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SPE-50669-MS

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SPE 50669 MS
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SPE50669ScreeningcriteriaforevaluatingthedevelopmentfieldsK.J.Weber,SPE,andH.Dronkert(Delft,UniversityofTechnology).Copyright1998,SocietyofPetroleumEngineersInc.Thispaperwaspreparedforpresanfationatthe1998SPEEuropeanPetroleumConferenceheldinTheHague.TheNetherlands,20-22October1998.ThispaperwasselectadforpresentationbyanSPEProgramCommitteefollowingreviewofinformationcontainedinanabstractaubmittadbytheauthor(s).Contentsofthapaper.aspresented,havenotbaanreviewedbythaSocietyofPetroleumEngineersandaresubjecttocmrsctionbytheauthor(s).Thematerial,aspresanted,doesnotnacesaarilyreflectanypositionoftheSocietyofPetroleumEngineers,itsofficers,ormambars.PaperspresentedatSPEmeetingsaresubjecttopublicationraviewbyEditorialCommitteesoftheSocietyofPetroleumEngineers.Elac.tronicreproduction,distribution.orstorageofanypartofthispaperforcommercialpurposeswithoutthewrittenconsentoftheSocietyofPetroleumEngineersisprohibited.Permissiontoreproducainprintiarestrictedtoanabstractofnotmorathan3tXlwcfdsillustratic+mmaynotbecopied.Theabstractmustcontainconspicuousacknowledgmentofwhereandbywhomthepaperwaspresented.WriteLibrarian,SPE,P.O.Box833838.Richardson,TX75063-3836.U.S.A.,fax01-972-952-9435,AbstractContinuingreservoirmanagementatmaturestagesoftenconcentratesondelineatingpocketsofremainingmobileoil.Thisisbecomingamajortaskforreservoirgeologistsandpetrophysicists.Manyoldfieldsarecomingupasinvestmenttargetsforreactivationandthereisgeneralconsensusthatmoderntechniquescanleadtoadditionalrecoveryofbetween10and20percent.However,therearemanypitfallswhichcanrenderanapparentlyattractiveprojectintoaneconomicfailure.Thispaperwilldiscussthenon-technicalscreeningcriteriarelatedtoreservoirarchitecture,accumulationconditionandproductionhistory.Mobileremainingoilcanbefoundinanumberofpredictablelocationsinreservoirsdependingontheirstructuralstyleandfacies.Atticoilalongfaultsisperhapsthemostsimpleconfigurationbutsizeablevolumesofremainingoilcanalsooccurasafimctionofreservoirstratificationandlateraldiscontinuity.Asystematicoverviewofthedifferenttypeshasbeencompiledbasedonstructuralorstratigraphiclateralcontinuityandverticalreservoirconnectivity.Thisleadstofourmaintypeswithsomesub-groupsforeachofwhichscreeningcriteriahavebeenderivedonthebasisoffieldexamplesandmodels.Thescreeningcriteriaspecifiminimumconditionswhichmayleadtoeconomicre-developmentwithhorizontalside-tracksfromexistingwells.InadditionrecommendationsaregivenwithrespecttodatagatheringtoconfmthepresenceofeconomicallySocletYofPetroleumErmineerspotentialofremainingoilinmatureviabletargets.IntroductionNumerousoilfieldswhichhavebeeninproductionformanyyearsarebeingreviewedtoevaluateoptionsforincreasingtheultimaterecovery.Thetaskinvolvesthedeterminationofthevolumeandlocationofmobileremainingoilandsubsequentlythetechnicalandeconomicassessmentofmethodstorecoverthisoil.Thefirstpartofthistaskisoftendifficultbecauseofthepoordataqualityassociatedwitholdfields.ThisisparticularlytrueforeasternEurope,formerRussianrepublicsandSouthAmericawheremanyfieldsareonofferforfarm-intypeventures.Certainbasicdataareusuallyavailableandthepurposeofthispaperistoprovidefirstroundscreeningcriteriabasedonthesedatainordertoselectthosereservoirsforwhichre-developmentschemesarelikelytobeeconomic.Fortheselectedreservoirsfurtherstudyandsomeadditionaldataacquisitioniswarranted.Thedatawhichmaybeexpectedtobepresentarewelllogs,limitedcoremeasurements,basicfaciesdescriptions,originaloil-in-placeandcumulativeproductionfigures,contourmapandwellpositions.Essentialisalsotohaveaccesstowellcompletiondata.Individualwellperformancedataareoftendifficulttoobtain.Theproposedscreeningschemeisbasedonaclassificationoftypesofremainingoilconfigurations.Oncesuchapotentialoilpockethasbeenrecognizedoneattemptstoassessitseconomicvaluebyestimatinganumberofparameterstoalimiteddegreeofaccuracy.Dip,originalaccumulationconditions,beddingthickness,reservoirprofile,porositydistributionandoriginaloilsaturationcanoftenbedeterminedsatisfactorily.Moredetailedreservoirarchitectureandparticularlypermeabilitydistributionaremoredifficulttoobtain.Theclassificationschemeformobileremainingoilpocketsconsistsofasplitintoreservoirswithahighandalowverticalpermeability/connectivityandafimthersubdivisionintotypeswithahighandlowhorizontalconnectivity.3452K.J.WEBER&H.DRONKERTSPE50669Thisclassificationcanusuallybecarriedout.Inthispaperfourmajortypesofmobileremainingoilconfigurations,representingthefourcombinationsofhighandlowverticalandhorizontalconductivityarebeingdiscussed.Thescreeningcriteriapresentedarebasedonre-developmentwithpairsofhorizontalside-tracksfromexistingwells.Acostofonemilliondollarshasbeenassumedperjobofrecenteringthehole,millingthecasinganddrillingandcompletionofthetwoside-tracksof300meterlengtheach.Thisisbasedonvariouscostestimatesobtainedforlandoperations,whichformthebulkoftheexpectedprojects.Theeconomicanalysisbasedonthismethodandcostlevelshowsaremarkablylargescopeforre-developmentofreservoirswithoil-rims,atticoilcasesinfaultedreservoirsandlayercakereservoirswithbedsofcontrastingpermeability.Fluviatilelabyrinthtypereservoirs’aremuchmoredifficulttore-developbutanumberofobservationsarebeingmadetoindicatethemorefavorableconfigurations.ClassificationofMobileRemainingOilConfigurationsTheretentionofmobileoilinsufficientlylargevolumestoalloweconomicre-developmentislargelycontrolledbythepresenceofheterogeneouspressuredistribution,andthefluiddensityandviscositycontrasts.Thispaperisonlyconcernedwithsandstonereservoirscontaininglightoilwithhavebeendevelopedwithverticalwells,producedunderreasonabledraw-downconditions.Inviewofthepotentialforrecompletionandinfilldrilling,themostimportantheterogeneitiesarefaults,boundariesofgeneticunits,largepermeabilitycontrastsandbafflestoflowsuchasshaleintercalations.Followingthesubdivisionsofelasticreservoirsintolayercake,jigsawpuzzleandlabyrinthtypesonecanalreadypredictanumberoftypicaloildisplacementpatterns.Consideringthemajorlargescaleheterogeneitieswecansubdividethereservoirsintotypeswithahighverticalconductivityandthoseinwhichthestratificationandlowpermeableintercalationsresultinalowverticalconductivity.Nextwecanfi.wthermakeadistinctionbetweenlayercakereservoirswithahighdegreeoflateralcontinuityofthebedsandreservoirsinwhichthelateralcontinuityislimitedbyfaultsorpinch-outsofthesandbodies.ThisleadstoaschemeasshowninFig.1.TothefirstcategoryAwecanattributeoilrimreservoirswithahighverticalconductivityinwhichunproducibleoilcolumnsareleftbetweentheverticalwellsasaresultofcuspingandconing.Poorlateralcontinuitycanbecausedbyanormalfault(B1)whicheven,whennon-sealingoverthejuxtaposedreservoirinterval,trapsoilintheup-thrownblockagainstthecaprockinthedown-thrownblock.Dependingonthethrowofthefaul~thestructuraldipandthedistanceoftheverticalwellsfromthefault,avolumeofoilwillremainbehindwhenthewellswaterout.Inlabyrinthreservoirsonefindsupdipstratigraphictraps(B2)especiallyinlownet/grosscases.Insuchcaseswewillalsoencounterpoorsweepefficiencyunlessthewellspacingissmall(D).Poorsweepcanalsoresultfromstratificationwithlargepermeabilitycontrastbetweenthebeds,particularlywhentheseareseparatedbyimpermeableintercalations(C).Thissituationisquitecommoninlayercakereservoirs.Evenwithouttheimpermeableseparationscross-flowmaybelimitediftheverticalpermeabilityofthelowpermeablelayerislow.ThissituationIi-equentlyoccursinfluviatilelabyrinthreservoirsandthiscanoccurincombinationwiththeconfigurationB1andD.ThecasesselectedforthispaperaretheAtypeoil-rimreservoir,theatticoiltypeB1,thelayercaketypeB2withvaryingpermeabilitycontrastsandbedthicknessesandthefluviatiletypereservoircombiningtheB2andCcharacteristicswhilealsotakingintoaccounttheinternalheterogeneityofthechannel-fillbodies.Exceptfortheoil-rimreservoirs,thecasesstudiedarelightoilreservoirsproducedbywaterdrive.Simulationofjigsawpuzzletypereservoirswithadjacentsandbodieswithlargepermeabilitycontrastshasshownthatretentionofsignificantvolumesofmobileoilinthelowpermeablebodiesrequiresverylargecontrastswhichrarelyoccurinnaturez.Thus,suchconfigurationshavebeenleftoutofconsiderationinthisstudy.OilRimReservoirs.Producingoilrimreservoirswithverticalwellswillalwaysleavebehindconsiderablevolumesofremainingmovableoilasaresultofcuspingandconing.Awayfromthedrainagepointstheoilcolumnmaybelargeenoughtobeproducedeconomicallywithhorizontalside-tracks.Ifconvenientlypositionedshalebreaksarepresentandthedipislow,individualverticalwellscansometimesproduceoilrimsquiteeffectively.Asmallwellspacingobviouslyleadstoahigherrecoveryfactorthanalargespacing.Datafrom29oilrimreservoirs,mostlywithfairlyhighverticalconductivityandwellspacingsof600to800mwerecollectedtoformabasisofcomparisonforourtheoreticalmodels.Consideringahomogeneousanisotropicreservoirproducedatamoderatesupercriticalratewithagivennumberofwells,thefinalconfigurationoftheremainingmobileoilmaybeassumedtobeindependentoftheoriginaloilcolumn.Usingadetailedreservoirsimulationofafieldasexample3,atheoreticalmodelwasdefinedbyiterationtofittheobservedtrendinrecove~factors(Fig.2a).Underconditionofastrongwaterdrive,thegascapessentiallystaysinplaceexceptfortheformationofcusptowardstheproducingwell.Thewaterwillmoveupuntilthetopofthewaterconeand/orthebaseofthegasconewillreachtheperforatedzone.Withacuspingzoneof5minwhichtherecoverycoefficientis0.2andaconingzoneof10mwitharecoverycoefficientof0.1,weobtainoverallrecoveriesthatfollowthetrendofthe29reservoirs(Fig.2b).Theactualrecoveryissomewhathigherformostfieldswhichmaybeattributedtothepresenceofshalebreaksholdingbacktheconingeffect4andtocarefulmonitoringoftheproductionrateduringthefinalstagesbeforeclosing-inthewells.Thisresultgaveencouragementtosetupageneralsimulationmodeltopredictfirstlytherecoveryfi-omagivenoilrimthicknessandsecondlytherecoveryfromapairofhorizontalside-tracksdrilledintotheremainingoilzone(Fig.346SPE50669SCREENINGCRITERIAFOREVALUATINGTHEDEVELOPMENTPOTENTIALOFREMAININGOILINMATUREFIELDS32c).Themodelconsistedofahomogeneousanisotropicsandstonewithahorizontalpermeabilityof1000mDanda-verticalpermeabilityof100mD.Theporosityis0.25andN/Gis0.80.Thedimensionsare800minthestrikedirectionand1200minthedipdirection,thicknessis40mandthedipis5degrees.Themodelissubdividedintovariablesizeblocksof10x1Oto80x100inthex-ydirectionswithaconstantthicknessof5m.Oilviscosityis0.3cp(0.3*10-3Pas).Theoilrimheightwasvariedfrom20to100m.SimulationhasbeencarriedoutwiththeIMEXadvancedreservoirsimulator(ComputerModellingGroup).Thegridsizeisreducedaroundthecentralverticalwellandalongthepathsofthesubsequenthorizontalside-tracks.Theresultinggraphofoilrecoverypercentageversusoilcolumnheightforverticalwellsshowsareasonablebutratheroptimisticfitwiththefielddata(fig.2c).ThisispartlyduetoacarefulchoiceofproductionrateasafunctionofoilcolumnandpermeabilitybasedonthestudiesofSchols5.Theverticalwellswereabandonedwhena90percentwatercutoraGORof1000m3/m3wasreached.Aftertheabandonmentoftheverticalwellthesimulationiscontinuedwiththeproductionofthepairofside-tracksasshowninFig.2c.Theabandonmentconditionswerethesameasfortheverticalwells.Thisresultsinrecoverieswhicharefrom8to10percenthigherthanthoseachievedwithverticalwellsonly(Fig.2b).Aneconomicassessmentoftheseresultsforanoilpriceof15US$/bbl,aninvestmentof1millionUS$andaveragefiguresforoperatingcostandgovernmenttake,showsthatside-trackingintoremainingoilzonesofoilrimreservoirsisattractiveforawiderangeofcases.Fora5yearproductionperiodofthehorizontalwells,netpresentvaluesoffrom1.5tonearly4MMUS$werecalculatedforrespectivelya20anda60mthickoriginaloilrim.Theconclusionisthatforreservoirsthatareclearlyquitehomogeneous,side-trackingisalmostcertainlyaviableproject.OilrimreservoirswithlowultimaterecoveryrelativetotheresultsshowninFig.2bmayalsobeinteresting.Apartfrompoorproductionpracticethislowrecoverymaybecausedbyparticulartypesofheterogeneitysuchascombinationsofthinhighpermeabilitystreakscoupledwithdipsabove5degrees.Forsuchcasesspeciallydesignedhorizontalside-tracktrajectoriescanbedesigned.Inthiscontextitisusefidtostudyanumberofrecentcasehistoriesofthedevelopmentofoilrimreservoirs67,8ForthemorehomogeneousreSWVohXwithhorizontalwells.thescreeningactivitiesshouldbeconcentratedonthepermeabilityprofiletoensurethattheremainingoildistributionresemblesthemodeledconfiguration.AtticOil.Thenextconfigurationforwhichscreeningcriteriawerederivedistheatticoiltype(fig.3a).Numerousoilfieldsareeitherboundedbyfaultsorareintersectedbycrestalnormalfaults.Ifoiloccursadjacenttothefaultsacertainvolumeofoilremainstrappedbetweenthefaultandthenearestwells.Thisvolumeisafimctionofthethrowofthefault,thedip,thethicknessofthereservoir,thepermeabilityprofileandthepositionofthewellstogetherwiththeproductionpolicy.Thusitisnecessarytostudytherelativeinfluenceofanumberofparameterstoobtainarealisticideaoftheremainingmobileoilvolumes.Togetanimpressionofthevolumesthatarecommonlyleftinattictraps,astudywasmadeofnumerousfieldswithstructureslikefaultedanticlines,tiltedblocks,andthoseformedbysaltdomesorwrenchfaulttectonics.Assumingahorizontaloil-watercontactattheleveloftheintersectionofthewellandthetopofthereservoironeusuallyobservesoilvolumesintherangeof100000to300000m3fordistancestothefaultsof100to300manddipsof1to10degrees.HoweverthesevolumesareinrealitymuchhigherwhenconingaroundtheproducingwellsistakenintoaccountasshowninFig.3a.AseriesofreservoirsimulationswerecarriedoutwiththeIMEXblackoilsimulator,usingalightundersaturatedcrudeandtheseriesofparameterpermutationsshowninTable1.Themodelconsistedofarectangulargridwith19x20x10blocksinthex-y-zdirectionswithasizeof920minthex(strike)and600minthey(dip)direction.AsdetailedinTable1,theverticalthicknessandpermeabilityprofilewerevariedtorepresentanaveragebarrierbar,fluviatileorhomogeneousanisotropicreservoir.Firstlytheproductionwasmodeledofaverticalwellsituatedinthecentreofthegridatdistancesof50to250mfromthefaultformingtheupperno-flowboundary.Theproductionwasstoppedwhenawatercutof90percentwasreached.Wellproductionlevelswereoptimisedtoobtainmaximumrecovery.Aftera200daytimelapsetostabilisereservoirpressure,thesimulationswerecarriedonwiththeproductionofapairofhorizontalside-tracksalongthefault,eachwithaperforatedlengthof280m.Againproductionwasstoppedwhena90percentwatercutwasreached.Astrongnaturalwaterdrivewasassumed.Theresultsofthesimulationsoftheverticalwellperformancesandtheremainingoilvolumespriortodrillingthehorizontalside-tracksshowalargediscrepancybetweenthevolumetricestimatesmadewiththeplanimeter.Especiallywhendipsarelowtheassumptionofahorizontaloil-watercontactresultsinasignificantunderestimationoftheremainingoil(Fig.3b).Theerrorcanbeaslargeasafactorof100forlowdipsandsmalldistancestothefault.Toobtainanestimateofrecoverableoilforthepairofside-tracksonehasfirsttodeterminethevolumebetweenwellandfaultusingahorizontalcontact,nextoneappliesthecorrectionindicatedbyFig.3b(differentforeachreservoirtype).Finallyoneusestherelationshipbetweendistancetofault,dipandrecoveryfactorfortheappropriatetype(Fig.3c).Theactualscreeningprocedureshouldincludeacriticalreviewoftheaccuracywithwhichthefaultpositionisknown.Ifthefaulthasnotbeenpenetratedneartheattictrap,t
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