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SPE_35520

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SPE_35520
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SPE35520WellPlacementOptimisationandRiskingusing3-DStochasticReservoirModellingTechniquesD.Seifert,SPE,J.J.M.Lewis,SPE,andC.Y.Hern,Heriot-WattUniversity,andN.C.T.Steel,BritishGasExplorationandProductionLtd.Copyright1996,SocietyofPetroleumEngineersThispaperwaspreparedforpresentationattheEuropean3-DReservoirModellingConferenceheldinStavanger,Norway,16-17April1996.ThispaperwasselectedforpresentationbytheSPEProgramCommitteefollowingreviewofinformationcontainedinanabstractsubmittedbytheauthor(s).Contentsofthepaperaspresented,havenotbeenreviewedbytheSocietyofPetroleumEngineersandaresubjecttocorrectionbytheauthor(s).Thematerial,aspresented,doesnotnecessarilyreflectanypositionoftheSocietyofPetroleumEngineersoritsmembers.PaperspresentedatSPEmeetingsaresubjecttopublicationreviewbytheEditorialCommitteeoftheSocietyofPetroleumEngineers.Permissiontocopyisrestrictedtoanabstractofnotmorethan300words.Illustrationsmaynotbecopied.Theabstractshouldcontainconspicuousacknowledgementofwhereandbywhomthepaperwaspresented.WriteLibrarian,SPE,P.O.8333836,Richardson,TX75083-3836USA,fax01-214-952-9435AbstractThecost-effectivedevelopmentofsmallfieldsrequiresathoroughdescriptionandevaluationofthereservoir,withastrongfocusontheestimationofassociateduncertainty.Thispaperdescribesanew,integratedmethodologyfordefiningandriskingoptimumhigh-angledevelopmentwellsasappliedtoaUKSNSbasin-margingasreservoir.SequentialIndicatorSimulation(SIS)realisationsofgeneticunitdistributionsweregeneratedforeachtargetreservoirzone,conditionedwithrelevantgeologicaldataderivedfromthereservoirandtheanalogueoutcrop.Themodelswere"pin-cushioned"withalargevarietyofwelltrajectoriesthatvariedintermsofinclination,azimuth,lengthandpositionwithinthezone.Geneticunitstringsarethenextractedfromeachwelltrajectoryandanalysedstatistically,resultinginrankedwelltrajectories.Thisintegratedapproachproducesrankedandriskedwellplanningoptions,onwhichrisk-consciousmanagementdecisionsarereadilymade.IntroductionDevelopmentdecisionsareincreasinglytakenwithlessdata,andtheuseofminimalfacilitiesrequiresthatoptimisationofdevelopmentwellsismorecriticalthanformerly.Theplanningofdevelopmentwellsincomplexreservoirse-quencesnecessitatesthedetailedintegrationofdatatorankpotentialwelllocationsandtrajectories.Utilisationofhybriddeterministic-stochasticreservoirmodels,enablesthisprocesstobecarriedoutefficiently,withquantificationofassociatedrisk.Thebenefitofoptimisingtheplacementofhighanglewellsisthattheriskassociatedwithwellplace-mentisminimisedandthatthemaximumvolumeofthereservoircanbeaccessedwiththeleastnumberofwells,whichinturnishighlycosteffective.ThemethodologyofthisstudyisoutlinedinFig.1.Detailedquantificationofthecontrolsonwellperformancewithinthereservoirwasfollowedbythedefinitionoftheappropriategeneticunits1,2.Inparallel,aperiodofsourcingandappraisalofpossibleoutcropanalogueswasfollowedbyadetailedoutcropstudywhichresultedinthederivationofquantitativedataoftheanaloguewithrelevancetotheFrobisherreservoir2.Integrationofthereservoirspecificandoutcropanaloguedataintoahybriddeterministic-stochasticmodelprovidedthebasisfordeterminationoftheoptimumwelltrajectoriesandsubsequentriskingoftheirsuccessinpenetratingpay.Thispaperwillonlybrieflyintroducethegeologicalanalysesofthereservoirandtheoutcropanaloguebutwillfocusindetailonthegenerationofthereservoirmodelandonthedeterminationandriskingoftheoptimumwelltrajec-tories.GeologicalAnalysisoftheReservoirTheFrobisherComplexisabasin-margin,LowerPermianRotliegendsandstonereservoir,locatedinUKCSblocks47/3b,3cand4a,oftheSouthernNorthSea(Fig.2).FrobisherissituatedontheplatformbetweentheEastMidlandShelfandtheSolePitTrough,withintheDowsingFaultZone.Thereservoircomprisesaheterogeneousmixtureoffluvio-aeoliansediments.Elevenverticallystackedpackagesofalternating"wetter"and"drier"sedimen-tationphasesarerecognisedfromcore,whichintheabsenceofbiostratigraphicmarkers,haveprovidedthebasisforreservoirzonation.Thesecyclesarethoughttorepresental-locyclic,climaticfluctuations.Ageneticunit(GU)isdefinedasabodyofrockthatisdistinctfromotherbodiesonthebasisofgeometrical,petrophysicalandspatialproperties3.FivegeneticunitsarerecognisedinFrobisher,basedontheverticalassociationandproportionoffacies(Table1).Probepermeabilitystudieshaveshownthatprimarydepositionalattributesarethemaincontrolonpetrophysicalvariation.Permeabilityvariationwithinthereservoirreflectsfaciesandgrainsizechanges,andtheassociationoffaciesintogeneticunitsresultsinacharacteristicpermeabilityprofileforeachgeneticunit.2WELLPLACEMENTOPTIMISATIONANDRISKINGUSING3-DSTOCHASTICRESERVOIRMODELLINGTECHNIQUESSPE35520Effectiveproductionreliesontheidentificationanddrillingofhighpermeabilitygeneticunits.Targetingofbet-terqualityreservoirintervalsisdependantonanunderstand-ingofthespatialdistributionofhighpermeability,aeoliandunegeneticunitswithinthereservoir.ThepermeabilitiesoftheDuneGUareonaveragetwoordersofmagnitudehigherthanthoseoftheothergeneticunits,foragivendiageneticsuite.GeologicalAnalysisoftheAnalogueTheCutlerFormation,exposedinSEUtah,isaPermianaged,mixedfluvio-aeoliansequence.TheUpperCutlerBedsoftheCutlerFormationwereselectedasananaloguetoFrobisher.SuitabilitytoFrobisherwasassessedbycompar-isonoffaciestypes,proportions,geneticunitthicknessesandstylesoffluvio-aeolianinteraction.Inaddition,thesizeofthesystemandregionalcontrolsondepositionalarchitec-tureweretakenintoconsideration.Quantitativedatawasac-quiredfromlayerswhichdisplayedsimilargeneticunitpro-portionsandstylesofinteractiontotargetZoneswithinFrobisher.Twelvekilometresofphotopanelwereacquiredfrom15localitiesbyhelicopter-basedphotography.InterpretationandStatisticsThephotopanelswereinterpretedinthefieldtoensureaccu-rateidentificationofambiguousfeatures.Theresultantlinedrawingswerepassedthroughanimageanalysispackagetoderivequantitativestatisticsonthesizeanddistributionofkeygeneticunits(Fig.3).Asthesizeandconnectivityofremnantaeolianbodiesisafunctionofthesize,stackingpatternanderosivecapacityofco-evalorsubsequentfluvialphases,emphasiswasplacedonacquisitionofquantitativedataandmodellingofthefluvialbodies.FluvialchanneldimensionsrepresentativeofFrobisherweredeterminedbyscalingoutcropderiveddimensionstoFrobisherdata.TheaveragethicknessofthefluvialchannelgeneticunitwascalculatedforthezoneofinterestineachwellwithinFrobisher.Anenvelopearoundthismeanvaluewascreatedandtheaverageaspectratiowithintheenvelopewascalculated(Fig.4).Thisvaluewasusedasthedefinitiveaspectratioforchannelswithinthatwell,forthatzone.ComparisonofFluvialChannelGeneticUnitthicknessbetweenCutlerandFrobisherindicatesthatFrobisherchan-nelsaregenerallythickerthanCutlerchannels.Thisproba-blyresultsfromdifferencesinthesizeofthetwodeposi-tionalsystems.TheCutlersystemismuchlargerthanFrobisher,therefore,toadequatelysampleasimilaramountofproximal-distaldepositionalvariabilityinchannelsizestotheFrobishersystem,alongerproximaltodistalareawithintheCutlerFormationshouldbesampled.Scalingandinter-polationbetweenCutlerFormationandFrobishergeneticunits,however,hasallowedgenerationofrealisticreservoirmodelsinthekeyreservoirzones.ReservoirModellingProceduresModellingTechnique.Traditionally,reservoirmodelsweredevelopeddeterministically,usingcorrelationtech-niques.Thisyieldedasinglereservoirmodel.Forthisstudy,however,stochasticdistributionsofthegeneticunitswereproducedwithinaconstructional(deterministic)correlationandzonationframeworkderivedfromregionalandlocalmodels.Stochasticmethodswereemployed,becausetherangeofoutcomevaluesderivedfromequiprobablerealisa-tionsprovidesameasureofuncertainty4.TheSequentialIndicatorSimulationmethod(SIS)wasadopted,mainlyforitsflexibility.Adiscussionofthepro'sandcon'softhistechniquewouldbebeyondthispaper,butcanbefoundinreferences4through9.However,itshouldbementioned,thatSISisapixel-basedtechnique.Thus,griddingofthemodelpriortosimulationisrequiredandmodelsizehastobeconsideredcarefully,sincecomputertimeneededtosimulateamodelisafunctionofmodelsize(intermsoftheamountofcellswithinthemodel).ThestochasticsimulationswereperformedusingStanfordUniversity's"GSLIB"software8.TherealisationswerethenloadedintoLandmark's"SGM"softwareforvisualisationandfurtheranalysis.Theaimofthemodellingwastoreproducethesedimen-tologicalheterogeneityofthereservoir;itthereforefocusedonthedistributionofgeneticunits.Inordertoachievethisattherequiredresolution,"generic"modelsoftheFrobisherreservoirweredevelopedinseveralkeylocationsaroundex-istingwells,whichreducedthearealsizeofthemodelsandthusallowedforsmallercellsizes.Itwasassumedthatthemodelledreservoirareawasnotfaultedandallexistingstruc-turewithinthereservoirhasbeenremoved.Petrophysicalmodellingwasnotattemptedatthisstage.Indeterminingthebestmodellingstrategy,itisimpor-tanttounderstandwhichgeneticunithasthemostsignifi-cantimpactontheconnectivityofthepay.Asindicatedabove,sedimentologicalanalysisofthewellandoutcropdataledtotheconclusionthatthegeometryandconnectivityoftheaeoliantargetsandsisgovernedbyincisingfluvialchannels,sourcedfromtheSW.Thus,itwasdeemedappro-priatetofocusonmodellingofthefluvialbodiesinaback-groundofaeolianratherthanviceversa.Thethicknessofthecellswaschosenwithrespecttothicknessesobservedinthewellsfortherespectivegeneticunits.ThecellsXandYdimensionswerechosenwithre-specttothewidthsofbodiesfortherespectivegeneticunitsasobservedintheoutcropanalogue.Asaresult,themod-elledportionofthereservoirforoneofthekeylocationsisasquareareaofapproximately4440x4440feet(1354x1354m),withathicknessof28feet(8.7m).Themodelconsistsof690,000cellsandthecelldimensionsusedinthismodelare20x20x2feet(6x6x0.6m).Thesedimentologicaldatafromthewellwasgriddedintotwofeetincrements,aftergroupingofthegeneticunitsintothethreemodellingcate-gories(Table1).Forthisstudy,imperialmeasurementswereusedtobecompatiblewithwelllogandcoredata.Animportantfactorinchoosingthebestmodellingap-proachwastheneedtoreproducelargescalelateraltrends(non-stationarity)thatareobservedwithintheFrobisherreservoir.Suchtrendsare,forexample,variationsintheaeo-liansedimentcontent,andchannelsizeacrossthemodel.SPE35520D.SEIFERT,J.J.M.LEWIS,C.Y.HERN,N.C.T.STEEL3Suchtrendsarederivedfromnearbywellsandtheregionalgeologicalframework.Theselateraltrendshavebeenincor-poratedbymodellingeachzoneasaseriesoffour"slices"(Fig.5).Differentsetsofstatisticalinputparametersweredefinedforeachofthefour"slices",usingthegeneticunitproportionsandgeometriesappropriateforthatvolumeofthereservoir(Fig.6).The"slices"weresimulatedsequen-tially,thefirst"slice"tobesimulated(S2)beingconstrainedbythewell,subsequent"slices"beingconditionedtothead-jacentfaceoftheprevious"slice"(Fig.5).InputParameters.Thegeometriesoftheaeolianandwetinterdunegeneticunitsandtheorientationsofallthreege-neticunitswerekeptconstantinallfourslices,becausetheyarebelievedtoremainmoreorlessconstantthroughoutthemodelledportionofthereservoir.Onlythefluvialgeome-trieshavebeenchangedinaccordancewiththeassumedlin-eartrends.Difficultywasintroducedwhenchoosingthevariogrammodel.Usingasimpletwo-dimensionalBooleanapproachfordistributingellipsoids,atest-modelofanarealsurfacewasgenerated(binarymodel),usingactualgeometriesasde-rivedabove.Thealternatingsequencesofchannelandaeolianresultedinhorizontal(semi)variogramsthatoscillatedupanddown,exhibiting"holes"(Fig.7).Thesameistruefortheverticalvariogramderivedfromwelldata.Intheabsenceofa(semi)variogrammodelthatcanaccommodatethe"hole-effect"asproposedbyJensen10,itwasdecidedtouseonlyasphericalmodelforthevariograms.Simulations.Fig.8showsanarealcross-sectionofthefirststochasticrealisationforoneofthekeylocations.Thefluvialchannels(grey)trendtowardstheNE.Notice,therearemorefluvialchannelspresentinthesouthernportionofthereservoirandthechannelstendtoamalgamatetowardsthesouth,whichresultsinwiderchannel-tracts.Thissub-stantiatedtrendsintheconceptualgeologicalmodel.Thewetinterdune(WID,white)areorientednorth-south,paralleltothedunecrests.Finally,inblack,theaeolianservesasthe"background"category,exhibitingthegeometryofremnanterosionalaeoliandeposits(determinedbyfluvialincision).Inthisrealisation,moreWIDandaeolianarepresentinthenorthcomparedtothesouthernareasofthereservoir,whichagainverifiestheconceptualgeologicalmodel.Forreference,Fig.9showsadifferentrealisation(realisation2)ofthesamecross-section,wheresimilartrendscanbeobserved.Figs.10and11showcross-sectionsinthealong-channelandacross-channeldirectionsforrealisation1.Thelocationsofthecross-sectionsareshowninFig.9.Fromthese,thehigherproportionsoffluvialgeneticunitsarevis-ibleinthesouthernareas,whereasmoreaeolianandWIDgeneticunitsarepresentinthenorthernareas.Itcanalsobeseenthatthechannelsarethickerandwiderinthesouthernareas(Fig.11),whichisconsistentwiththetrendimposedbytheinputparameters.Thesecross-sectionsdemonstratecompartmentalisationoftheaeoliansandbodiesbyfluvialchannels.Expectedsedimentologicalfeatures,suchasfluvialincisions,aeolianoverriding,andthecomplexinterplayofaeolian,fluvialandWIDarealsoreproducedinasedimento-logicallycoherentfashion.WellPlacementProceduresObjectives.Themainpurposeofthisstudywastode-velopandapplyanewapproachfortheplanningofsuccess-fulhorizontalandhighlydeviatedwells.Themainadvan-tagesofdrillingsuchwellsaretheincreasedhorizontaldis-placement(extendedreachwell)andtheincreaseddrainagearea.Furtheradvantagesaretheincreasedlengthofthecom-pletionzoneperwell,andthepossibilityofconnectingsev-eralvolumesofpay,thusaddingsignificantlytotheproduc-tivityofthewell11.Consequently,thetwo-foldobjectiveofthiswellplace-mentstudywasto:1)findtheorientationofthewelltrajectorywhichislikelytocontainthehighestproportionsofaeoliange-neticunits(tomaximiseproductivity),andatthesametimeintersectsthemostaeolianbodies(tomaximisedrainage);and2)riskthelikelihoodofwellsinthisorientationinter-sectingthesehighproportionsofaeoliangeneticunits.Toeffectthis,agreatnumberofnumericalwellsweredrilledthroughthereservoirmodelandthegeneticunitpro-portionsandthetypesofbodiesintersectedwereextractedfromeachwelltrajectory.Thetrajectorieswerethenrankedinorderof"success".PincushioningtheModels.Inordertocomprehen-sivelysampletheentirereservoirmodel,tenlocationsforwellclusters(Figs.12and13)wererandomlyselectedwithinthevolumeofeachrealisation,foreachreservoirmodel.Awellclusterisapointfromwhichnumericalwellsare"drilled"inalldirections.Foreachwellcluster,wellsweredrilledasfollows:1)Ahorizontalwellevery15°azimuthwithawelllengthof1600ft(well-type1).Forthisdataset,thesewellclusterswereestablishedatdifferent,randomlyselectedelevations(Fig.12).2)Asetof24inclinedwells(inclinationof1°)every15°azimuth(well-type2)and1600ftlength(Fig.13).3)Anotherwellclusterwasestablishedintheverycentreofthemodel.Originatingfromthiscluster,horizontalwellsweredrilledevery15°ofazimuth,withineveryoneofthe14celllayersinthereservoirmodel(well-type3).Eachwellwasdrilledwiththethreedifferentlengthsof"long","medium"and"short"(Fig.14).4)Forthesamecluster,asetofinclinedwells(inclinationof<1°)weredrilledatevery15°azimuth(well-type4)andwithvariousdifferentlengths(Fig.15).Insummary,atotalof956numericalwellswere"drilled"forthemodelalongwhichthegeneticunitproportionsandthebodiesintersectedwereextracteda
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