SPE 164108

Shale Inhibition: What Works?Sandra Gomez and Arvind Patel, M-I SWACO, A Schlumberger Company

Copyright 2013, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE International Symposium on Oilfield Chemistry held in The Woodlands, Texas, USA, 8–10 April 2013.

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 beenreviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessar ily reflect any position of the Society of Petroleum Engineers, itsofficers, 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 toreproduce 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.

AbstractUnderstanding the behavior and responses of shale to shale inhibitor additives of drilling and completion fluids has been achallenge for many years because of the numerous and complex chemical and physical variations present in these type offormations. This paper presents the results of laboratory experiments that show how the integration of the geological aspectscan help to understand the different responses of the shale to fluids and improve the selection of chemical additives for clayinhibition.

Many of the chemical inhibitor additives have been developed in the laboratory through systematic methods with the useof “standard clay-rich rock samples”. These rock samples have relatively simple geologic characteristics and predictablebehaviors. The responses of these types of rock samples tend to be consistent and appropriate for evaluations wherechemistry of the additives is manipulated constantly. However, the application of these additives goes beyond this approach.In the field, applications target formations that can vary significantly in composition and structure, and simplicity is anuncommon condition. Therefore the challenge for clay stabilization is to integrate many more aspects that can influence theresponse of the shale formation to fluids.

The paper will present the preliminary process of analysis and understanding of the geologic characteristics of the rocksamples and the importance of this fundamental step to achieve the chemical clay stabilization objectives. The paper willinclude various cases of the inhibitor selection process for shale samples with different characteristics. Chemistry andgeology perspectives will be used to analyze the results of the experiments including cases with typical and atypicalresponses of shale samples. This approach is used to provide a more comprehensive and meaningful interpretation of the rockstructural failures due to rock/fluid chemical interactions and, in the same manner, explain how chemical solutions can beused to minimize and control rock instability.

IntroductionExtensive work has been done for many years to evaluate and find solutions to mitigate and control the interaction of drillingfluids and clay-rich formations (shale, mudstone, claystone, and other fine-grained clastic rocks). Significant advances in thechemistry of clay inhibitors have shown excellent results in problematic shale formations around the world. Conventionally,different type of salts are added to the drilling fluids or combined in completion brines to control reactive clays. Potassiumchloride (KCl) has been for many years the most recommended type of salt for inhibition purposes. Other inorganiccompounds such as soluble silicates also are recognized for their inhibition capability which uses a combination of chemicaland physical stabilization mechanisms.

Most recent developments in the chemistry of inhibitors concerned organic compounds. Amines compounds are widelyused to provide inhibition properties to water-based drilling fluids and completion fluids. These amine compounds weredeveloped to replace salts in environmentally sensitive areas and improve the performance of the inhibitor additives. Someother organic additives are also available to provide a source of potassium in the fluids and stabilize the clays.

Multiple types of other additives are also used to control the invasion of fluids in natural or induce fractures, natural flowchannels, or pores. Most of these additives are inert or non-reactive products that work by creating a physical plug. Differenttypes of asphaltenes, graphite, calcium carbonate, and other solid products with specific particle sizes are available to createan effective seal. The latest developments in sealing additives include nano particle products (Ji et al. 2011) and copolymers(Mehtar et al. 2010). Technological advances with the use of polymers for shale control include different types of polymericproducts to control the dispersion of shale cuttings by encapsulating the clays and preventing contact with water. Most ofthese products prevent the invasion of water in the clay structures by an encapsulation mechanism. This paper focuses

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primarily on shale inhibitors that interact chemically with the clay minerals. Additives that act physically to seal or plug theformations are referenced briefly in some examples.

Although different types of clay inhibitors provide excellent performance in specific formations, there still is not a simpleor general chemical solution to control the instability of shale formations. A specific combination of chemicals for clayinhibition can be successfully used for a specific shale but not for another shale even though the two formations have thesame amount and type of clays. For this reason, the selection of the inhibitors becomes a “customized process” where types,amounts, and combinations of chemical additives are carefully optimized using multiple laboratory experiments using asingle shale sample to represent the problematic formation. This testing optimization process is very effective not onlybecause the actual shale formation can be used for the experiments but because it can integrate and cover many more aspectsrelated to the rock material beyond type of clays, amounts, and reactivity. These other aspects are related to the structuralrock properties which can be as important as the chemical interactions between clays and fluids. The structural properties ofthe shale formations can vary significantly to the point that in some cases they are more important to the instabilitymechanism than the impact of the mineral composition of the rock.

Chemical Inhibition AdditivesWater-based drilling fluids systems and completion brines are formulated with inorganic or organic additives, mostly in theform of salts to enhance the shale inhibition properties. Potassium chloride is a widely used inorganic shale inhibitor whilequaternary amine salts are favored organic shale inhibitors. These shale inhibitors can be classified in two categories:temporary shale inhibitors, also known as clay-controlling additives, and permanent shale inhibitors, also known as clay-stabilizers. Both of these classes of shale inhibitors have their pros and cons.

Inorganic Shale InhibitorsSalts such as sodium chloride, calcium chloride, and potassium chloride are clay-controlling additives and provide temporaryshale inhibition. These salts are only effective as long as the water-based drilling fluids containing these salts are in contactwith the clays. However, as soon as the salts are depleted or the salt-containing fluid is displaced by fresh water, the clay willagain hydrate and swell, destabilizing the drilled formation. The concentrations of salts between 2 and 37% are frequentlyused in drilling fluids.

Salts retard the swelling of the clay through a variety of mechanisms. The cation exchange reaction reduces the amount ofwater that can be absorbed by hydratable cations on the surface of the swelling clays. This mechanism however becomesineffective when clays contain little or no exchangeable cations. Alternatively, a large quantity of salts, such as heavy brinesor other electrolytes, have been used to increase the ionic concentration of the water phase in order to retard osmotichydration. This method of shale inhibition becomes very effective in shale formations where Smectite is the predominantclay mineral.

Inorganic salts are relatively inexpensive and are extensively available throughout world. Also, they can be applied invariety of drilling environments, such as high temperature and high pressure, and in wide range of pH conditions as they arechemically very stable. However, in large quantities these salts may adversely affect the chemical biological ecosystems. Forexample, above certain limits, these salts have an adverse effect on water quality and soil quality for agriculture use. Thisimpact by fluids with high salt concentrations has led to their undesirability in environmentally sensitive land-based drillinglocations and surface disposal restrictions imposed by environmental authorities. In exploratory drilling, high conductivityfrom these salts interferes with the sensitivity of induction logs. These salts also bring other restrictions, such asincompatibility with other drilling fluid additives and limited flexibility in drilling fluid formulations. Nevertheless,potassium salt has been the workhorse in shale inhibition for many generations either used alone or in combination with othertype of inhibitors.

Silicate additives also have been used in drilling fluids for many years. The soluble silicates are metal salts of silicic acidand exist in multiple polymeric forms in solution. Solid silicate products are also available. The three main components in thesilicates are: silica (SiO2), Alkali (Na2O or K2O) and water. The molecular structures of the silicates can vary from simplechains of monomers to complex structures. The inhibition mechanism of the silicate consists of a combination of twochemical reactions: gelation and precipitation. The gelation consists of the self-polymerization of the silicate structures toform a gel structure. This chemical reaction is controlled by pH. The polymerization begins at pH below 10.5. Precipitationof silicate is the cross-linking of silicate by multivalent cations. During the drilling process of shale formations, the pH of thedrilling fluid systems will drop below 10.5 when it is in contact with the formation fluids (pH~7). The precipitation also willtake place when multivalent cations present in the formation fluids react with the silicates to form insoluble precipitates (vanOort 1996). The gelation and precipitation take place on the surface of the shale blocking the influx of fluids and pressureinto the formation. Sealing of microfractures also is attributed to the silicate reactions in the wellbore which prevent furtheropening and weakening of the rock along fractures or bedding planes.

Organic Shale InhibitorsTo combat the environmental impact and performance-related issues associated with salts, particularly with potassiumchloride, a number of alternative cationic sources have been evaluated. This was the starting point for the development ofsome exotic cationic amine compounds as shale inhibitors. These shale inhibitors are classified as a more permanent type of

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shale inhibitors and are also known as clay-stabilizers. These shale inhibitors chemically interact with shale by multiple orsingle cation exchange mechanism, either by entering in to the shale matrix or reacting on the surface of the shale. Theseamine-based shale inhibitors are very effective on shale with high cationic exchange capacity. Various types of cationicorganic amine salts have been used in the field; however, these salts were unstable at high-pH and high-temperatureconditions typically encountered in the field. Upon decomposing, these amine salts produce an obnoxious ammonia odor andare rendered ineffective as shale inhibitors. To alleviate the problems associated with toxicity, odor, stability and performanceof some of these simple amine salts, a vast number of exotic amines and quaternary amine compounds have been developedand applied in the field. Depending on the structure and chemistry, these compounds are classified in three major categories:monomeric, oligomeric and polymeric amine shale inhibitors.

Monomeric Amine Shale Inhibitors.In monocationic amine-based shale inhibitors, the sodium ions in the clay are substituted with cationic amine species by acationic exchange process. The low-molecular-weight monocationic amines (Fig.1) enter into the clay platelets and reducethe amount of water adsorbed by the clay, thus reducing the clay swelling due to hydration. Another advantage ofmonocationic amines is the reduced level of treatment required to attain a high level of inhibition as compared to inorganicsalts. However, most of these monocationic amine shale inhibitors have a lower level of shale inhibition and higherammoniac odors when compared with oligomeric or polymeric amine shale inhibitors and, therefore, their application asshale inhibitors is limited. Various monomeric shale inhibitors, such as ammonium chloride, tetra-methyl ammoniumchloride, choline chloride and other ammonium derivatives have been developed and applied in the field with varied success.

Figure 1. Monomeric amine shale inhibitor.

Oligomeric Amine Shale InhibitorsTo mitigate the HSE-related issues and improve the shale inhibition properties of monomeric species, various oligomericamine shale inhibitors have been developed and applied in the field. These oligomeric amines, with two to four nitrogenfunctionalities, work in a similar fashion to that of monomeric amines due to cationic character and size – they enter into clayplatelets and exclude water molecules from entering and hydrating the clay. However, oligomeric amines unlike monomericamines have one main advantage in that they provide more permanent clay stabilization due to multiple active sites adsorbedon the clay simultaneously and therefore are less susceptible to reversing the adsorption.

Oligomeric amines provide more flexibility in designing and optimizing the molecule for the desired application. Forexample, polyether amines (Fig. 2) were designed and optimized for their application as shale inhibitors in the early 2000s(Patel 2002). The uniquely optimized molecular structure of these diamines perfectly fits between clay platelets (Fig. 3) andgreatly reduce the clay’s tendency to imbibe water from the aqueous environment. During the past five decades, variousoligomeric cationic amines have been introduced in the drilling fluid industry as shale inhibitors.

Figure 2. Oligomeric amine shale inhibitor. Figure 3. Molecular modeling of shale inhibition.

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Polyamine Shale InhibitorsPolymeric amine shale inhibitors are claimed to be more permanent shale stabilizers due to multiple active amine sites, inexcess of 100, available along the polymer chain (Fig. 4) for bridging with clay surface. However, polymeric cationic amineshave a large molecular size and cannot penetrate the clay layers as effectively as oligomeric quaternary amines. As a result,adsorption occurs mainly on the surface of the clay resulting in a less effective shale inhibitor in highly swelling clay types.Consequently, the large number of cationic sites is less significant when compared to oligomeric cationic amines, which enterthe clay plates easily and provides better shale inhibition. Various additional issues, such as compatibility with other anionicdrilling fluid additives and limitations in drilling fluid formulations due to high viscosity and toxicity, make these polymericcationic amines less attractive. In any case, various polymeric cationic amines have been developed and used in the field.

Figure 4. Polymeric amine shale inhibitor.

Laboratory Development of Clay InhibitorsDifferent types to laboratory tests are conducted to evaluate the performance of clay inhibitor products in the experimentalphase. Several methods include rock-fluid interaction tests where “reference or standard” reactive rock samples are used forthe evaluations.

MethodsTraditional inhibition tests in the drilling fluids industry are selected to evaluate the effectiveness of chemical additives incontrolling clay interactions with fluids (Stephens et al. 2009). Dispersion, bulk hardness, and bentonite tests are the mostcommon methods. The test procedures are relatively simple and can be carried out for multiple fluids simultaneously. Theresults generally help to differentiate the inhibition performance of the additives with relatively small variations in theconcentrations. In general, rock samples are prepared and exposed to the fluids for a period of time. Then differentmeasurements are conducted to determinate the effect of the fluids in the rock material or the impact on fluid properties.

Rock SamplesClay-rich rock samples, which exhibit strong interaction with freshwater and non-inhibited fluids, are usually selected toevaluate the inhibition performance of chemical additives. Some of these rock samples are available from outcrops or mines.Table 1 shows the mineral composition by x-ray diffraction of these common clays and typical cation exchange capacity(CEC) of each clay.

Table 1. XRD/CEC of Clay-Rich Rock SamplesOxford

ClayArneClay

NorwayShale

WyomingBentonite

Smectite* (%) 28 16 20 41Kaolinite (%) 15 25 35 5Illite (%) 15 25 10 -Dolomite (%) 1 1 - -Calcite (%) 3 1 1 10Halite (%) - - 3 -Hematite (%) - - 1 -Pyrite (%) 2 - - -Siderite (%) - 1 - -Feldspar (%) 1 1 - 9Quartz (%) 35 30 30 35CEC, meq/100 g 23 13 19 33

*Includes Smectite/Illite mixed layers

The clay content of these rocks is relatively high 45-65%. Reactive clay minerals are present in these rocks: Smectite,Illite, and Kaolinite. The CEC data indicates that these samples have moderate to high reactivity (Stephens et al. 2009).Another important feature of these rocks is the homogeneous structure. The thin-section photograph (Fig. 5) shows themassive structure of the rocks (lack of bedding or poor laminated structures) which makes these samples excellent forrock/fluid interaction studies. Consistency of the rock properties is important to be able to identify the effect of concentrationor chemical composition of the inhibitor additives.

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Figure 5. Thin section images of clay-rich rocks

In general, the structural characteristics of these rocks are simple. Minor variations in the mineral composition and size ofthe grains are observed not only in the micron scale but also in larger scales (centimeter, meters, or more).

Figure 6 presents the results of bulk hardness tests conducted with the Oxford clay and different inhibitors. This test isused to determine the hardness of the rock sample after exposing to a test fluid.

In this test, fragments of rock are exposed to the fluid under hot-rolling conditions. Then the fragments are recovered andextruded through a perforated steel plate utilizing the bulk hardness tester. The data represents the number of turns of a feedscrew and the torque required for each turn. Depending upon the efficiency of shale inhibitor and hardness of the shale, thetorque may reach maximum torque of 225 inch·lb and the sample form a disc-shaped extrusion. Highly efficient inhibitoradditives will yield harder rock indicated by higher torque readings. In this case the results show that the fluid formulationscontaining oligomeric shale inhibitors performed better than other chemical combinations.

Figure 6. Bulk hardness test results for different clay inhibitors using Oxford clay

Structural Features of Clay-rich FormationsMultiple variations in the sedimentary structures can be found in the clay-rich formations. In general, some of the beststudied structures in sedimentary rocks are parallel bedding, cross-bedding, ripples, and mudcracks (Boggs 2011). One of themost important structural features in the fine-grained clastic rocks is bedding. Bedding is defined as tabular or lenticularlayers of sedimentary rock that have lithologic, textural, or structural unity that clearly distinguishes them from layers aboveand below (Boggs 2011). Additionally, one single bed may contain subdivisions such as plane laminae which could be alsodifferent in composition, texture, color, and cementation. The thickness of the beds varies from 1 cm to hundreds of

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centimeters. Beds less than 1 centimeter are called lamclaystone and mudstone exhibit massive

Significant variations in the clay content and consequently in the reactivity with fluids can be found instructures. Figure 7 shows dark thin layers along the bedding direction and other light layers. The darker layers have higherclay content with higher potential for reactivity with fluids and opening of the bedding plane.structure (Figure 8), the clay is homogenously distributed

Figure 7. Laminated structure

Examples of pieces of rock with laminated and massive structuresin the Figures 9 and 10. Note that the direction of the cracks and fracturesrock.

Figure 9. Rock with strong laminated structureimmersed in a solution of 3

inhibitor.

Other features such as pre-existing natural fractures can also have aFractures in the rock are open channels for the fluids to intrude and react with the rock producing a weakening of thestructure. The Figure 11a-b show shale samples which failed mainly along the bedding direction butexisting fractures as well. The cross-bedding fractures intersectscenario. The impact of laminated bedding, fissures, and fractures on wellbore instability is closely related to the penetrationof drilling fluids (Gomez 2011).

Figure 11. Cracking of shale samples during the exposure to fluids. Multiple breaks occur along the beddingdirection. Each rock also broke along a cross

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centimeter are called lamina. Shale formations generally exhibit laminated beddings while theclaystone and mudstone exhibit massive (structureless) beddings.

Significant variations in the clay content and consequently in the reactivity with fluids can be found inigure 7 shows dark thin layers along the bedding direction and other light layers. The darker layers have higher

clay content with higher potential for reactivity with fluids and opening of the bedding plane.genously distributed and the rock can crack in different directions.

Figure 7. Laminated structure Figure 8. Massive structure

Examples of pieces of rock with laminated and massive structures exposed to an oligomeric amine inhibitor are presented. Note that the direction of the cracks and fractures is related to the structural characteristics of the

Figure 9. Rock with strong laminated structureimmersed in a solution of 3 vol% amine-based

inhibitor.

Figure 10. Rock with massive structureimmersed in a solution of 3

inhibitor.

existing natural fractures can also have a substantial impact in thetures in the rock are open channels for the fluids to intrude and react with the rock producing a weakening of the

show shale samples which failed mainly along the bedding direction butbedding fractures intersect other multiple fractures creating a more complex instability

The impact of laminated bedding, fissures, and fractures on wellbore instability is closely related to the penetration

Figure 11. Cracking of shale samples during the exposure to fluids. Multiple breaks occur along the beddingdirection. Each rock also broke along a cross-bedding natural fracture.

0.5 inch

Shale formations generally exhibit laminated beddings while the

Significant variations in the clay content and consequently in the reactivity with fluids can be found in these laminatedigure 7 shows dark thin layers along the bedding direction and other light layers. The darker layers have higher

clay content with higher potential for reactivity with fluids and opening of the bedding plane. In the sample with massiveand the rock can crack in different directions.

Figure 8. Massive structure

oligomeric amine inhibitor are presentedrelated to the structural characteristics of the

Figure 10. Rock with massive structureimmersed in a solution of 3 vol% amine-based

inhibitor.

the failure behavior of the rock.tures in the rock are open channels for the fluids to intrude and react with the rock producing a weakening of the

show shale samples which failed mainly along the bedding direction but also cross bedding pre-multiple fractures creating a more complex instability

The impact of laminated bedding, fissures, and fractures on wellbore instability is closely related to the penetration

Figure 11. Cracking of shale samples during the exposure to fluids. Multiple breaks occur along the beddingbedding natural fracture.

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Complex rock structures are also present in formations with significant clay content and potential for instability. The shaleformations are generally found close to oil traps creating the seal for the reservoirs. It is common to find irregularstratification and cross-laminations in the transition zones near to the reservoir zone. Interbedding structures of shale,siltstone, and sandstone are also important to evaluate because the clay content can vary significantly from high to lowcontent within a few centimeters. Figures 12 and 13 show an example of interbedding structures. The structure of the rockexhibits lenses with predominance of silt-size quartz (light color material) and clay-rich beds (dark brown areas).

Figure 12. Direct photograph of lenticular beddingstructure formed by shale and cross-laminatedsiltstone.

Figure 13. The thin section image showing theinterbedding structure.

Clay Inhibitor SelectionThe selection of the clay inhibitors for field applications is a process that starts with the understanding of the characteristicsof the shale formation(s) to be encountered and exposed to the drilling fluid, reservoir drill-in fluid, or completion fluid. Inthis part of the process, it is important to find the type and amount of clay minerals present in the shale. However, asmentioned before, this information is only one piece in the rock characterization process. Many other features of the rock areimportant to understand the instability mechanisms of the shales and to select the additives that will control the rock/fluidinteractions.

The following cases present examples of shale samples from actual formations encountered during drilling operations indifferent areas. Some of these formations exhibit complex characteristics. The response of the rock samples to the fluids inmultiple cases can be related reasonably with their geologic characteristics. Some cases show atypical or unexpected behaviorof the rock samples in fluids.

Case 1. Laminated Shale Formation with Smectite-Rich LayersA slab core section from a shale formation from northeast USA was used to design the inhibition chemicals for the drillingfluid. Direct examination of the rock material indicated that the sample had a strong laminated structure. Based on the visualexamination of the sample, most of the material along the core had similar color, texture, and in general similar properties.However, some interbedding layers of a different type of material with other characteristics were identified in different partsof the core (Figure 14 and 15). Analyses were conducted to identify the differences between the types of rock materials.

Figure 14. Predominant type of rock material in the coresample. Strong laminated structure, dark gray color andrelatively hard consolidation.

Figure 15. Type of rock found in some parts of thecore. Massive structure, light gray color, andrelatively soft consolidation.

It was found that the clay content of the two types of rock was very different. The dark gray rock had approximately 26%clay while most of the light gray color rock had approximately 57% clay minerals. The highest clay content in a few layers

1 cm

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was up to 88%. Predominance of smectite was identified in all the samples. Based on these analyses, it was necessary toredirect the clay inhibition design of the drilling fluid to the most critical factor in this rock. The presence of highly reactivelayers of clay was evidently the initial target in this case. The testing method selected for the evaluation of the different fluidswas the linear swelling test (Stephens et al. 2009). This method specifically measures the swelling tendency of rock sampleswith drilling fluids and is certainly the most appropriate technique when the rock samples have a significant amount ofswelling clays. Figure 16 shows the results of the swelling test. The data shows the final tests results after multiple testsconducted for optimization of the salt concentrations. Based on the results, sodium chloride (NaCl) had a better inhibitionperformance than the traditional KCl. Excessive swelling of the clays was observed with the use of a non-inhibited fluid.

Figure 16. Swelling test with smectite-rich core sample.

Case 2. Strongly Laminated Illite-Rich Shale FormationsMultiple analyses of the illite-rich shales showed that these rocks have specific responses to the fluids. These typically lowreactive, fissile, and hard formations tend to fracture when they are exposed to fluids (Darley 1969). This type of formationhas been of great interest during the last few years. Most of the gas shale reservoirs in North America and other areas aroundthe world are illite-rich shales. The extent of fracturing in these shales can vary significantly according to the amount of illitein the rock. Other factors that also influence the stability of these formations are the presence and amount of other mineralsand components such as quartz, calcite, and organic matter.

The unique combination of the minerals and their distribution in the rock are important factors that affect the overallreactivity and interaction of these rocks with fluids. Figure 17 shows rock samples with different amounts of illite. Thefracturing tends to increase significantly when the illite content is high. Generally, these formations do not interactimmediately with fluids. Gradual opening of the laminated bedding structure and opening of pre-existing fractures is thetypical response of these rocks. Multiple fracture development can produce significant rock structure weakening and wellboreinstability problems.

Many illite-rich shales respond negatively to poor or non-inhibited fluids. Fracturing can be observed in the samples afterimmersing the rock pieces in different fluids. Appropriate selection of the clay inhibition additives help to minimizefracturing and weakening of the rock structure. Figure 18 shows pieces of shale with ~48% of illite. The samples wereexposed to the three fluids with different types of clay inhibitors. It was found that the fluid containing K-Silicatesignificantly controlled the opening of the rock along the bedding plane.

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Figure 17. Three illite-rich shales after fluid exposure. Development of fractures mainly along the bedding plane is observed inthese samples. The extent of fracturing varies from severe to minimum from left to right. Illite content also varies from high (~48%)to low (~6%) from left to right.

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The use of sealing additives for a physical plugging of natural or induce fractures can be also prevent the invasion offluids into these hard and brittle shale formations. A combination of chemical inhibition and physical plugging is generallyrecommended for gas shale applications.

Case 3. Reactive Silt Shale Formation with Sandstone LensesInterbedded shales present in the sandstone reservoir represent a challenge for reservoir drill-in fluid design and also forcompletion fluid which needs to provide inhibition and control of clays in openhole completion operations. Unstable shale inopenhole sections may cause premature screenouts, reduction of the permeability of gravel packs, and inability to runcompletion tools and assemblies (Shenoy 2008).

Samples from the interbedded silt and shale section of the reservoir zone in Equatorial Guinea were evaluated with thecompletion fluids to understand the potential behavior of the rock material in contact with the fluids. The rock samples hadapproximately 40 to 46% clay. Additionally, the rock had a significant amount of localized clay-rich areas close to permeablesandstone lenses. Invasion of the brine through the permeable sandstone to reach zones with reactive clays was a concern forthe completion operation. Thin section images of the rock are presented in Figures 19 and 20.

Figure 19. Most of the sample exhibited the silt shalestructure and composition presented in this image.

Figure 20. Interbedding fine-grained sand is present inthe silt shale sample.

The white grains in Figure 19 are mainly quartz and feldspar. The brown background is clay. The samples have somefractures that may be caused by natural dryness of the unpreserved rock. The sandstone lenses in the samples were not wellconsolidated. The blue areas in the Figure 19 are pores. Clay-rich areas are located close to the sandstone lenses.

The samples were exposed to freshwater, base brine, and base brine with a monomeric amine inhibitor additive. Testresults are shown in Figure 21. The results of the test with water showed that the sample had a strong reactivity and potentialfor instability. The sample dispersed in water in few minutes. Unexpectedly, the samples remained intact in both brinesolutions. The calcium chloride (CaCl2) brine provided the right inhibition to this rock regardless of the multiplecharacteristics of the rock which represented potential risks of instability.

Figure 18. Illite-rich shale samples exposed to different fluids. From left to right 1) Freshwater/Gel/Polymer system(non-inhibited fluid) 2) Polymer/amine system (inhibited fluid), and 3) K-Silicate system (inhibited fluid).

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Figure 21. Silt shale with sandstone lenses (light color areas in the rock pieces) exposed to different fluids: from left to right 1)Freshwater 2)10.2-lb/gal CaCl2 and 3) 10.2-lb/gal CaCl2 with 8-lb/bbl monomeric amine.

Other ObservationsOther particular cases showed that some formations do not behave as expected according to the structural characteristics.Although the preliminary analysis of the rock indicates that the strong laminated structures were potential factors for openingfractures along the bedding plane, the response of the shale were different in the experimental phase. Highly laminated shaledispersed totally in presence of freshwater instead of the anticipated breaking of the rock along the bedding. Development offractures in other directions different to the bedding plane was observed in the rock after the exposure to potassium formatebrine. Figure 22 shows an example of atypical behavior of a shale sample in fluids.

Figure 22. Strong laminated shale core pieces before and after fluid exposure. From left to right 1) Sample before fluid exposure. 2)Sample exposed to freshwater. 3) Sample exposed to Potassium formate brine.

ConclusionsUnderstanding the individual properties of the rock and fluids is an important step to achievinf chemical control of clayminerals using inhibitor additives. The mechanisms of interaction of the different type clay of inhibitor products are generallywell understood. However the properties and responses of shale formations to fluids are difficult to generalize and classify.Each rock is a unique system with a unique combination of properties. The mineral composition, structure, grain distribution,consolidation, and other geologic properties of clay-rich formations can vary significantly. Additionally, rock properties,reactivity, and stability can not only change within a few feet along the shale sections but also can change within a fewmillimeters especially in fine laminated shales. Complex interbedding structures, localized clay-rich zones, or the presenceof natural fractures represent potential factors for wellbore instability. Identification of these properties can help to select theinhibitor additives, other clay control products, and the evaluation methods for the drilling and completion fluid design.

Although many of the rock/fluid interaction studies show that clay-rich formations behave according to what is expected,some cases show that some formations can have atypical responses. In these cases, the data collected in the laboratory can bethe first piece of information to anticipate unexpected wellbore instability events, or it can be used to adjust the drilling plansaccordingly.

AcknowledgmentsThe authors thank the management of M-I SWACO, A Schlumberger Company for permission to publish this paper.

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