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PETSOC-03-05-02

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PETSOC 03 05 02
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May2003,Volume42,No.561IntroductionHydraulicfracturingisanecessarytechniquetodeveloplowpermeabilityreservoirs.Fracturingfluidtakesaparamountroleinfracturingtreatments.Inmostcases,cross-linkedfluids(CLF)canmeetthetreatmentrequirements.However,severeformationdam-agefrequentlyoccurredwhencross-linkedguargel(CLGG)wasused.ThecoreflowtestsbyDevineetal.(1)showedthattheper-meabilityreductionbyCLGGwas56%~71%forcoresof100~200mD,and12%~35%forcoresoflessthan1mD.Thefluiddamagetothetightsandwaslessseverethanthatofapermeableformation(1,2).TheexperimentstudybyAlmond(3)at120˚Fwith20/40meshsandpacks(simulatingfractures)showedthattheflowreductionbyCLGGdamagecouldbeashighas100%.Withoutdoubt,damageofthisorderofmagnitudewillgreatlydecreasetheproductivityoffracturedwells,whetherthedamagehappensinformationsorinproppedfractures.Therefore,fluidimpairmentbyCLGGcouldbeasignificantproblemadverselyinfluencingthesuccessoffracturingoperations.Fluidcostisanothercriticalfactoroffracturing,especiallyinlowpermeabilityandtightreservoirswherethejobsizeislargeandthefluidcostishigh,butthewellproductivityislowanddeclinesfasterthanthatoffracturedwellsinconventionalreser-voirs.Fluidcostsavingisakeyfactortoeconomicallydevelopthesereservoirs,inparticularwhentheyaremarginalreservoirs.Theapplicationofpolymer-freefluidsevidentlydecreasedfluiddamageandpartlyreducedfracturingcostsinceitneededsmallerproppantandfluidvolumeforthesamefracturesizethanCLFfracturing(4,5).Butitwouldnotbeaneconomicalsolutionforlowpermeabilityandtightreservoirswhereinmostcasesjobsizesarelargeandthefluidcostsarehigh.WaterfracoperationsintightgasreservoirshaverecentlybecomeverypopularintheUSA(6-8),mainlybecauseoftheanticipatedlowoperationalcostsandlowfluiddamage.Theapplicationresultsshowedthatwaterfraccaneffectivelyreducethefracturingcostupto50%ormorewhiletheproductivityoffracturedwellsissimilartothewellsfracturedwithCLGG.Intheabove-mentionedwaterfracapplications,treatedwater(with10lbs/Mgalgeloronlywaterwithfrictionreducer)oralineargel(either10lbs/Mgalgelorwaterwithfrictionreduceraspadfol-lowedby20lbs/Mgalgel)wasused.Thepadusuallyconstituted50%ofthejobandtheslurryusesverylowsandconcentrations(aconstantconcentrationof0.5lbs/galwithatrialof0.5~2lbs/galrampforthelast1%~5%ofthejoborjust0.5~2lbs/galramp).Infact,waterfracisneitheranewconceptnoranewtechnique.Itsinceptiondatesbackto1964attheYumenoilfieldinnorth-westChina,whereithadbeenextensivelyusedwithtechnicalandeconomicsuccessinlowpermeabilityshallowoilreservoirs(9).Initially,thefracturingfluidwassurfactantwaterbutatpresent,itisalowconcentrationpolymersolution.Theslurryhasbeenramp-typewithsandconcentrationssimilartothoseusedincon-ventionalfracturingwithCLGG.Experimentalstudyhasshownthatthesuccessofwaterfracwithlowsandconcentrationisdifficulttopredictbecauseitdependsonformationpropertiessuchasthedegreeoffracturedisplacement,thesizeanddistributionofasperity,andtherockmechanicalproperties(10).However,theuseofproppantconcen-trationsatconventionalfieldlevelscouldhelptoovercomethedependenceonformationproperties(10).Thispaperpresentsanew,practical,andeconomicalfracturingfluidsystemforlowpermeabilityshallowreservoirs.Thenewfluidsystemisanalternativeforwaterfracoperationsanddesignedtoaccommodatefracturingrequirementsatconventionalsandconcentrations.FluiddesignconsiderationstominimizefluidAPracticalandEconomicalFracturingSolutionForLowPermeabilityShallowReservoirsF.GU,E.KURUTheUniversityofAlbertaZ.ZHAO,Z.MOPetroChinaAbstractFluidcostsavingiscriticalforfracturingoperationsinlowpermeabilityreservoirswheretheproductionrevenuesarelowbutthejobsizeisrelativelylargeandthefluidcostishigh.Cross-linkedfluids(CLF)areusuallythefirstoption.However,theymaycausesignificantdamagetobothproppedfracturesandformations,andarenotthecheapestoption.Polymer-freefluids,ontheotherhand,causemuchlessdamagebuttheyareexpensiveandfluidcostsmayimpairtheeconomicresultsoffracturing.Waterfracwouldbeacompromisesolutionforlowpermeabilityreservoirssinceitsfluidsarecheapandfluiddam-ageislow.Thesuccessofwaterfracwithlowslurryconcentra-tionsis,however,difficulttopredict.Thispaperpresentsanewfluidsystemthatwasformulatedtomaximizetheeconomicreturnoffracturingwellsinlowperme-abilityshallowoilreservoirs.Itisasolid-free,linear,syntheticpolymer-basedsystemwithaverylowformationdamagechar-acteristic.Thenewfluidsystemcanmeetavarietyoffracturingrequirements,includingslurryconcentrationsofconventionalfieldlevels.Moreover,itismuchcheaperthancross-linkedguargel(CLGG).Themethodfordesigningfluidcomponentsandtheproce-dureforpreparingthefluidtoachieveminimumformationdam-ageandminimizethecostaredescribed.Acomparisonoftheproductionperformancesfromthesamewelloradjacentrefer-encewellsfracturedwiththenewfluidsandCLGGismade.Thereservoirgeology,fluidtype,andoperationdataoffractur-ing,andwellperformancesfrommorethan300successfulwellsinthreedifferentlowpermeabilityshallowoilreservoirs(800~1,500mdepth)arealsopresentedindetail.PEERREVIEWEDPAPER(“REVIEWANDPUBLICATIONPROCESS”CANBEFOUNDONOURWEBSITE)damageandcostarediscussed.Thegeologicalandfracturingoperationdataofmorethan300wells,whichhavebeensuccess-fully(technicallyandeconomically)fracturedwiththenewfluidssince1996inthreelowpermeabilityshallowoilreservoirs,areincludedinthispaper.ANewFluidSystemConsiderationsofFluidDevelopmentThenewfluidsystemwasinitiallydesignedtoimprovefractur-ingefficiencybyreplacingpastsurfactantwaterfluid(someplus3%KClasclaystabilizer)butnotgreatlyincreasethefluidcostoftheCase1reservoir.Becausethereservoirisveryoldandinitslatedevelopmentperiodwithhighwatercut,alargeincreaseinthefluidcostwouldresultineconomicfailure.Aftercarefullyanalyzingthepropertiesofpastfracturingfluidsandtheforma-tion,itwasconcludedthatthereweretwomainfactorsaffectingthefracturingefficiency:thesandladencapacityofthefracturingfluidandtheswellingofformationclay.Sincethesandladencapacityofafluidisnormallyproportion-altofluidviscosity,itisnecessarytoaddsomepolymersintothefluidtoincreaseitsviscosity.Ingeneral,largemolecularweightpolymershavehigherviscositybuildingcapability.Thismeansthat,ascomparedtolowmolecularweightpolymers,asmalleramount(i.e.,lowerfluidcost)ofhighmolecularweightpolymercanbuildthesameviscosity.However,thestudybyGalletal.showedthathighmolecularweight(>8×106)andintermediatemolecularweight(1×106~6×106)polymerscausedextensiveformationdamage(11).Polymerfragmentswhicharedesolubilizedfromthegelledfluidnolongercontributetofluidviscositybutdo,unfortunately,contributesignificantlytoproppantpackdamage(12,13).Thereforeinordertoavoidtheseveredamage,molecularweightsofpolymersmustbekeptwithinan“optimumrange”bycompromisingfluidcostandfluiddamage.Whenselectingpolymersforthenewfluidsystem,specialcarewastakentobalancethefluidcostwiththeanticipatedformationdamage.Inordertocontrolmolecularweightsandavoidthefor-mationofanyinsolublesolidresidue,anewsyntheticpolymerwasformulated.Itisapartiallyionizedlinearpolymer.Thefinalproductisamixtureoffinesolidandliquidwherethesolidisthepolymerandtheliquidisanagentusedtoincreasethedispersioncapacityofthefinesolid.Themixturecancompletelydissolveinwaterleavingnoresiduetodamagetheproppedfracture.Becauseitisinstantaneouslysoluble,thefluidcanbepreparedatwellsitesusingcementpumpsbeforefracturingtreatments.Thiscansaveonsomeofthefluidpreparationandtransportationcosts.Besidesbuildingfluidviscosity,thepolymeralsohasclay-sta-bilizingcapability.Iftheclaycontentisnothigh,thenewpoly-mersolutioncanworkverywellwith3%KClandnospecialstabilizerisneededtopreventtheclayfromswelling.Sincethepolymerissynthetic,thereisnoworryaboutbacterialdegrada-tion.Thesurplusfluidofonefracturingjobmaybetransportedtoanotherwelltosavecosts.FracturingFluidsTwofluidsweredevisedfordifferentwellconditions.Fluid1isusedforrelativelyshallowandlowtemperaturereservoirsandFluid2isusedfordeeperandhighertemperaturereservoirsorfracturingwithsmallerpumprates.Fluid1:ThickenerA+surfactant+claystabilizerFluid2:ThickenerB+surfactant+claystabilizer+delaybreakerTheconcentrationofThickenerAis0.4%~0.5%byweightandtheconcentrationofThickenerBis0.5%~0.6%byweight.Theconcentrationofdelaybreakeris0.1%~0.15%byweight.ThickenerBhasstrongerviscositybuildingabilitythanThickenerA.Typesandconcentrationsofsurfactantandclaystabilizershouldbeselectedbyexperimentsbasedontheprice,formationfluids,clayminerals,andcompatibilitywithotheradditives.FluidPropertiesandCostThefluidlosscoefficientandfluiddamagetoformationsweretestedforthethreeoilreservoircasesandareshowninTable1.TheresultsshowthatthedamageofthenewfluidsismuchlessthanthatofCLGG.FluidCostGenerallythecostofnewfluidsisabout40%~60%ofCLGG.Thecostwouldchangewiththetypesandconcentrationsofthick-enersandadditives.FieldApplicationsThenewfracturingfluidswereusedinthreedifferentfields.Case1isaveryoldreservoirandinitslatedevelopmentperiodwithhighwatercut.Previously,thereservoirwasfracturedwithsurfactantwater.Case2isaneconomicallymarginalreservoirwithmulti-layers.HydraulicfracturingwithCLGGwasattemptedbutdidnoteconomicallysucceed.Case3isanewreservoirstillinitsexplorationstage.Thebiggestchallengeisthatthisreservoirhasabottomwaterlayerwithoutanybarriertoseparateitfromthepayzone.Sothefracturingpumpratemustbeverysmalltoavoidbreakingthroughthebottomwaterlayer.Case1ReservoirGeologyThereservoirofCase1iscalledReservoirMandislocatedattheYumenoilfieldinthenorthwestofChina.Itsareais10.65km2.Theformationdepthis500~1,200m.Thereservoirwasdiscoveredin1945andputintodevelopmentin1955.Horizontally,thereservoirhasthreeproductionareas:TopArea,SideArea,andLowProductivityArea.Vertically,ithas62JournalofCanadianPetroleumTechnologyTABLE1:Propertiesoffracturingfluids.FluidLossFormationDamageCoefficient,CIIIK0KDamagedKReductionCaseFluidCoreNo.×10-3×10-3µm2×10-3µm2%1115.773362725.022.919705915.712110.44710.65770.565614.032112.4510.4116.39221.0417.2218.1630.2642CLGG[1]100~20056~71m/minTABLE2:BasicformationdataofCase1.AreasItemUnitTopSideLowProd.Gaspermeability×10-3µm218.528.214.7NetM1m13.914.610.0ThicknessM2m7.28.36.6M3m3.14.73.2threeoil-containingintervals:M1,M2,andM3.Theaveragewellspacingis200mandthereservoirisdrivenbyinjectedwater.Theaveragegaspermeabilityoftheformationfromcoretestis24.2×10-3µm2.Theclaycontentishigh,from10%~30%.Themainclaymineralismontmorillonitewithanaverageclaycontentof52%.Theaverageinitialoilsaturationwas54%.Theinitialfor-mationtemperatureandpressurewereabout32˚Cand9.31MPa,respectively.Thebubblepointpressureoftheoilwas6.27MPa.Atpresent,thewatercutis20%~60%.Theotherdataofoil-con-tainingintervalsareshowninTable2.FracturingHistoryThefirsthydraulicfracturingwassuccessfullyattemptedinReservoirMin1955withcrudeoil.ItwasalsothefirsthydraulicfracturingintheChinesepetroleumindustry.Inthefollowingyears,variousfracturingfluids,suchasacid-in-oilemulsion,water-in-sulfonated-oilemulsion,causticsodawaterandlimewater,etc.,werealsotried.Butallthesefluidswerenotextensive-lyusedbecauseofhighcosts,insufficientsupply,orpoorfractur-ingresults.In1964,engineerstestedfracturingwithfreshwater,nowadayscalledwaterfrac,andhadgreatsuccess.Sincethen,waterfracwaswidelyusedinReservoirM.Duringthe1980s,manygelsandcross-linkedfluidssuchaspolyacrylamide,CMC,andCLGGwerealsotested.Thesefluidshowever,werenotwidelyappliedbecausetheywereusuallyexpensiveandtheproductivityoffracturedwellsusingthesefluidswasnotsignifi-cantlyhigherthanthoseusingwaterfrac.Surfactantwater(plus3%KClasaclaystabilizer)andtreatedproducedwaterweretheonlytwofluidswidelyusedinReservoirMbecauseoftheirlowcostandgoodfracturingresults.FracturingWithFluid1WhilethedevelopmentofReservoirMenteredintothelatestageduringthe1990s,theeconomicperformanceofwaterfracwithsurfactantwaterdecreased.Themaincharacteristicswereshorteffective-periods(average91days),lowoilincreases(aver-age83.5m3),andloweconomicratioofinputtooutput(average1:1.43).Inordertoimprovethefracturingeffectiveness,aresearchprojectwasinitiatedinearly1996.Anewfluid(Fluid1)wasdevisedandrelatedfracturingtechniques,suchasminifractestsforfluidlossandfracturingoptimization,werealsodeveloped.Inthemiddleof1996,sixtestwellswerefracturedusingFluid1.FracturingresultsshowninTable3indicatethatFluid1hasanexcellentperformance.VerygoodapplicationresultswerealsoobtainedwhenFluid1wasextensivelyusedinReservoirM.Thestatisticaldataof65fracturedwellsinthemiddleof1998showedthattheeconomicratioofinputtooutputroseto1:4.17.Uptonow,morethan250wellswerefracturedwithFluid1.FracturingparameterswithFluid1inReservoirMweresimilartothatofCLGG.ThefracturingparametersareshowninTable4.Case2ReservoirGeologyThereservoirofCase2iscalledtheNanyishanshallowreser-voirandislocatedattheQinghaioilfieldinthenorthwestofChina.Itsareais7.0km2.Theformationdepthoffracturedwellsis800~1,500m.Thereservoirwasdiscoveredin1957andnotputintodevelopmentbeforebecauseofitslowproductivityandhighdevelopmentcost.Itisaneconomicallymarginalreservoir.Vertically,theformationhastwooil-containingsections:sec-tionIIIandIV.Everywellhas2~6oillayers.Thewellspacingis200~350m.Thereservoirisdrivenbysaturatedgas.Thefor-mationrocksaremainlyshaleandshalycarbonate.Theabsoluteclaycontentisgreaterthan30%.MainformationdataarelistedinTable5.FracturingHistoryInJuly1998,thefirsthydraulicfracturinginthisreservoirwasdoneinwellQian3-2usingCLGG.Theformationdepthwas1,240~1,278m.Injectedfluidvolumewas36m3andproppantvolumeofquartzsandwas4m3.Thepumpratewas3.0m3/min.Theinitialoilproductionrateafterfracturingwas2~3m3/day.Technically,becauseofthedamagefromtheunbrokenpolymer,insolublesolidinthefluid,clayswelling,andthesmalljobsize,thewelldidnotgettheexpectedproductionrate.Theevalua-tionshowedthatthetreatmentfailedeconomicallybecauseofthehighfluidandequipmentcostandthelowproductionrate.May2003,Volume42,No.563TABLE3:FracturingresultsofsixtestwellsinCase1.BeforeFracturingAfterFracturingEffectiveOilRateWaterOilRateWaterPeriodIncreasedWellAreasInterval(m3/day)cut(%)(m3/day)cut(%)(days)oil(m3)B148LowProd.M30.99592.0120480446H207TopM32.75204.5225494713J257SideM31.72354.4440486295F149SideM30.73534.1320210736L259LowProd.M2;M31.84273.75304491307I186SideM2;M30.58551.6358330389TABLE4:FracturingparametersinCase1.ParametersValueUnitPumprate1.8m3/minPumpingpressure8~27MPaFluidvolume60~90m3/layerProppanttypeQuartzsandAveragesandvolume15m3/layerSandsize0.5~0.8mmPreflushfluid/totalfluid27~30%Averageslurryconcentration450kg/m33.75lbs/galTABLE5:FormationdataofCase2.SectionParametersValuerangeIIIPorosity(%)14.72~27.56Permeability(×10-3µm2)0.1~10Oilsaturation(%)50~64.2Totalnetthickness(m)1.4~7.4Depthoflayer(m)836~920Formationpressure(MPa)10.80~11.92Formationtemperature(˚C)48.29~52.27IVPorosity(%)9.25~21.75Permeability(×10-3µm2)0.1~10Oilsaturation(%)50~69.0Totalnetthickness(m)2.8~7.5Depthoflayer(m)1173~1278Formationpressure(MPa)15.29~16.68Formationtemperature(˚C)64.23~69.20Oilcompressibility(1/MPa)4.0×10-4Gassaturation(m3/1,000kg)8.77~82.04Oildensity(g/cm3)0.7511~0.8511Averageoilviscosity(mPa•S)5.32FracturingWithFluid1LargereservesofoilmadeitattractiveforpeopletodeveloptheNanyishanshallowreservoir.In1999,aresearchprojectwasstartedtostudyeconomicalandfeasiblefracturingtechniquesforthereservoir’sdevelopment.Theresearchinclu
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