Oil Review Africa Issue Four 201372 www.oilreviewafrica.com

IIN A THOROUGH study of shalecharacterisation and inhibition testing,seven drilling fluids—including six water-base muds (WBM) and one synthetic-base

mud (SBM)—were evaluated to determine a simpleranking for technical and environmentalperformance as well as treatment and disposaloptions for each fluid.

The SBM was the strongest technically, followedby an amine-modified high-performance WBMsystem. Both systems can be combined with techno-economically feasible treatment and disposal optionsthat minimise environmental impact.

IntroductionTullow requested a fluid selection study with coresfrom fields of interest. The study was conductedto support the drilling fluid design andrecommendations for future development of anEast African basin. A total of 108 core sampleswere available, with 15 selected based on depthand fields with the most similar properties as thezones of interest.

Selected core samples from two wells wereused to evaluate the rock/fluid interaction offormation samples with different drilling fluids.Six laboratory techniques evaluated the stabilityof the rock samples. Samples from Well A wereused for testing with base fluids (brines) and thesamples from Well B were used for testing withthe fully formulated drilling fluids.

Testing was split into two phases, with thefirst examining formation material and surmising

the best fluid-rock interaction. The second phasedetermined the rock behavior with the selectedfluids.

Shale characterisation In Phase 1, shale characterisation was determinedby the shale mineralogy and Cation ExchangeCapacity (CEC) data using the semi-quantitativeXR Diffraction (XRD) analytical method. Thesetests provided information on minerals and theirrelative abundance in the shale and shalereactivity. Thin section analyses of core samplesfrom the field provided a more qualitativedescription of the rock with respect to principalmineral, mineral with distribution of the rock,qualitative description of the rock, presence offractures and orientation, and grain size ranges.

Phase 2 included a shale inhibition andstability study. With the XRD/Thin Sections andCEC results, the fluid formulation was optimisedfor enhanced shale inhibition study. Tests toevaluate the formation and drilling fluidsincluded examination of bulk hardness, dispersiontesting, accretion testing, linear swelling,

immersion testing, capillary suction tests, X-RayDiffraction, and CEC.Fluid systems for study included:6 Fluid 1: Fresh Water/PHPA/PAC/XCD WBM 6 Fluid 2: Fresh water/potassium

Acetate/PHPA/PAC/XCD WBM6 Fluid 3: Amine-base high performance WBM6 Fluid 4 : SBM6 Fluid 5: High performance WBM-low

conductivity (modified amine)6 Fluid 6: Fresh water/potassium

chloride/PHPA/PAC-polymer mud 6 Fluid 7: Fresh water/low potassium

acetate/PHPA/PAC/XCD polymer WBM

The team applied elements of several qualitytargets from various published discharge guidelinesin the country and also applied internationallyrecognised treatment targets for determining theappropriate conditions of solid and liquid effluentreleased to land and water. Regulatory guidelinesallowed the team to compare the fluids against abenchmarked goal during development andassessment.

Common elements of concern found in drillingand completions fluids when assessing theenvironmental impact of the waste included: totalsuspended solids (TSS), salts/ions, hydrocarbons,heavy metals, waste minimisation and treatment ofthe waste, the methods available to treat thewastes, and identifying benefits of the treatment.

To consider environmental impacts, the study

Figure 1:Drilling fluids selected for study, including drilling performance and environmental rating.

Common elements of concernwere found in drilling andcompletions fluids when

assessing the environmentalimpact of the waste.

Drilling in environmentally sensitive areas of East Africa requires operators to consider both technical andenvironmental criteria when treating and disposing of cuttings and excess drilling fluids. In less technicallydemanding exploration wells, Tullow Oil sought drilling fluids with green credentials, but recognised that a systemwith improved technical performance was necessary to drill more demanding, high angle and extended reach wells.

Drilling fluid selection for environmentallysensitive areasTe

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focused on the two phases of the system, the solidcuttings and the liquids (mud and water) that areused, separated and disposed of during the operation.

Immersion testFor the immersion test, the sample in theparaffin-base oil was not affected by the fluid. Thesample in the 10 per cent KCl was the lessaffected for water-base fluids. Immersion testingshowed that 10 per cent KCl minimum wasneeded to keep the sample intact. The addition ofamine improved inhibition. The paraffin-base oilshowed the best inhibition.

Fluids containing amine and the SBM showedno or negligible accretion. The potassiumacetate/low molecular weight PHPA (LMW-PHPA)exhibited 8.1 per cent accretion while thefreshwater/low molecular weight PHPA fluidshowed 37.2 per cent.

Bulk hardness testing showed that Fluid 4 andthe Fluid 3 system exhibited the best results. Highdispersion tendency in freshwater and a significantincrease in recoveries were obtained with thedrilling fluids. Data showed that the PHPA/PACsystem had the lowest performance.

Lowest conductance was exhibited by Fluid 4and the high performance Fluid 5, 0.5 μS/cm and1.2 mS/cm, respectively. The lowest CapillarySuction Test (CST) values were obtained utilizingfluids: 5 ppb potassium acetate in freshwater,25ppb potassium acetate in freshwater, and 25 ppb(seven per cent wt) KCl.

Thin sections analyses indicated pore sizesmainly ranged from 50 to 130 microns. Themaximum pore size is ~150 microns. XRDs showedthat the formation consisted mostly of quartz andkaolonite. The cores were unconsolidated plugswith the presence of clay. The presence of kaoloniteis generally the cause for fines migrations whenproducing the well.

A variety of fluids were considered technicallycapable of drilling the zone, but the teamquestioned whether their environmental rankingscomplemented the practicality after consideringconstituents of concern in each fluid, the totalwaste volume and the treatment options. Thefluids all shared many of the same components

with one or two substitutions that affected fluidproperty changes in terms of technical andenvironmental performance.

Fluids were assessed based on their componentsbefore coming into contact with the formation,which could contribute additional contaminants notcontained in the products, but that could negativelyalter the environmental ranking.

Fluid rankingsFluid 1 was given an environmental ranking of an Abecause of its low electrical conductivity (ioncontent), the high potential for biodegradation ofthe suspended solids in the form of polymer, andthe minor impacts that would occur from anunplanned release or spill of the fluid. Negativepoints were given because of the potentially large

volume created and required for disposal.Fluid 2 received a D ranking. Although it has a

high potential for degradation and a high rank forinhibition, the electrical conductivity is very high,especially in the liquid phase. With low intensitytreatment on site it would take several months totreat the water to acceptable discharge levels,making it impractical for operations.

Fluid 3 received a B ranking. The electricalconductivity is originally much lower than theacetate while still providing good inhibition.However, the fluid would need to be treated toremove residual conductivity and the biochemicaloxygen demand (BOD) and chemical oxygendemand (COD). The treatment may not be able toachieve the low values set for water discharge in areasonable timeframe.

Fluid 4 had a rating of 10. Lab tests indicatedthat the SBM provides a very high amount ofinhibition. The expected washout in the hole wouldbe very low (roughly 10 per cent or less dependingon the formation), greatly reducing the cuttingsgenerated. With the high inhibition, the amount ofdilution to maintain the low solids in the activemud system should be low and solids controlshould be able to remove the majority of solids,allowing the base mud to be reused for other wells.

A rating of nine was given to Fluid 5 as the labtest indicated that the amine-base highperformance WBM provides a high amount ofinhibition. The expected washout in the hole wouldbe in a low range (roughly 15 per cent or less),reducing the cuttings. With the high inhibition, theamount of dilution to maintain the low solids in theactive mud system should be low.

A rating of eight was given to Fluid 6. Lab testsindicated that Fluid 6 provides high inhibition. Theexpected washout in the hole would be in a lowrange (15 per cent or less) with reduced cuttingsand low dilution required to maintain low solids.

The environmental ranking is a D due to thehigh initial electrical conductivity of the fluid andthe low potential for biodegradation of the ionsonce they enter the environment. The ranking also

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Figure 2: Accretion results for Fluids 1-7.

Fluids were assessed based ontheir components before

coming into contact with theformation.

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Figure 3: Visual depiction of accretion results for Fluids 1-4.

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Oil Review Africa Issue Four 201376 www.oilreviewafrica.com

re�ects the lack of adequate treatment to removethe source of contamination.

The rating of eight to nine in lab tests of Fluid 7indicated that the WBM provides a high amount ofinhibition with an expected washout in the hole of15 per cent or less, with reduced cuttings and a lowamount of dilution required. The �uid was given anenvironmental ranking of C. Even after lowering thepotassium acetate content the initial exchangecapacity was still high. The minimisation of theadditive improved the ranking from the previousformula. A biological treatment onsite is requiredbefore discharge. The acetate is organic and willbiodegrade. There is minimal waste generation. Asummary of environmental and drillingperformance can be seen in Figure 1.

RecommendationsDrilling performance of the �uids was evaluated onthe shale inhibition (bulk hardness, dispersion,accretion and swelling tests) results while theenvironmental evaluation was based on chloridecontent, conductance and waste disposal methods.

From the summary of the results of thedi�erent shale inhibition testing with all the �uids,the Fluid 4 was rated 9.7 of a maximum 10 pointsas the most inhibitive drilling �uid, followed byFluid 3 rated at 8.0.

Fluid 6 was rated 6.5, while Fluid 5 was rated 6.4and Fluid 6 scored 6.0. Fluid 7 scored 5.6, whereasthe least inhibitive WBM, scoring 2.8, was Fluid 1.

Treatment and disposalDewatering and water treatment can be utilised onlocation to reduce waste generation and recyclewater on location for Fluid 1. Treatment of mud toreleasable water would be reasonably easy. Sometreatment for excess polymers and some aerationmay be required to attain most regulatoryparameters for release.

Unless a signi�cant amount of barite has beenused and high levels of heavy metals are detected,then the cuttings can be blended with soil and landfarmed or buried with little to no impact due tocontaminants. Cuttings with barite may requiresome blending with soil to reduce theconcentration to an acceptable level.

For Fluid 2, the dewatering and water treatmentprocess will not remove the excess acetate in thetreated water, requiring a secondary bioremediationand aeration process that can take as long at 20 to45 days per batch. Onsite and timely treatmentmay require a centralised treatment plant to treathigh concentration potassium acetate mud. Drillcuttings will retain a level of potassium acetatethat can be bio-remediated with some soil blendingand soil enhancement with amendments.

For Fluid 3, onsite dewatering and watertreatment will not remove the amine in the treatedwater, which would have a conductivity level of6,000 to 8,000 mS/cm, above the suggested levelof 2000 mS/cm or lower in many countries. Pastprojects have shown that a dilution of 70 +/-percent will bring the conductivity to a level of2,000 mS/cm. Aeration can assist in reducing theamine but will take time. If dilution can’t beperformed, a secondary bioremediation process willbe required to promote the removal of the aminesin the water. The drill cuttings will retain a lowlevel of amine that will be able to be bio-

remediated with some soil blending and soilenhancement with amendments. Burial of thestabilized cuttings can be done with quick lime,cement or �y ash.

For SBM �uids such as Fluid 4, solids below 10microns and the water phase can be removed with achemical additive and centrifugation. Commerciallyavailable systems can recover base oil from usedmud. Drill cuttings with SBM can be bio-remediated.Other options for cuttings treatment or disposal arethermal treatment and waste injection, or evenstabilisation/solidi�cation. Waste injection disposesof all OBM waste �uids and cuttings.

For Fluid 5, onsite dewatering and watertreatment process will not remove the amine in thetreated water, thus there will be a conductancelevel of 2,000 +/- mS/cm, which is in the range fordischarge in many countries. Drill cuttings willretain a low level of amine that can be bio-remediated with some soil blending and soilenhancement with amendments. Stabilisation/solidi�cation is a feasible option.

For Fluid 6, high chlorides not removed throughonsite dewatering and water processing must bereduced by reverse osmosis or distillation. However,both create a high-chlorides sludge for disposal.Dilution would require rates of 60 to 70 to one ormore, which is not feasible. High chloride �uids andwastes can be disposed of through injection. Drillcuttings will retain a level of chloride that will beinhibitive to bioremediation.

For Fluid 7, a secondary bioremediation processwill be required to remove acetate in water treatedon site, which can take 20 to 45 days per batch. Acentralised treatment plant may be required to treathigh concentration potassium acetate mud. Drillcuttings will retain a level of potassium acetate thatwill be able to be bio-remediated with some soilblending and soil enhancement with amendments.

ConclusionBased on the above analysis, Fluid 4, the SBM, wasthe strongest technically, followed by Fluid 5. It wasshown that both can be combined with techno-economically feasible treatment and disposaloptions that minimise environmental impact.

Authors: Paul Burden and Klisthenis Dimitriadis,Tullow Oil; Kayli Clements, Chau Nguyen, TonyStaples, Seye Thomas; M-I SWACO, ASchlumberger Company

Lab tests indicated that theSBM provides a very high

amount of inhibition.

Figure 4: Visual depiction of accretion results for Fluids 5-7.

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