and for the impact on the resulting flow capacity damage for a screen only horizontal completion. The drill-in fluid was pumped at a constant shear rate at the formation surface of 166 s-1 for four hours followed by four hours of static fluid loss. A clear transition in fluid loss rate can be seen when the mud circulation is stopped. In this case the differential for the dynamic and static portions of the tests was the same at 500 psi. Figure 5 shows one of the formation samples when it was removed from the cell. The filtercake can be seen on top of the formation pack.

Cell Cell

Fluid Loss

System Pressure Control

Permeability Fluid -Production Direction

Back Pressure Control and Volume Measurement for Permeability

Mud Pills, Spacers, Flushes,

Cell

Discharge Cell with Pressure Control

Pump

Fluid Loss

Fluid Loss

SPE 112497 5

Figure 5: Formation Material after Test Figure 4: Dynamic and Static Fluid loss on Unconsolidated Formation Material Effect of Fluid loss Additive Concentration. The dynamic fluid loss test is especially useful in determining the effects of solids in the mud on fluid loss into the formation material. In this example, the test conditions shown in Table 2 were repeated with one of the sizes of formation material from Figure 1. The same DIF was used in the test, but with three concentrations of calcium carbonate fluid loss material. The particle size distribution of the calcium carbonate in the DIF was kept constant. Figure 5 presents the fluid loss data versus total calcium carbonate concentration. For this combination of formation particle size and DIF, the highest concentration of fluid loss control material gave the least amount of fluid loss. The next step of the process is then to evaluate the flow capacity damage to the screen or gravel pack completion for the different DIF formulations with and without cleanup treatments. Figure 6: Dynamic and Static Fluid loss with Three Concentrations of Fluid Loss Control Additives Flow Capacity Damage to Screens. Each of the filtercakes created in the tests with increasing fluid loss control additives were evaluated for screen damage. The cells containing the filtercakes were treated with a displacement system containing a push pill and flush. The formation screen was then placed into the cells and the remaining filtercake and formation material stressed against the screen. The results showed significant flowcapacity damage to the system with increasing calcium carbonate concentration in the DIF and indicated that a cleanup system capable of dissolving calcium carbonate would be necessary to remove the fluid loss material in the remaining filtercakes.

Filtercake

Fluid Loss vs. Time

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ss (g

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50 ppb Calcium Carbonate25 ppb Calcuim Carbonate0 ppb Calcium Carbonate

Fluid Loss versus Time

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Figure 7: Retained Permeability of the Formation plus Screen vs. Fluid loss Additive Concentration Evaluation of Cleanup Systems for a DIF / Formation / Screen System. In the next example, filtercakes were once again created under dynamic fluid loss conditions and then treated with ether push pills and spacers or push pill, spacer and organic acid. Figure 8 is the time for breakthrough for the filtercake treated with the organic acid. After about 600 minutes, the cleanup treatment had degraded the filtercake to the point where fluid loss control could no longer be maintained. After allowing an additional 48 hours soak time, the remaining filtercake and formation material was stressed into the screen and the final permeability of the formation plus screen and screen only were determined. The results are given in Table 3. The acid system was effective in increasing the permeability of the screen and formation to 51% of the clean formation plus screen values. Figure 8: Breakthrough Time for the Acid Cleanup Soak

% Retained Permeability after Cleanup Treatment

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0 10 20 30 40 50 60ppb Calcium Carbonate

% R

etai

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l)

CellA

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Screen Permeability (darcy)

Formation plus Screen Permeability

(md) Cleanup Stages

Initial Final Initial Final

Retained %

Push Pill / Spacer / Organic Acid

384 339 97 49 51

Push Pill / Spacer / no acid 357 166 120 2.2 1.8

Table 3: Permeability Results Filtercake Mass. The mass and composition of drilling fluid filtercake remaining on the formation material is a function of several variables including formation pore size and mud composition including solids concentration, particle size and shape. The filtercake can be very damaging to the flow capacity of screen only or gravel pack completions and be challenging to remove. The flow conditions in the annulus during mud circulation along with the composition of the mud determine the equilibrium filtercake thickness and composition that forms under flow conditions. Figure 9 illustrates filtercake mass remaining on the formation surface at the end of dynamic fluid loss stage at three different shear rates for a particular mud. The composition of the filtercake is shown in Table 4. Different mud compositions and flow regimes can greatly alter the amount of filtercake deposited on the formation as well as the filtercake composition.

Filtercake Analysis for 310sec-1 Test (wt. %)

Calcium Carbonate 20.6

Total Polymers (Xanthan plus Starch) 6.9

Drill Solids 73

Table 4: Filtercake Composition Figure 9: Filtercake Dry Weight for a Water-based DIF Filtercake Plugging of Gravel Packs. Filtercake plugging of screens and gravel packs has been shown to be an important consideration in designing sand control treatments1. The current method may also be used to evaluate gravel packed completions. The primary difference in the method is that an extra test is required to measure the initial gravel pack permeability as well as the gravel pack plus formation + screen permeability. This data is necessary to compare to the tests with filtercake. Figures 10 and 11 illustrate damage to gravel packs and screens from remaining filtercake. The filtercake was not removed before stressing the gravel pack and screen into the formation material.

FilterCake (lb/ft2) vs. Shear Rate

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Filte

r Cak

e W

eigh

t (lb

/ft2 )

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Figure 10: Filtercake Damage to Gravel Pack Permeability Figure 11: Filtercake Damage to Screen

Summary The challenge has been to design a small scale laboratory dynamic fluid loss test using unconsolidated formation material that could create DIF filtercakes under appropriate flow, temperature and differential pressure conditions without washing out the formation material. This test method and equipment has been used extensively to create filtercakes on unconsolidated formation material at shear rates, temperature, and differential pressures expected in the wellbore. By isolating the test cells in the mud flow loop, up to four different cleanup treatments can be run on filtercakes that have been identically prepared. This feature results in better comparative data between systems and also considerable time savings. The method evaluates the resulting flow capacity of the sand control system when stressed against remaining DIF filtercakes and formation material. The design of the system allows complete tear down and cleaning to be performed between tests resulting in a reasonable test cycle time. Conclusions The test method requires that attention to detail be performed in each step of the process. Filtercakes formed during the mud flow and fluid loss stage of the method create filtercakes more representative in composition and thickness of those formed under flow conditions in the wellbore. The method is also able to evaluate the flow capacity damage resulting from remaining DIF filtercake on the formation and the sand control screen or gravel pack. The effectiveness of filtercake cleanup systems can also be determined. References

1. Burton, Robert C., Hodge, Richard M., “The Impact of Formation Damage and Completion Impairment on Horizontal well productivity,” paper SPE 49097, SPE Annual Technical Conference and Exhibition, New Orleans, LA, 1998

2. Burton, Bob, Hodge, Richard, “Drilling and completion of Horizontal Wells Requiring Sand Control,” paper SPE 52812, SPE IADC Drilling Conference, Amsterdam, The Netherlands, 1999.

3. Bennett, J. M., Hodge, R. M., “Design Methodology for Selection of Horizontal Open-Hole Sand Control Completions Supported by Field Case Histories, paper SPE 65140, SPE Asia Pacific Oil & Gas Conference and Exhibition, Jakarta, Indonesia, 2001

4. Chesser, B.G., Clark, D.E. and Wise.: “Dynamic and Static Filtrate-Loss Techniques for Monitoring Filter-Cake Quality Improves Drilling-Fluid performance”. SPE Drilling & Completion, (Sept.1994)189

5. Navarret, R.C., Cawiezel, K.E. and Constien. V.G.: Dynamic Fluid Loss in Hydraulic Fracturing Under Realistic Shear Conditions in High-Permeability Rocks”, SPE Production & Facilities, (Aug. 1996)138

Acknowledgements The author would like to acknowledge the contributions of Valerie Skidmore in designing the instrument and Albert Chan and Richard Hodge for their encouragement and support of dynamic fluid loss testing.

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Appendix A: Test Procedure The test consists of the following steps:

1. Formation preparation 2. Initial screen permeability measurement 3. Mud Preparation and viscosity check 4. Initial formation + screen permeability measurement 5. Fluid loss (dynamic and static) 6. Clean-up Treatment fluid loss and soak 7. Production direction flowback and final formation + screen permeability measurement 8. Final screen permeability measurement 9. System Cleanup

Each of these steps will be discussed in detail. Formation Preparation: The formation material is cleaned if necessary and then sample is run through a sample splitter to divide the material down to the weight needed for each test. A laser particle size analysis is then done on each sample prior to use. Screen permeability measurements: Screen samples are cut to the desired diameter, cleaned and mounted into the holder. The initial permeability of each sample is determined before starting the test, and again at the conclusion of the test. Mud Preparation and QC: For field mud samples, the viscosity, particle size distribution and solids composition is determined. For laboratory prepared samples, QC checks for viscosity and fluid loss on ceramic discs are completed, if needed. Drill solids are added at the desired concentration of shale and sand. Particle size distribution of solids in the mud may also be determined. Formation + screen permeability measurements: The formation material is loaded into the test cell and the screen sample stressed against the formation material. Flow is initiated in the production direction to obtain the system (formation plus screen) permeability. For gravel packed tests, the permeability of the screen plus gravel pack is determined and then screen plus gravel pack plus formation material permeability is measured for the initial permeability measurements without filtercake. Fluid Loss (dynamic and static): After the initial formation + screen permeability measurements are determined, the cells are connected to the mud flow system. Fluid loss lines are attached and the system loaded with the test mud. The pressure in the system is increased and mud circulation begun. The system is heated to the test temperature. The fluid loss lines are opened. The mud is circulated across the formation material for the dynamic fluid loss time while recording leakoff. If there is also a static fluid loss time, the pump is shut off and fluid loss continued for that time. Differential pressure and temperature may also be adjusted for the static fluid loss conditions. Cleanup Treatment fluid loss and soak: At the end of the mud fluid loss, the cells may be flushed with push pills, spacers, and reactive flushes if desired. The flow rates across the filtercakes are designed to simulate the circulation rates. Differential pressure is maintained across the formation material at all times during the cleanup flushes. If a soak time is desired with a reactive flush, the fluid loss catchers are set to only allow the maximum allowable volume to leakoff.

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Production direction flowback and final formation + screen permeability measurement: At the end of the cleanup flushes or reactive cleanup treatment, the screen sample is inserted into the cell and stressed against the formation and remaining filtercake and flow initiated in the production direction. For gravel packed tests, the gravel pack material is loaded into the formation material followed by the screen and then stressed. Flowback is initiated in the production direction at increasing pressure drop until the desired pore volumes of flow have passed through the formation and sand control system and the permeability is measured. Final Screen Permeability: The screen samples are removed from the cells and the final permeability measured. System Cleanup: At the end of the test, the entire system is taken apart and cleaned. Each line, valve, and cell body must be thoroughly washed to remove all traces of remaining drilling fluid and solids.