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flenTXarticleinfoReceivedinrevisedform7March2016Accepted27May2016Availableonline28May2016abstractetal.,2014),therewillbemoreoilreservoirstobecomethetargetsDuringCO2injectionprocessforEOR,itisnecessarytouseinWeyburnandChristo-theearliestCO2(Xie,1991),andinJiangsuPro-).Recently,Jilinhasbeenin-onCCS-EORinseparatedfroma5–30%(Liuetal.,qingzijingnearSongyuanCity,JilinProvince,China.Theoilre-ContentslistsavailableatScienceDirectlJournalofPetroleumScienceJournalofPetroleumScienceandEngineering145(2016)328–335http://dx.doi.org/10.1016/j.petrol.2016.05.040systemictechniquestomonitortheprocessforunderstandingtheservoirislocatedinthemiddleofacentralfaultzone,whichisofverylowpermeabilityandwithwell-developednaturalfractures,extendedfromeasttowestdirection.Theblockiswithmultipleoilbearingstratainverticaldirection,butitssinglepaylayer0920-4105/&2016ElsevierB.V.Allrightsreserved.nCorrespondingauthor.E-mailaddress:ssrr123@hotmail.com(R.Shaoran).ofCO2-EORinthefuture.2010;Zhangetal.,2015;Renetal.,2011).JilinOilfieldandthetargetoilblockH59arelocatedinDa-TheestimatedOOIP(originaloilinplace)isof53.7billiontons(t)intightoilreservoirsinChinaaccordingtotherecentChinaResourceReview(Hu,2009),whichneedstobedevelopedusinginnovativetechniques,andCO2floodingcanbeabetterchoiceforthesereservoirswithverylowpermeability(Khosravietal.,2014;Zhaoetal.,2014;Wangetal.,2014).IthasbeenestablishedthatCO2injectionisaneffectiveEOR(enhancedoilrecovery)methodandamaturetechnology(Gozalpouretal.,2005;Godecaetal.,2011;WangandGu,2011).AlongwiththedevelopmentoftheCCStechnology(carboncaptureandstorage)(Zhaoetal.,2014;WangCO2(Renetal.,2010;Guetal.,2008),suchasductedinthewell-knownCO2EORandCCSprojectOilfield,Canada(WILSONandMONEA,2004;Jamespher,2005;Whittakeretal.,2004).InChina,projectwascarriedoutin1960sinDaqingOilfieldapilotprojectwasconductedinCaosheOilfieldvince,whichwaswelldocumented(Wuetal.,2009OilfieldCompany(abranchcompanyofPetroChina)itiatingthefirstlarge-scaledemonstrationprojectChina,targetingtightoilreservoirsandusingCO2nearbynaturalgasreservoirwithCO2contentof11.IntroductioninteractionbetweenCO2andcrudeoilandavoidingtheleakageofithasbeencon-Keywords:CO2injectionCO2storageMisciblefloodingMiscibilityeffectLowpermeabilityreservoirH59blockinJilinOilfieldwasselectedasapilotsiteforCCS(carboncaptureandstorage)project.Theblockisofalightoilreservoirwithaveragepermeabilityof3.5mDandwell-developednaturalfractures.CO2separatedfromanearbynaturalgasreservoirwasinjectedintotheoilblockduringsixyearsop-eration,withnearly0.26milliontonsofCO2(0.32HCPV)hasbeeninjected.InordertoevaluatethemiscibilityeffectofCO2flooding,injectionandwellproductiondatawereanalyzed,includinginjectionpressure,bottomholeflowingpressure(BHFP)ofproducerwells,CO2breakthroughtimeandproducedoilandgascompositions.ReservoirpressuredistributionduringCO2injectionwasfiguredoutbasedinjectionpressureandBHFPusingtheKriginginterpolationmethod,andthevaryingmiscibleregionwasillustratedinpressurecontours.TheanalysisresultsshowthatCO2injectioncansignificantlyincreaseoilproduction,butthesizeofthemiscibleregioncanbegreatlyreducedafterearlyCO2breakthrough,whichcancauseBHFPandoilproductiondeclining.IncomparisonwiththeexperimentaldataofcorefloodingwithCO2atdifferentmodes,theproducedoilcompositiondataindicatethatCO2floodinginthefieldwasmorelikelyinnearmiscibleorimmisciblemodes,thoughtheinjectionpressurewashigherthantheMMP,whichcanbeattributedtounexpectedearlyCO2breakthroughandlowpermeabilitynatureofthereservoir.&2016ElsevierB.V.Allrightsreserved.Articlehistory:Received1December2015AssessmentofmiscibilityeffectforCO2permeabilityreservoirHuangFenga,HuangHaidongb,WangYanqinga,RHassanButta,RenShaorana,n,ChenGuolidaSchoolofPetroleumEngineering,ChinaUniversityofPetroleum(Huadong),Qingdao266580,bSchlumbergerLimited(China),Beijing100000,ChinacDepartmentofPetroleumandGeosystemsEngineering,TheUniversityofTexas,Austin,dPetroChinaJilinOilfieldCompany,Songyuan138000,Chinajournalhomepage:www.eoodingEORinalowJianfenga,ZhangLianga,RenBoc,China78712,USAsevier.com/locate/petrolandEngineeringpilotH.Fengetal./JournalofPetroleumScienceandEngineering145(2016)328–335329thicknessisthin.Theoilbearingareaoftheblockis4.5km2,anditsOOIPisof2.16C2106t.Theexpectedultimaterecoveryfactorviawaterfloodingis21%,withrecoverablereserveof0.454C2106t.Fig.1.LocationCO2EORTheoilviscosityatreservoirconditionisof1.85mPas.Thesalinityoftheformationwaterismoderate,rangedfrom10,229to19,320mg/LwithpHvalueof6.8.Theaveragepermeabilityoftheoilbearinglayersisof3.5mD.ThenorthpartoftheH59block(Fig.1)wasselectedasCO2floodingtestingsitewithanareaof1.7km2,andOOIPof0.86C2106t.Thecentralpartoftheoillayerisburiedinthedepthof2440m,andtheoriginalreservoirpressurewasaround24MPa.Thebubblepointpressureisof7.31MPa,whichisfarbelowtheMMP(22.9MPa)andthecurrentreservoirpressure(around20MPa).SointhefollowingCO2floodingprocess,theeffectofbubblepointpressureongasproductionisnotsignificant.Thereservoirtemperatureisabout98.9°C.TheoriginalGORinthereservoirwasaround100m3/m3.Theprimaryproductionofthereservoirwasdrivenviaelasticenergyanddissolvedgas,inwhichonly5%ofOOIPwasrecoveredfrom2004to2008duetolowpermeabilityandhighhetero-geneityfeaturesofthereservoir.WaterinjectionstartedinApril2008,butpoorwaterinjectivityandperformanceduringwaterfloodingmadetheoperatortoseekothertechniquesforEOR.SoCO2injectionwasselectedincombinationwithaCCSoption.IntheCO2pilotsite,thereare5gasinjectionwellgroups,in-cluding5injectionwellsand19productionwells,whichareshowninFig.1.Duetothelowpermeabilityoftheblock,are-versed7-pointswellpatternwithashortwelldistancearound300mwasappliedintheblock.CO2injectionstartedinApril–June2008withinjectionratesof30–50m3/d.DuringtheCO2injection,oilproductionrateofmanyproducersincreasedevidently.Lateron,anewinjectorwellgroup(H-59-1)locatedinthesouthofthepilotsitebegantoinjectCO2inordertoincreasethereservoirpressure.Inthisstudy,asystemicmethodhasbeenappliedforfieldmonitoringanddataanalysisinordertoevaluatethemiscibilityeffectofCO2flooding.TheinjectionpressureandbottomholeflowingpressuredatafromtheproducerwellsareusedforthesiteinJilinOilfield.estimationofreservoirpressureduringCO2flooding,whichcanbeagoodindicationformiscibleorimmisciblefloodingmodes.ProducedoilandgascompositionalongwiththeoilproductionrateandCO2breakthroughtimingarealsousedtoanalyzethemechanismsandperformanceofCO2flooding.2.EORmechanismsofCO2floodingAfterCO2isinjectedintotheoilformation,eitherafirstcontactmiscibilityoramultiplecontactmiscibilitycanbeachieved(Tangetal.,2005;YangandLi,2007).Thefirstcontactmiscibilitymeansthattheinjectedgascanbedirectlymixedwithoilinanypro-portionundertheprevailedreservoirpressureandtemperaturecondition.ThemultiplecontactsmiscibilityisaprocessinvolvingcontinuouscontactsbetweenCO2andoil,reducinginterfacialtensiongradually,andfinallyacompletemiscibilityisachievedviacontinuousCO2dissolutioninoilandenrichmentoflightoilcomponentsingaseousphases.CO2floodingcanbeconsideredasaprocessofmultiplecon-tacts.Suetal.(2001)InordertoachieveCO2miscibleflooding,crudeoilmustcontainasufficientamountoflightcomponents(C2C24C6),soastoensurethattheinjectedCO2canextractthelightcomponentsofthecrudeoiltoachievecompletemiscibility.Therefore,theEORmechanismsofCO2floodingforthetargetedblockatprevailingreservoirconditionscanbemainlyattributedtoreducingtheinterfacialtensionbetweenoilandCO2,causingoilswellingandviscosityreductionandformingadissolvedgasdrive.(Wangetal.,2010;Zhao,2001;CaoandGu,2013).H.Fengetal./JournalofPetroleumScienceandEngineering145(2016)328–33533001020304000.511.5Oilcomponentcontent,mol%CO2injectionquantity,HCPVC2-C10miscibilefloodingC20+miscibileflooding010203040Oilcomponentcontent,mol%C2-C10immiscibilefloodingC20+immiscibileflooding2.1.ReducinginterfacialtensionIntheprocessofCO2flooding,withenrichmentofCO2andlightoilcomponentsintheoilzone,theinterfacialtensionbe-tweeninjectedgasandoilcanbereduced,finallyreachingtoamisciblestate,inwhichtheinterfacialtensioncanbereducedtonear0gradually.Itisexpectedthatviscous-fingeringandearlygasbreakthroughcanbeeliminatedduringamisciblefloodingprocess.2.2.OilswellingandviscosityreducingThevolumeofoilcanbeincreasedduetodissolvedCO2,andtheoilvolumeexpansioncoefficientcanbeover1.3atrelativelyhighpressures.ThedissolvedCO2inoilalsocangreatlyreducetheviscosityofcrudeoil,improvingthemobilityratio.OilswellingandviscosityreductioninducedbyCO2notonlycanreducetheresidualoilsaturation,butalsocanincreasethereservoirenergythatisofgreatimportanceforenhancingoilrecovery.2.3.EffectofdissolvedgasdriveDuringoilproductionandafterCO2breakthrough,thereservoirpressurecanbereducedtoorlowerthanthesaturationpressure,whichmakesCO2separatedoutfromtheoleicphasetoformagasdriving.Inaddition,therapidflowofCO2gascandisplacemoreoiloutfromthereservoir,andreducetheresidualoilsaturation.ThiscanbeanimportantoildisplacementmechanismforCO2flooding,butearlygasbreakthroughcanreducethemiscibilityeffectandtheusageefficiencyofCO2.00.511.5CO2injectionquantity,HCPVFig.2.ProducedoilcompositionversusHCPVinjectedunderCO2miscibleandimmisciblemodesduringCO2floodingexperiments;thestraightlinesindicatingCO2breakthrough.3.Evaluationmethodsformiscibilityeffect3.1.VariationsofformationpressureTheinjectedCO2underacertainpressureandtemperaturecanbemixedwithcrudeoilintheformation.Thechangeofthefor-mationorreservoirpressuredirectlyaffectsthemiscibilityeffect,andacompletemiscibilitybetweenCO2andoilcouldbeachievedatpressureabovetheminimummisciblepressure(MMP)(ArshadandKFUPM,2009;Zendehboudietal.,2013;Shokrollahetal.,2013).ThemeasuredMMPfortheH59crudeoilisof22MPa.IfthereservoirpressureisgreaterthantheMMP,thecrudeoilandCO2willbecompletelymiscibleviaamultiplecontactprocess.DuringCO2injection,thebottomholepressureofmostinjectionwellswasgreaterthan24MPa,sotheBHFPoftheproducersisim-portantfortherealizationofthemisciblefloodingmode.TheBHFPcanbemeasuredusingdownholepressuretransducers.WhethertheCO2floodingisinthemisciblemodecanbepreliminarilydeterminedviatheformationpressuremonitoringandanalysis.3.2.DynamicanalysisofinjectionandproductiondataDynamicanalysisofproductiondataisaconventionalanalysismethodintermsofreservoirengineering,whichmainlyincludestheanalysisofoilandgasproductionrates,gas/oilratio(producedGOR,includingCO2andoriginalnaturalgas),andoilandgascompositions.Duringoilproduction,theBHFP(bottomholeflowingpressure)washigherthanthebubblepointpressure,sotheeffectnaturalgasreleaseonGORwasnotimportant.DuringCO2injection,CO2canbedissolvedincrudeoiltomaketheoilswellingandviscosityreduction,whichcanimprovethegas-oilmobilityratioandincreaseoilproduction.DissolutionofCO2incrudeoilalsocanproduceeffectofdissolvedgasdriving,sotheproducedoilwillbeaccompaniedbythedissolvedCO2evenbe-foreCO2breakthrough,andtheproducedgasoilratiomayin-creasealongwiththeincreaseinoilproduction.3.3.AnalysisofproducedoilcompositionIntheprocessofmiscibledisplacementofCO2,CO2canextractandvaporizethelighthydrocarboncomponentsfromthecrudeoil(Bou-MikaelandPalmer,1989).Athigherpressures,CO2hasastrongerextractioneffect,whichcanincreasetherelativefractionsoflightoilcomponents,andthedensityoftheproducedoilwillbereducedaccordingly.Atlowpressures,thoughtheextractionabilityofCO2isweak,thedensitiesoftheproducedoilmayhavelittledifferencebetweenCO2floodingandwaterflooding,buttheextractioneffectofCO2ismuchhigherthanthatofwater,sothechangeofoilcompositioncanbestillobvious.TheextractionabilityofCO2canbepositivelycorrelatedwiththeformationpressure.IntheprocessofCO2miscibleflooding,alotoflightcomponentsincrudeoilcanbeextracted,sothecompositionandpropertiesoftheproducedoilcanbeanalyzedtoevaluatethemiscibilityeffectofoilandCO2.Fig.2showstheexperimentalresultsofcorefloodingviaCO2atdifferentpressurestosimulatemiscibleandimmisciblefloodingmodes,inwhichthefractionvariationsoflightcomponents(C2–C10)andheavycomponents(C20þ)oftheproducedoilwereanalyzed.Theexperimentswereconductedat98.9°C(reservoirtemperature),andcoresamplesfromthereservoirformationwereused.Thedisplacementpressureformiscibleinjectionwassetatover25MPa,andbelow10MPawasappliedtosimulateanim-miscibleflooding.Thedataindicatethat,underhighpressureoratamisciblefloodingcondition,asignificantvariationofthepro-ducedoilcompositionwasobservedafterCO2breakthrough.TherelativecontentofC2–C10intheproducedoilvariedgreatlybuttendedtoincreaseinitially,andthendecreased,whilethecontentofC20þtendedtodecrease.Incontract,underlowpressureoratanimmisciblefloodingmode,oppositephenomenawereob-served,inwhichtheheavycomponents(C20þ)showsanobviousincreasingtrend,whilethelightcomponents(C2–C10)inhibitadecreasingpatternastheinjectionvolumeincreased.ThefielddatawillbeanalyzedinthefollowingsectiontodeterminetheCO2floodingmodeunderreservoirconditions.4.Resultsanddiscussions4.1.ReservoirpressureanalysisIntheprocessofCO2flooding,thedownholepressuredataofproductionandinjectionwellswereobtainedbydirectdownholemeasurementsorcalculatedfromthewellheadpressure.TheH-12-6i29.8130.2132.0132.91––H.Fengetal./JournalofPetroleumScienceandEngineering145(2016)328–335331H-10-421.7251611.611.7–H-10-8i27.1130.0629.1128.6124.71–H-8-4i–30.4629.1134.51––H-8-65.820.71613.515.712H-8-813.49.99.610.110.931.11H-6-6i29.7130.5131.6131.6130.4132.41H-4-1107.79.68.613.631.71H-4-2i27.6627.0526.7127.9124.71–H-4-410.68.412.310.413.330.61(Note:“i”indicatesagasinjectionwelland“wi”meanswaterinjection,andtherestbottomholeflowingpressuremeasuredforsomewellsareshowninTable1.Inthepilotzone,eachinjectionwellstartedCO2injectionaroundApril–May2008,whileproductionwellswereshut-induringthefirstfewmonthsoftheinjectioninordertoincreasethereservoirpressuretofacilitatemiscibleflooding,andproductionwasresumedintheendof2008.Thevariationsoftheformationpressuredis-tributionduringCO2injectionintermsofpressurecontourmaps(asshowninFig.3)werefiguredoutusingtheSurfersoftwareac-cordingtotheKriginginterpolationmethod.FormationpressureinsidetheareaenvelopedbythewhitedottedlinewasgreaterthantheMMP,inwhichamisciblefloodingmodecanbeassumed.Fig.3(a)and(b)showthepressuredistributioninJuly2008andJanuary2009,respectively,aftertwoandsixmonthsofCO2injectionbeforeCO2breakthroughfromtheproducers.Theevo-lutionofanenlargingmiscibleregioninthenorthernandcentralpartsofthepilotzoneduringCO2injectioncanbeobserved,whilemostofthesouthernpartwasinalowpressureregion(lessthan15MPa).Thiswasduetothelowpermeabilityandpoorinjectivityfeaturesofthesouthernpart.TheobservedbreakthroughofCO2fromeachproducerstartedinFebruarytoApril2009,andtheformationpressurebegantodeclineconsequently.AsshowninFig.3(c)and(d),themiscibleregionwassignificantlyreducedtoasmallareajustaroundtheinjectionwells,andtheBHFPofthemostproducerswasmuchlowerthantheMMP.Itshouldbementionedthat,afterCO2injection,oilproductionwasgreatlyincreasedbeforeandalongwithCO2breakthrough,whichwillbeanalyzedindetailsinnextsection.UptoJune2012,asshowninFig.4(a),theformationpressurehadbeendecliningafterCO2breakthroughfromalltheproducers,Table1Bottomholeflowingpressuremeasuredatdifferenttimes.WellnumberBottomholeflowingpressure,MPa2008.72009.12009.62010.12012.62014.6H-14-10wi2826.122.726.7127.81–areproducers)althoughsomemeasureshadbeentakentoimprovethesweepingefficiency
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