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SEG-2009-3944

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SEG 2009 3944
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Remainingoilthicknessandwellpositioningusing4DatAlbafield,NorthSeaAliTura*,StevenDobbs,KevinDavies,OliverHermann,andJenniferZhang,ChevronUpstreamandTechnologyCompaniesSummaryHighquality4Dseismicshotin2008overtheAlbafieldallowsustoestimatetheremainingoilthicknessfrom4Damplitudes.Thisisusedtomodifyplannedwellsandreduceriskfornewproductionandwaterinjectionwells.The4Dallowsbetterverticalpositioningofhorizontalwellsusingthearealinformationof2008oil-watercontact(OWC)andtopsand.Further,the4Ddataalsoshowstime-shiftsacrossthereservoirthatmatchesinjectorandproducerlocations.IntroductiontoAlbaTheAlbafieldintheCentralNorthSeawasdiscoveredin1984withfirstoilin1994.Cumulativeoilproductionreached350mmbblin2008fromafieldSTOIIPof1000mmbbl.ThefieldisEoceneageturbiditechannelfilldeposits.Sandsareunconsolidatedandfinegrainwithdiscreteshaleflows.Averageporosityis35%,averagepermeabilityis2.7darcy,andtheoilisheavywith20API.Waterdepthisonaverage140mandreservoirdepthis2km.Thefieldisdevelopedbywater-floodandgravelpackedhorizontalwells.ThelargestuncertaintyinthefieldispresentdaymappingoftheOWC.Weseethatthe4Ddatacanhelpreducethisuncertainty.4DAcquisitionFourlegacy3DsurveysincludingstreamerandOBCwereanalyzedforbaselinesuitability.Considerationwasgiventoproductionperiod,4Drepeatabilityandcost.Themostrecenttowstreamersurveyacquiredin2002with8streamers100mapartand25mflipflopsourcewasselectedasthebaseline.Threefacilityundershootsformedpartofthe2002surveyandwereincludedinthemonitor.Navigationdatafromthebaselinealongwithlocalseastatedatawereusedtostatisticallypredictrelativefeatherbetweenthebaselineandmonitor.Thebaselineshowedan70%probabilitythatfeatherwouldnotexceed±3degrees.ProbabilityNRMSfunctionsfordifferentoffsetsandacquisitionconfigurationswerecalculated.4Dqualityandcostwereevaluatedfornumberofdifferentmonitorscenarios,includingtheuseofdenserstreamerspacing,overlappingandsteerablestreamers.Theeffectsofseismicinterference,anticipatedduringthemonitor,weremodeledandusedtodeterminetimesharecostimplications.Theanalysisshowedthat20%NRMSuptoamaximumtargetfrequencyof70Hzwasanticipatedforacombinedsource,receiverrepeatabilityof15m,excludingtheeffectsofnonsourcegeneratednoise.Steerablestreamersweredeterminedasthemostcosteffectivesolutiontoachievethisrepeatability.Thesurveywasacquiredtominimizethecombinedsourceandreceivererrorswhilerepeatingthemonitorvesselheadings,includingundershoots.Postsurveyrepeatabilityanalysisinvolvedin-lineandcross-linerelativesourcesandreceivererrors.95%ofthecross-linesourceerrorswerewithin4.5mforstraightlines,14mforstronglydeviatedlinesaroundfacilities.95%ofrelativereceiverfeatherwaswithin3degreesforstraightlines,6degreesfordeviatedlines.4DProcessingThreesurveyvintages,1991,2002and2008wereprocessedthrougha4Dsequence.Allthreesurveyswereresampledearlyintheflowtocommonsamplingintheshot,receiver,midpointandoffsetdomains.Instrumentsensitivitycorrectionswereappliedtodeterministicallymatchthesurveyamplitudes.2Ddirectionaldeconvolutionwasusedtoremovethesource,receiverghostsandarrayeffectsforeachofthesurveys,includingundershootconfigurations.QCdemonstratedthatthedeterministicapproachdidresultinconvergenceoftheangledependantamplitudeandphaseresponses.Particularattentionwasfocusedonbincentreregularizationand4Dtracepairselection.Binnedtracesinthemonitorthatwereabsentinthebaselinewereremoved,evenattheexpenseofcoverage.Testingclearlyshowedthatsourceandreceiverrepeatabilitywasparamounttoestablishingaquality4Dimage.Afasttrack4Dcubewasavailablewithin2monthsoffinalshotandwasusedtoguidethemainprocessingaswellasenablingearlyinterpretation.Final4Dproductswereavailablewithin6monthsoftheacquisition.Keytothesuccessoftheacquisitionandprocessingwasextensivelogisticalandtechnicalplanning,meticulousQCandadherencetogoodrepeatability.Qualityof4DDataatAlbaTheaverageNRMSbetweenthebaseline2002andmonitor2008dataisabout20%whichisverygood4DdatarepeatforstreameracquisitionintheNorthSea(Figure1).TheNRMSvaluesapproach30-40%nearthethreeundershootplatformareas.Cautionisexercisedininterpretationintheseareas.InsomepartsofthefieldNRMScanbeaslowas10%andthe4Dseismicdataqualityintheseareasareexcellent.The4DdifferencebetweenbaselineandmonitordatashowsclearchangesinthereservoirzoneduetoOWC3944SEGHouston2009InternationalExpositionandAnnualMeetingAlbaRemainingOilfrom4Dmovement(Figure2).Outsidethereservoirzonenotime-lapsechangesareobservedindicatingthehighqualityofthe4Ddata.Mappingof2008OWCThequalityofthe4Dseismicdataallowsustopickthe2008OWCdirectlyfromthedifferenceseismic.4Dchangesfrom2002to2008aremostlydominatedbywatersaturationchanges.SyntheticseismicmodelingshowsthatwatersaturationchangesarelargeandcanmostlybeseenbyanalyzingtheP-impedancechanges(Figure3).ThismayseemcontrarytoconventionalinterpretationatAlbasincetheP-impedancecontrastbetweentheshalesandoilfilledsandsissmall,andSimpedancecontrastislargesoPSdataisusedtomapthereservoir.However,in4Dmode,themainsignalistheP-impedancecontrast.Figure3showsaverysmallimpedancechangeduetothesmallporepressurechangebetween2002and2008(about+/-400psi).Asaresult,theP-impedanceorstackeddifferencedatawillbefluidsensitiveandcanbeusedtoobtainthe2008OWC.Watersaturationchangefromthehistorymatchedsimulationmodelbetween2002and2008isshowninFigure4a.Thetransitionfromwaterfilled(red)tooilfilled(white)sectionisindicatedbytheblacklinelabeled‘08OWC’.4Dsyntheticseismicdataisgeneratedusingourin-house4Dmodelingsoftware.Thesynthetic4Ddifferencestackeddataaftera-90degreephaserotationisshowninFigure4b.Thezero-crossing(bluetored)inFigure4bcorrespondstothe2008OWC,asshowinFigure4a.Figure1:NRMSvaluesoverAlbabetween2002baselineand2008monitorafterfasttrackprocessing(scaleis0%to60%).OverallNRMSoffasttrackstackdataisabout20%whichisverygoodforstreamerdataintheNorthSea.TheNRMSislargerinthethreeareaswithobstructionsandinterpretationintheseareaswillneedtotakethisintoconsideration.Figure3:Synthetic4DmodelingindicatesthatP-impedancechangeswillshowchangesinwatersaturationΔSw.PressurechangesΔPparesmallat+/-400psibetween2002and2008andwillnotbedetectableinthe4Ddata.Figure4:a)Watersaturationchangefromthesimulationmodel,theoriginaloil-watercontact(OOWC),and2008OWCisshown.b)Zerocrossingofthe4Dstackedsyntheticseismicquadraturedata(bluetored)indicatesthe2008OWC.3945SEGHouston2009InternationalExpositionandAnnualMeetingAlbaRemainingOilfrom4DRemainingOilThicknessThe2008OWCisobtainedfromthefielddatabypickingthezerocrossingofthe4Dstackeddataafter-90degreephaserotation.Thetimedifferencebetweenthe2008OWCpickandtheAlbatopsandpickindicatestheremainingoil.InFigure5,thebluelineisthe2008OWCpickfromfielddataandthegreenlineistheAlbatopsandpickasobtainedfromconvertedwavedata.TheremainingoilisindicatedinFigure5astheareabetweenthesehorizons.Coningfrom4DandMatchtoSimulationModelThehighpermeabilityandporosityatAlbacauseswaterconingundertheproducers.The2008OWCobtainedfromthe4Ddatacanspatiallymaptheconingregions.InFigure6,weshowtheproducingwellsandwaterconingfrom4Dasindicatedbytheorangecircles.Theconingobservedfromthe4Ddoesnotmatchtheconingpredictedformtheproductionhistorymatchedreservoirsimulationmodelinmanyplaces.Asaresult,thesimulationmodeliscurrentlybeingupdatedtomatchboththe4Dseismicandtheproductiondata.Thismoreaccuratedynamicmodelcanbeusedtoreliablyplacenewwells.OptimizationofWellLocationsThetwo-waytime(TWT)betweenAlbatopand08OWCcanbeconvertedtoRemainingOilColumnThickness(ROCT)asfollows:ROCT=TWT*Oil_Sand_Velocity/2Further,theRecoverableOilThickness(ROT)canbecomputedasfollows:ROT=ROCT*Avg._Porosity*Oil_SaturationFinally,theRecoverableOilVolume(ROV)canbecomputedas:ROV=ROT*Area/BoiThe2008RecoverableOilThickness(ROT)overAlbaisshowninFigure7basedonthe4Ddata.Notethegoodmatchbetweenthewaterinjectors(bluespheres)andnoROT(i.e.swept).TheROTinformationisusedtoreduceriskonfutureinjectorsandproducersthatwillbeplaceinthefield.BetterverticalplacementofwellsisaccomplishedthroughoptimizingthewelllocationusingtheAlbatopsandandthepicked2008OWC.Matchof4DEstimatedProductionvsWellProductionComparingthe4Dseismicestimationofoilvolumeproducedinaclosedregiontotheactualproductionfromthewellscanindicatethequantitativereliabilityofthe4Dinformation.A4DestimateoftheproducedoilvolumeinacertainregioncanbeobtainedbycomparingtheoriginalOWCand2008OWC(orangeandbluelinesinFigure5)andcalculatingtheproducedoil(similartoROVcalculation).ForthiscalibrationweusetheAlba12Area(indicatedinFigure7).Thisareahaslow4DNRMSandisawayfromtheobstructionareas.TheactualoilproductioninArea12fromwellsis27mmbls.The4Destimatedproductionbasedonpickedcontacts,averageporosityandoilsaturationchangesis28mmbls.The4Dseismicisabletopredicttheproductionwithin4%.Figure6:Opacityfiltered4Dquadratuerdifferencedisplay(bluesurface)highlighitngthesweptzone(redareainFigure5)showsthespatialextentofconingundertheproducersasindicatedbytheorangecircles.Figure5:The2008OWC(inblue)canbeobtainedfromthe4Dquadraturedatabypickingthezerocrossingasshown.WealsoshowthetopAlba(green),thebaseAlba(cyan),andtheOOWC(orange).Theareabetweenthe08OWCandAlbaTopisremaingoil.3946SEGHouston2009InternationalExpositionandAnnualMeetingAlbaRemainingOilfrom4D4DTime-shiftsatAlbaAtAlba,time-shiftsgeneratedoverthereservoircanbeclearlyobserved(Figure8).Thesetime-shiftvaluesaresmall(+/-2to3ms)andcorrelatewellwithlocationofinjectorsandproducers.Producers(greenlines)showaspeed-upfromhardeningduetoincreaseinwatersaturation.Injectors(bluelines)showaslow-downfromsofteningduetoincreaseinporepressure.Thecross-sectionviewinFigure8bclearlyshowsgenerationoftime-shiftsacrossthereservoirinterval.Thesetime-shiftsarecurrentlybeingincorporatedintothebackgroundmodelofasimultaneousinversion.ConclusionsIn2008,a2002baselinestreameracquisitionwascarefullyrepeatedovertheAlbafieldintheNorthSea.Analysisofthisdatashows:NRMSvaluesfromacquisitionindicatehighquality4DdataClear4DamplitudechangescanbeseeninthereservoirfromOWCmovementPickingthe2008OWCallowsestimationofzonesofsignificantremainingoil,regionsofconingunderproducingwells,andcalibrationofthe4DdatawithproductiondatainisolatedregionswithgoodNRMSvaluesThereservoirsimulationmodelisbeingupdatedtomatchboththeproductiondataatthewellsandtheinterpreted2002and2008OWC’sfromtheseismic4Dfield-widedataThe4Ddataisusedtodeterminenewwelllocations,reducewellplacementrisk,anddetermineverticalplacingofthenewwells.The4Ddatawasusedtoconfirmthetwo2009drillingtargetswithinfivemonthsofrecordingandiscurrentlybeingusedtodevelopthe2010drillingtargets.AcknowledgementsTheauthorswouldliketothankChevronmanagementandAlbaco-venturer’sConocoPhillips,StatoilHydro,BP,Total,Cieco,andEndeavourforpermissiontopublishthiswork.WethankCGGVeritasforapprovalstopresentthe2002data,andWesternGecoforacquiringandprocessingthedata.Figure7:Recoverableoilthickness(ROT)overAlbaasof2008.Areaswithredindicatelargeremainingoil(upto12moilcolumn).Theproducedoil(whitespheres)andinjectedwater(bluespheres)volumebetween2002and2008areindicatedbythespheresize.ThisROTinformationisusedtodesignproducerandinjectorwelllocations.Figure8:a)Mapviewof4Dtime-shiftsoverAlba(blueindicatesupto3msslow-downandredindicatedupto3msspeed-up).Lighttodarkbluelinesindicatedperforationzonesofinjectorsandlighttodarkgreenlinesindicatetheperforationzonesofproducers.Thematchbetweenthespeedupneartheproducersandtheslowdownneartheinjectorsisclearlyseen.b)Cross-sectionthroughthe12areashowsspeedupinthereservoirduetoincreaseinwatercontent.Time-shiftsaregeneratedonlyacrossthereservoirzone.3947SEGHouston2009InternationalExpositionandAnnualMeetingEDITEDREFERENCESNote:Thisreferencelistisacopy-editedversionofthereferencelistsubmittedbytheauthor.Referencelistsforthe2009SEGTechnicalProgramExpandedAbstractshavebeencopyeditedsothatreferencesprovidedwiththeonlinemetadataforeachpaperwillachieveahighdegreeoflinkingtocitedsourcesthatappearontheWeb.REFERENCESEbaid,H.,A.Tura,M.Nasser,P.Hatchell,F.Smit,N.Payne,D.Herron,D.Stanley,J.Kaldy,andC.Barousse,2008,Firstdual-vesselhigh-repeatGoM4DsurveyshowsdevelopmentoptionsatHolsteinField:TheLeadingEdge,27,1622–1625.Hanson,R.,P.Condon,R.Behrens,S.Groves,andM.MacLeod,2003,Analysisoftime-lapsedatafromtheAlbafield4C/4Dseismicsurvey:PetroleumScience,9,103–111.3948SEGHouston2009InternationalExpositionandAnnualMeeting
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