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FullLengthArticleAnalysisofcapillarypressurelow-salinitywaterfloodingXiaoWanga,VladimirAlvaradoa,aDepartmentofPetroleumEngineering,UniversityofWyoming,bDepartmentofChemicalEngineering,UniversityofWyoming,articleinfoArticlehistory:Received29November2015Receivedinrevisedform23March2016Accepted8April2016tobetterexplainAllrightsreserved.1.IntroductionCapillarypressureandrelativepermeabilityarekeytodescribemultiphaseflowinporousmedia.Understandingoffluiddistribu-tionsisnecessarytobetterestablishconnectionswithfluidmove-mentinreservoirs.Inthissense,rockelectricalresistivityservesas⇑Correspondingauthorat:DepartmentofChemicalEngineering,UniversityofWyoming,Laramie,WY82071,USA.E-mailaddress:valvarad@uwyo.edu(V.Alvarado).Fuel180(2016)228–243ContentslistsavailableFueljournalhomepage:www.elseandrock–fluidinteractions,includinggeochemicalreactions,needstobeaccountedforlow-salinitywaterfloodingmechanisms.C2112016ElsevierLtd.http://dx.doi.org/10.1016/j.fuel.2016.04.0390016-2361/C2112016ElsevierLtd.Allrightsreserved.pressureandrelativepermeabilitycurvesatreservoirconditionsresultingfromacombinationofsteady-andunsteady-stateexperiments.Capillarypressureresultsconfirmthathysteresisismoreprominentunderlow-salinityconditionsandapparenthigheroiltrappingisobservedduringimbibition,comparedtohigh-salinityconditions.Unsteady-statecorefloodingresultsalsoshowthatlow-salinitybrineisnotconducivetoenhancedoilrecoveryinlow-salinitywaterflooding.Geochemicaleffectsappeartonega-tivelyimpactbeneficialinterfacialmechanismsproposedtobenefitoilrecoverysuchastheformationofmoreviscoelasticinterfacesunderlow-salinityconditions.Weconcludethatcouplingoffluid–fluidAvailableonline13April2016MSC:00-0199-00Keywords:CapillarypressurehysteresisRelativepermeabilityhysteresisElectricalresistivityhysteresisLowsalinitywaterfloodingUnsteady-statecorefloodingHistorymatchandrelativepermeabilityhysteresisunderconditionsb,⇑Laramie,WY82071,USALaramie,WY82071,USAabstractInthiswork,weanalyzemeasurementsofdrainage,spontaneousimbibitionandforcedimbibitioncapillarypressurecurvesinconjunctionwithtwo-electroderesistivityonsandstonecoresamplesunderlow-andhigh-salinitywaterfloodingconditions.State-of-the-artlaboratoryequipmentabletoworkwithactualreservoirfluidsatreservoirconditionswasdesignedandbuilttoconductthesemeasurements.Unsteady-statecorefloodingexperimentsundersimilarexperimentalconditionstothoseinthecapillarypressuretestswerealsocarriedout.Ablack-oilreservoirsimulationmodelwassetuptohistorymatchexperimentalproductionandpressuredatatoobtainmulti-phaseflowfunctions.TwoexperimentswereconductedonMinnelusaformation(eoliansand)rocksamplesat93C176CwithTCcrudeoilandsyntheticbrines.Placementofanoil-wetmembraneononeplugendandawater-wetdiskontheotherendguaranteedthatonlyonephasewasabletoflowthrougheachsampleendatanygiventime.Inoneexperiment,a57,491ppm-brine(High-Salinity)wasusedduringtheimbibitionprocess,whilea20-folddilutionoftheHigh-Salinitybrine(Low-Salinity)wasusedintheotherexperiment.Correspondingly,twounsteady-stateexperimentsatfixedinjectionratewerecompletedonMinnelusaformationrocksamplesplacedinthecorefloodingsystemusingcomparableexperimentalconditionsasthoseinthecapillarypressureexperiments.Comparisonofhigh-andlow-salinityexperimentalresultsshowsthatmorenoticeablecapillaryhysteresistowardwater-wetnessaroseinthelow-salinityexperi-ment.High-salinityexperimentresultsshowedthattheimbibitionresistivityindexwashigherthanthatcorrespondingtodrainage.Historymatchingofthetransientproductiondataincapillarypressureexper-imentsalongwithend-pointsobtainedfromunsteady-statecorefloodingexperimentswasusedtoobtainrelativepermeabilitycurves.Availabilityofhigh-qualitycapillarypressuredataatreservoircon-ditionsimprovedtheaccuracyofrelativepermeabilitycurvesobtainedfromhistorymatchingunsteady-statecorefloodingexperiments.OurresultsshowasubstantialimprovementinobtainingbothcapillaryatScienceDirectvier.com/locate/fuelFuelapowerfultooltoevaluatethesefluiddistributionsinporousmedia.Thesemultiphaseflowfunctionsdependnotonlyonsatu-ration,butalsoonthesaturationpathandhistory,aphenomenonusuallyreferredtoashysteresis.Multiphaseflowhysteresiscanoriginatefrom(1)contactanglehysteresis,whichalludestotheadvancingcontactangle(inimbibitionprocess)beinglargerthantherecedingcontactangle(duringdrainage);(2)trappingofthenon-wettingphase;(3)wettabilityalterationafterarockiscon-tactedwithcrudeoil,especiallyatahighreservoirtemperature[1].Inpractice,allsourcesofhysteresisarepresentandnoteasilydistinguishedinexperiments.Leverettpointedouttheimportanceofcapillarypressureinhiswell-knownpaper[2].Significanteffortshavebeenmadetomea-surecapillarypressureinthelaboratory.AreviewofthecapillarypressuremeasuringtechniqueshasbeenpresentedbyJennings[3].Ofthevarioustechniques,therestored-statemethodintroducedbyMcCulloughetal.[4]isthemostaccurateone,whichcanbemodifiedtodeterminehysteresisloopsduringprimarydrainage,spontaneousimbibitionandforcedimbibition,inturn.Ontheotherhand,obtainingrelativepermeabilitycurvesisacomplexandtime-consumingprocess.Generally,therearetwotypesofmeasuringtechniques:steady-stateandunsteady-statemethods.Thesteady-statemethod,inwhichflowratesaremain-taineduntilsaturationandpressuredropreachsteady-stateandDarcylawisusedtocalculateonepointinrelativepermeability.Thismethodisreliable,buttime-consuming.Moreover,thismethodtypicallyrequireslongplugsorcompositecores(stackedshortplugs)andhighflowratestomitigatecapillaryentryeffects[5].Incontrast,theunsteady-statemethodisfast,butrequirescomplexinterpretationmethods.Thusrelativepermeabilityisoftenobtainedthroughhistorymatching.Electricalpropertiesmeasuredinwelllogshavebeenusedtoestimateformationporosityandinsituwatersaturationinhydro-carbonreservoirs.TheinterpretationofthesemeasurementsisbasedonArchie’sFirstandSecondLaw.Archie’sFirstLawgoesasfollows:F¼Ro=Rw¼/C0mð1ÞwhereFistheformationfactor,Roistherockresistivitywhenfullysaturatedwithwater,Rwisthewaterresistivity,/istheformationporosity,andmisthecementationorporosityexponent.Archie’sSecondLawisasfollows:I¼Rt=Ro¼SC0nwð2ÞwhereIistheresistivityindex,Rtistherockresistivitywhenitispartiallywatersaturated,Swisthewatersaturation,andnisthesat-urationexponent.Archie’sFirstLawrelatestheelectricalresponse-formationfac-tortoporosity,whereasthesecondlawrelatestheresistivityindextothewatersaturation.RtandRocanbeobtainedfromwelllogs,whereasmandnhavetobemeasuredinthelaboratory.Manystudieshaveshownthatthesaturationexponentncanbeafunc-tionofwatersaturationandsaturationhistory.Moreover,manyfactorscouldinfluencethesaturationexponentresponse.Onefac-torthathasbeenthefocusofmoststudiesiswettability.Theexpo-nentnhasbeenfoundtobehigherandthehysteresismoresignificantinoil-wetsystemsthaninwater-wetones[6].TangandMorrow[7]firstreportedlow-salinitywaterfloodingwiththeintentofincreasingoilrecoveryovertraditionalwater-floodinginsandstoneformations.Sincethen,manylaboratoriesandorganizationshavetakenanactiveinterestinreproducingthelowsalinityeffect[8].Inthemeantime,severaloperatorshaveX.Wang,V.Alvarado/thistechniqueinthefield,e.g.BP[9–12],Shell[13,14]andStatoil[15].Despitethepotentialofthistechnique,somelaboratoryorfieldtrialshavefailedtoincreaseoilrecovery[15].TangandMorrow[16]proposedconditionstoincreaseoilrecoverybylow-salinitywaterflooding:(1)existenceofsignificantamountofclay,(2)presenceofinitialwatersaturation,and(3)contactwithcrudeoiltohavemix-wettability.However,MorrowandBuckley[17]admitthattheseconditionsareinsufficienttothesuccessoflow-salinitywaterflooding;therearemanycasesmeetingtheseconditionsthatarenotconducivetoincreaseoilrecovery.Asaresult,theunderlyingmechanismsremainthesubjectofcontro-versy.Themajorityoftheresearchhasfocusedonfluid-rockinteractions,andproposedmechanismsincludewettabilityalter-ation,insituemulsificationandfinesmigration.BothLigthelmetal.[18]andLageretal.[19]explainedhowdecreasingthebrineconcentration,especiallybyreductionofmultivalentcations,couldturntherockmorewater-wet.Ithasbeensuggestedthatloweringthesalinitylevelcanincreasetheelectricdoublelayerandreducethemultivalentcationbridgesbetweenclaysandcrudeoil.Oncetherepulsiveforceexceedsthebindingforce,oilisdesorbedfromtheclay,andthustherockturnsmorewater-wet.RezaeiDoustetal.[20]suggestedasalting-ineffect,inwhichthesolubilityoforganicmaterialincreasesassaltsareremovedfromwater.InAlvaradoetal.’spaper[21],analternate,anadditionalimportantfluid–fluidinterfacialmechanismwasproposed.Alvaradoetal.suggestthatasthebrinesalinitydecreases,theoil–waterinterfa-cialviscoelasticityincreases,whicheventuallyhinderssnap-off,reducingoiltrapping.Asaresult,theoilphasebecomesmorecontinuousandahigheroilrecoveryisrealized.Inthepastfewyears,manyresearchershaveinvestigatedtheeffectivenessoflow-salinitybrineusingMinnelusarocksamplesandcrudeoils.TheMinnelusaformationinWyomingisanevapor-itefoundtooftencontainanhydrite.Somestudiesindicatethatlow-salinitywaterfloodingcouldincreaseoilrecoveryinthesereservoirs.Forexample,Puetal.[22]foundincorefloodingexper-imentsthattheinjectionoflow-salinitybrineproducedadditional5.8%OOIPintertiarymode.Insecondarymodeexperiments,theoilrecoveryforhigh-andlow-salinityinjectionswas36.4%and47.7%,respectively.Theauthorattributedthiseffecttoanhydritedissolu-tion.Theyalsofollowedthereleaseofdolomitecrystalscausedbyunder-saturatedlow-salinitybrine.Whereas,Gamageetal.[23]foundthatwhenlow-salinitywasinjectedintertiarymode,therewasnoadditionaloilrecovery;whenusedinsecondarymode,anoilrecoveryincrease(1022%OOIP)wasobserved.However,therearealsosomestudiesthatdidnotshowincreasedoilrecoverywithlow-salinitybrineinjection.Forexample,Thyneetal.[24]claimedthatlow-salinitybrineincreasedoilrecoveryonly1%OOIPintheirlaboratoryexperiments.Theyalsoanalyzed51Minnelusafieldsdataanddidnotfindacorrelationbetweenoilrecoveryandbrinedilution.Theseresultsindicatethatlow-salinitywaterflodingisatleastuncertainintheMinnelusaformation.Fresh-waterinjectioninthisrockhasbeenshowntocauseanhydritedissolution,whichinturnalterswatercompositiontowardshighersalinityandcalciumcontent[25].Inthispaper,wediscussresultsofquasi-staticcapillarypres-sureandresistivityhysteresisexperimentsatreservoirconditionsusingamodifiedporousplatemethodthatincludesanoil-wetmembraneattheupperendofacoresampleandawater-wetceramicdiskatthelowerend.Alongside,unsteady-statecoreflood-ingexperimentsarecarriedoutatsimilarexperimentalconditionstothoseofthecapillarypressureexperiments.CMG-IMEX(ComputerModellingGroupblack-oilsimulator)inconjunctionwithCMG-CMOST(ComputerModellingGroupsoftwaretoolforexperimentaldesign,samplingandoptimization;thetoolisusedforSensitivityAnalysis,HistoryMatching,OptimizationandUncer-taintyAnalysis)areemployedtohistorymatchresultsofcapillary180(2016)228–243229pressureandunsteady-statecorefloodstoobtaintherelativepermeabilityhysteresis.Inallexperiments,theconnatebrinesarethesame,whiletheinjectingbrinewaseitherhigh-orlow-salinity.Thus,theeffectsoflow-salinitybrineoncapillarypressure,relativepermeabilityhysteresisandoilrecoveryaremonitoredtounderstandunderlyingmechanismsinlow-salinitywaterflooding.isrepeateduntiltheirreduciblewatersaturationisreached.Theimbibitionprocessisinitiatedbyloweringtheoilphasepressure.230X.Wang,V.Alvarado/FuelTable1Disk/membraneproperties.TypeDiameter(mm)Thickness(mm)/(%)K(mD)Pec(psi)Water-wetdisk38.17.250.300.0052172.MaterialsandmethodsTocharacterizemulti-phaseflowinporousmedia,twotypesoflaboratoryexperimentsarecarriedout,(1)quasi-staticcapillarypressureandresistivityhysteresis,and(2)unsteady-statecore-floods.Inaddition,analysisofproducedbrinesincorefloodingexperimentsaswellasgeochemicalmodelingofthetransportreactionbetweenthecoreandtheinjectedbrineareconducted.Toestimatetherelativepermeabilitycurves,numericalsimulationisusedtohistorymatchtheresultsoflaboratoryexperiments.2.1.CapillarypressureandresistivityhysteresisexperimentsAlaboratoryequipmentusingactualreservoirfluidsatreservoirconditionswasdesignedandbuilttomeasuredrainage,sponta-neousimbibitionandforcedimbibitioncapillarypressurecurvesinconjunctionwithatwo-electrode(2T)andfour-electrode(4T)resistanceatfivefrequencies(Coretest,CP-355).Inourstudy,toobtainthefullcycleofcapillarypressurecurves,i.e.primarydrai-nage,spontaneousimbibitionandforcedimbibitioncurves,awater-wetceramicdiskandanoil-wetmembraneareused.Thepropertiesofthewater-wetdiskandtheoil-wetmembranearelistedinTable1.Thecoresampleisplacedverticallyinthecoreholder;theoil-wetmembraneisplacedinamembraneholderontheupstreamend,whilethewater-wetdiskisplacedonthedownstreamend.Alayeroftissuepaperisplacedbetweenthecoresampleendfaceandtheoil-wetmembranetoprovidecapillarycontinuityandasprotectionfortheoil-wetmembrane.Thedetailsoftheoil-wetmembraneassemblyandthecoreholderareshowninFigs.1and2,respectively.Aresistivitymeterisinstalledinthecapillarypressuresystemtoenablereadingbrineresistanceandthe2Tresistanceofthecoresampleandtheceramicdiskand4Tresistanceofasub-sectionofthecoresampleatfivedifferentfrequencies.Capillarypressureand2Tresistivityexperimentsarecarriedoutonastandardreservoircoreplug(1.5indiameterand3inlength),labeledWY1-115,whichwascollectedfromtheMinnelusaSandintheTCoilfieldofWyoming.Thistypeofreservoircontainsanhy-driteanddolomitecementations.Fig.3,athinsectionobservedunderopticalmicroscopy,isoneexampleshowingthecements.TheCT-scanningimageofWY1-115inFig.4alsoshowstheanhy-driteanddolomitedistributionandtellsustheheterogeneityoftherocksampleinanotherway.Tocarryouttwocapillarypressureand2Tresistivitymeasurements,WY1-115iscutinhalvestogen-eratesamplesWY1-115-1andWY1-115-2.Samplespropertiesareveryclosetooneanother,asshowninTable2.ThefluidsusedintheexperimentsareTCcrudeoilandsyn-theticbrinewithasalinityof57,491ppm(referredtoasHigh-Salinitybrineinthefollowing).TCcrudeoilisproducedfromtheMinnelusaFormationinaWyomingsandstonereservoir.ItsbasicOil-wetmembrane38.10.20––29Thedifferencebetweenthesetwocoresamplesisthatduringtheimbibitionprocess,theimbibedwaterforWY1-115-1isHigh-Salinitybrine,butforWY1-115-2isLow-Salinitybrine.2.2.Unsteady-statecorefloodingexperimentTwounsteady-statecorefloodingexperimentsareconductedonMinnelusarocksamples.Afterestablishingtheinitialwatersatura-tionbyfloodingtherocksampleswithTCcrudeoil(inprimarydrainageprocess),High-andLow-Salinitybrinesareinjectedintotworocksamples,respectively.Theexperimentalconditionsareverysimilartothoseofthecapillarypressureand2Tresistivityexperiments,exceptforthebackpressure,whichisatmosphericandtemperatureis70C176Cinthecorefloodingexperiments.TheschematicofthecorefloodingsystemisshowninFig.5.Thecorefloodingsystemisplacedinanoven,sothattheexper-imentcanbecarriedoutatthedesiredtemperature.Therearetwocoreholdersinthesystem,inasetupthatallowscarryingouttwoexperimentsfaster,byallowingsaturationandageingoftwocoresatthesametime.Threetransfer-vesselsareusedtostoretheinjectingfluids:TCcrudeoil,High-SalinityandLow-Salinitybrines.AnISCO500Dpumpisusedtodrivethepistonsinthetransfervesselsthatinjectfluidsintothecoresamples.Theproducedfluidsarecollectedinvialsatdifferenttimesusinganautomatedfractioncollector.Threepressuretransducers,placedinside
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