SimultaneousAcquisitionofDistributedAcousticSensingVSPwithMulti-modeandSinglemodeFibreOpticCablesand3C-GeophonesattheAquistoreCO2StorageSiteD.E.Miller1,T.M.Daley2,D.White3,B.M.Freifeld2,M.Robertson2,J.Cocker4,M.Craven4CorrespondingAuthor:DouglasEMiller,douglas.miller@silixa.com,224Marlborough#52Boston,MA02116

1Silixa,LLC2LawrenceBerkeleyNationalLaboratory3GeologicalSurveyofCanada,NRCan,4Chevron2WorkbyLawrenceBerkeleyNationalLaboratorywaspartiallysupportedbytheCCSMRprojectfortheAssistantSecretaryforFossilEnergy,OfficeofCoalandPowerSystemsthroughtheNationalEnergyTechnologyLaboratory,oftheU.S.DepartmentofEnergy,undercontractNo.DE-AC02-05CH11231.DraftofFebruary12,2016;AcceptedforCSEGRecorderspecialeditiononDAS/BoreholeGeophysics

AbstractAdynamite3DVSPsurvey,aVibrator2DVSPsurveyandrelatedcalibrationsurveysattheAquistoreCO2storagesiteinSaskatchewanservedasatechnologytestfordistributedacousticsensing(DAS).DASdatawasacquiredasaverticalseismicprofile(VSP)ontwocodeployedfibres,onesingle-mode(SM)andonemulti-mode(MM),simultaneouslywitha60-level3-componentwirelinegeophonearray.A2Dgridofexplosiveshots,usedforbaseline4Dsurfaceseismic,provided3D-VSPdatafromallsensors,whilevibratorsourceswereusedforasingle2DVSPline.TheDASfibreswerecementedinplaceontheoutsideofthewellcasingduringtheoriginalwellcompletionandextendtoadepthof~2.8km.GoodqualitydatawasacquiredbyallsystemswithcomparableSMandMMVSPresults.DASdataconvertedtoparticlevelocityandgeophonedatahavecomparableresponses.BothexplosiveandvibroseissourcetypesgivegoodqualityDASdatawithexpectedimprovementfromtwoversusonevibroseissource.Weobservevariablecoherentboreholenoiseattributedtovariablecementquality.GoodqualityandcomparablemigratedimagesareobtainedfromDASandgeophonedata.Followingthesetests,weconcludeDAS3DVSPisaviablecandidatefortime-lapsemonitoring.

IntroductionLocatedinsouth-easternSaskatchewan,nearEstevan,AquistoreisCanada’sfirstdeepsalinecarbondioxide(CO2)storageproject.ManagedbytheRegina-basedPetroleumTechnologyResearchCouncil(PTRC),AquistoreisthestoragecomponentofSaskPower’sBoundaryDamCarbonCaptureandStorage(CCS)integrateddemonstrationproject,andisoneofahandfulofactiveCO2storageprojectsintheworld.Itisdesignedtobebothanimportantdemonstrationprojectandacriticalresearchsiteandindustriallaboratory.TheAquistorestoragesiteisa3x3kmareaofreclaimedpasturelocatedthreekilometerswestoftheBoundaryDamplant(Figure1a).Two3.4kmdeepwellsatthecenteroftheAquistoresite,oneforinjectingCO2andtheotherformonitoringtheCO2plume,weredrilled150mapartwithinstrumentationinstalledbehindcasingineachwell.ThetargetreservoirforBoundaryDam’sindustrialCO2storageisat3.2kmdepth.ThereservoircomprisesthelowerWinnipegandDeadwoodformationswhichconstitutea150mthickCambriansedimentarylayerwithbrinyporefluid.

Figure1(a,top)AerialviewofAquistoresiteshowinglocationofseismicmonitoringsourceshotpoints(bluedots)andburiedgeophonesensors(greendots),alongwithwells(blackcircles),pipeline(red)andthenorth-southlineofvibratorpositions(orange).

(b,bottom)(left)Monitoringwelldiagramwithhorizonandinstrumentationlocations;(right)photooffibercabledeploymentonmonitoringwellcasing.

MultipleinstrumentlinesweredeployedoncasingduringtheobservationwellinstallationinNovember2012includingafibreopticcablecontainingonemultimode(MM)fibreloopfordistributedtemperaturesensing(DTS)

measurementsandonesingle-mode(SM)fibrefortelemetrytodownholegaugesandforperformingdistributedacousticsensing(DAS)measurements.Thebottomsectionofthefibrecablewasdamagedduringthewellcasinginstallation,causingoverallshorteningoftheboththeSMandMMfibresinthecableandtheseveringofthelowerMMfibreloop.Opticaltime-domainreflectometry(OTDR)testsconfirmedthattheresultingthreesingle-endedfibres(1xSMand2xMM)intheobservationwellarecontinuousandremainfullyfunctionaltoadepthofnearly2.8km.Figure1bshowsaschematicoftheobservationwellandthelocalgeologicalformationstogetherwithaphotoofthecasingdeployment.

DataAcquisitionAnimportantaspectoftheprojectistheefforttocharacterizeandmonitortheinjectionprocessusingtime-lapse3Dseismic.SeismicactivitiesareexpectedtocontributetogeologicalcharacterizationoftheCO2storagereservoir,demonstratethesafetyoftheinjectionprocess,andprovideongoingverificationofthesecurityoftheinjectedCO2.GiventhegreatdepthoftheAquistorereservoirandthelimitedarealextentexpectedfortheCO2plume,therewillbeasignificantchallengefortime-lapseseismicmonitoring.However,theincreasedthicknessofthereservoir,itsclasticcomposition(sandstoneratherthancarbonate),andthepresenceofasingleresidentporefluid(brine)allbodewellforapplicationoftime-lapseseismicmethodsformonitoring.AninitialsetofVSPrecordingswiththeSilixaiDASsystemweremadeduringabaselinesurfaceseismicsurveyconductedinMay2013.Basedontheresultsoftheinitialsurvey,amoreelaboratesetofrecordingsweremadeinconjunctionwitharepeatbaselinesurfaceseismicsurveyinNovember2013(Harrisetal,2015).ConsistentwithAquistore’sstatedgoaloffunctioningbothasademonstrationprojectandacriticalresearchsiteandindustriallaboratory,theNovembersurveysincludedawiderangeofcomparisonstudies.Inthesectionsthatfollow,wehighlightconclusionsdrawnfromsomeofthesecomparisons.ThedatarecordedinNovember2014includedsimultaneousacquisitionofsingle-modefibreDASVSPdataforthefull3Dsurfaceexplosivesurveyandofmulti-modefibreDASVSPdataforasignificantfractionofthesurvey.Additionally,asubsetofrecordingwasdedicatedtovibratorsourcetests.Allexplosiveshotdatawererecordedbyasurfacegeophonearray(providing3Dsurfaceseismicdata),aboreholewirelinearrayof3-componentwall-lockinggeophones,aswellasDASrecordingsfromtheopticalfibres.Thetotalnumberofexplosivesourceswas683

fortheSMsurvey,withshotpoint(SP)spacingofabout75-100mwithinthe3x3kmstudyarea(Figure1a)andloadedwith1kgofexplosiveat15mdepth.379oftheshotsweresimultaneouslyrecordedwithcomparableDASsetupparametersonaninterrogatorconnectedtotheMMfibre.Inaddition,Vibratordatawerecollectedusingtwo45,000pound-forcetrucksonone3north-southlineof52vibrosesispoints.A60-leveldigitallytelemeteredthree-component(3C)geophonearray(SercelMaxiwave)with15mverticalspacingwasdeployedandclampedintheobservationwell.54operational3Cgeophonesrecordeddataatlevelsbetween1470mand2355mdepthintheobservationwell.

DataAnalysis

iDASSignalandNoiseDaley,etal(2015)providesatechnicaldescriptionofDASsignalandnoiseproperties,illustratedwithdatarecordedataCO2sequestrationsitenearCitronelle,Alabama.Keypointsofthatanalysisincludethefollowing:

1. TheSilixaiDASinterrogatorusesopticalbackscatteringtomonitor,inamovingwindow,thetimedifferenceinopticalpathlength(u)changebetweentwosectionsofthefibrethatareseparatedbyalengthdzwhichisthe‘gaugelength’.Assuch,theiDASoutputcanbeequivalentlyregardedeitherasanestimateofthefibrestrain-rate !

!" (!"!")orasanestimateofthe

spatialderivativeoffibreparticlevelocity, !!" (!"!")ascalculatedby

differenceoperatorsappliedintime(t)oraxialdistance(z)respectively.

2. WecanobtainameasurementofstrainfromtheiDASnativestrainratesinceintegrationwithrespecttotimeconvertsstrain-ratetostrain(typicallyfollowedbyasuitabletemporalbandpassfilter).Moreover,forapropagatingsignal,integrationwithrespecttodistanceisequivalenttointegrationwithrespecttotimefollowedbymultiplicationbythepropagationspeedalongthefibrecable(apparentvelocity)withasigndeterminedbydirectionofpropagation.

3. Systemnoisestemsfromrandomquantumeffectswithintheopticalsystem

(somewhatanalogoustoresistornoiseinelectronicsystems).Totalnoisepowerisstablebutthedistributionofnoisepowerisnotflat:Onanychannel,rawnoisepowerincreaseslinearlywithtemporalfrequency;atanytime,afewchannelsaremuchnosierthanothers;atanychannel,noisepowerdriftsrandomlywithtime.

4. Anoptimalmethodtomaximizesignal-to-noiseratio(SNR)andflattenthenoisespectruminthesignalbandconsistsof:

• Tracknoisepowerwhilerecordingmultipleindependentchannelswithnearlyidenticalsignal

• Formoutputchannelsasoptimallyweightedaveragesoftheindependentinputchannels(i.e.weightedstacking)

• Integrateintimetoflattennoiseandconvertsignaltodimensionlessstrain

Figure2PanelAshowstherawDASrecording.PanelBshowstheresultofapplyingthenoise-reductionprocessingasarunningweightedaverageover11adjacentchannels(20m).PanelCshowstheresultoftime-integratingthewaveformsinPanelB.Thelower-rightpanelshowsaveragedspectraforthevariousboxes.

Figure2illustratesthesestepswithdatarecordedatAquistoreusingthemethodofDaleyetal(2015).Thedatacamefromasinglenear-offsetdynamiteshotandwererecordedfromtheSMfibre.Ineachofthesepanelsathin-lineboxidentifiesanoisewindowandathick-lineboxidentifiesasignalwindow.Notethatthenoise-reductionoperatorreducesthenoisebyabout10dBwithoutaffectingthesignal.Thetime-integrationflattensthetemporalspectrumofthenoiseandconvertsthe

signalfromstrain-ratetostrain(orequivalently,fromparticleaccelerationtoparticlevelocitywhenscaledbythepropagationspeed).Itshouldbenotedthatthesedatawereacquiredwiththebestacquisitionpracticeavailableatthetimetheywererecorded.LargelymotivatedbytheresultsshownhereandinDaleyetal(2015),thebestacquisitionpracticehasbeenmodifiedtoreplaceaninitialsimpleaverageoverneighboringchannelswithanoptimallyweightedaverage.Thishasresultedinalowernoisefloorthanwhatisseenhere.Bysamplingat.25m/channelandapplyingtheoptimallyweightedaveragetoresampleat2m/channel,thecurrentbestpracticeatthetimeofthiswriting(January2015)achieves.03nanostrain(RMS)ina10-100Hztemporalband.Thesystemissubjecttoarapiddevelopmentcycleandimprovedphotonicsarelikelytoreducethesystemnoisefurtherinthenearfuture.

SMvs.MMComparison

Figure3.ExplosivesourceDASrecordingsfornear(top)andfar(bottom)shotpointsforsinglemode

(left)andmultimode(right)fibercables.Sourceshotmapshownincenterinsetwithlineconnectingsourceandwelllocations.

Figure3showsrepresentativecomparisonsofsimultaneousDASrecordingsmadebyinterrogatorsconnectedtothesingle-modefibre(left)andmultimodefibre(right).Themiddlepanelineachdisplayshowsthedynamiteshotlocation.Somewhattooursurprise,nosignificantdifferencebetweenthequalityoftherecordingswasseen.TheSMdataseemstobeslightlynoisier,butthatislikelytobeduetoaslightdifferenceininterrogatorunitsratherthanadifferenceinfibretype.Evidently,atleastforthegood-qualityfibresintheAquistoreobservationwell,theacquisitionsetupprocedureadequatelytunesthesystemtocompensateforanydifferencesduetofibretype.WhileDASrecordingisitselfrelativelynovel,theuseofMMfibreforDASinanyfieldacquisitionisquitenew.MMfibreDASissignificantbecauseofthelargenumberofboreholeMMfibresdeployedfordistributedtemperaturesensing(DTS),implyingthepotentialabilitytoutilizetheseexistingMMdeploymentsforseismicmonitoringandimaging.TheSilixasystemhassubsequentlybeenusedwithequalsuccessonbothmultimodeandsingle-modefibres.Itshouldbenoted,however,thatiffibretypesaresplicedtogetherinaserialconfiguration,theopticalreflectionandsignallossatthesplice(duetothemismatchofopticalimpedance)willsignificantlyreducethesignal-noiseratiofromthedistalsegmentregardlessoftheorderinwhchthemismatchedfibresarecombined.

DASvs.GeophoneComparisonInordertocompareDASsignalwithgeophoneoraccelerometersignal,onemustconvertfromstrainorstrain-ratetoparticlevelocityoraccelerationandtakeintoaccountthat,likeultrasonictransducers,radarantennas,orspatiallydistributedgroupsofgeophonesorhydrophones,theDASsignalintrinsicallyrepresentsanintegratedarrayresponseofconceptualpointsensitivities.

Figure4ComparisonofDASandgeophonedataformulti-tracegather(left)andsingletrace(right)fortwodepthzones.

Figure4comparesDASandgeophonerecordings.TheleftpanelsshowDASdatathathasbeennoise-reduced,time-integrated,andscaledbytheapparentvelocity(±3000m/sec),overlainbygeophonedatafroma150minterval(10sensors).Thepanelsontherightshowsinglegeophonetraces(red)overlainwithcorresponding15maveragesofDASdata.UpanddowngoingDASsignalswereseparatedbythesimpleup/downfilterdescribedinMiller(2012)andMarzetta,etal(1988)andrecombinedafterscalingwiththesignedvelocitytomaketheleftpanels.DAStracesintheright-handpanelsaresimple15maveragesandwerenotshiftedtoalignupordowngoingsignalsbeforeaveraging.Thegeophonerecordswereprovidedasdigitized20bitintegerswithoutinformationaboutgainthathadbeenapplied,soasinglescalingfactortoconvertgeophonedatatovelocityunitswascalculatedfromthematchtooneoftheDAStraces.ThissinglecalibrationfactorissufficienttomatchthegeophoneandDASsignalsoveralldepthsandoverthelargedynamicrangeofsignalamplitudes.

CouplingandSourcestrength

Figure5VibroseissourceDASdatafor1sourcetruck(left)and2sourcetrucks(right).Datahasalowpassfilterof55Hz.

CarefulinspectionofFigure5showsanincreasedsensitivitytoboreholeguidedwaves(tubewaves)above1500mdepth.Recallingthatthefibreisinacementedannulus,thisisasignofachangeincouplingbetweenfibrestrainandborehole-guidedpropagation.Thecementingoftheannulusinvolvedseveralstagespumpingcementslurry.Oneofthosestageswasunderstoodtobemoreproblematicthanothers.Figure6showsvibratordata(aftercorrelationwiththesweep)thatwasrecordedduringapreliminarytest.Itshowstheexpectedsignalgainwhentwovibratorsareusedtogether.Italsoshowsclearlythebandofincreasedcouplingtoambienttubewaveenergyintheboreholethatisassociatedwiththerelativelypoorcementzonebetween1000mand1500m.

Imaging:DASandGeophoneVSPMigrationThecontinuousspatialsamplingoveralongextent,andstablereceptionpropertiesinherenttotheuseofDASwithfibreinacementedannulusmakeitastrongcontenderforuseintime-lapse(4D)applicationssuchastheCO2injectionmonitoringplannedforAcquistore.Sincedatawasacquiredbothwitha2DgridofdynamiteshotsandalineofvibratorpointsandrecordedwithbothDASandgeophones,thereareampleopportunitiesforcomparison.ItisparticularlyinterestingtodeterminewhethertheaddedstackingfoldfortheDASrelativetothegeophonesissufficienttocompensateforthelowerSNRpermetersampled.FormigrationoftheDASdatawehaveadaptedaprocessingchainandmigrationoperatorthathaslongbeeninuseforVSPimaging(e.g.Christie,etal,1995)andthathaspreviouslybeendemonstratedtoworkwellwithDASrecordings(Miller,et

al,2012).WehaveusedanintegraloperatorthathasbeenimplementedspecificallyforusewithiDASdata.Theprocessingchainissimple:

1. Separateupanddowngoingsignal2. Deriveadeconvolutionoperatorfromthedowngoingsignalandapplyitto

theupgoingsignal3. Migratethedeconvolvedupgoingusingaweighteddiffractionstack

Asmentionedabove,ourup/downseparationisastraightforwardimplementationofasimpleequationinvolvingHilberttransformsappliedinbothtimeandchanneldimensions.Thelongspatialarrayandcontinuousspatialsamplingmakethisarobustoperation.Ourdeconvolutionmethodisadata-adaptiveWienerdeconvolution(Haldorsenetal,1994;Chenetal,2008)inwhichthenoisepowerateachtemporalfrequencyisderivedfromthedata.Ithasthepropertythattheresultisindependentofpriorlinearoperators(suchastimeintegration)thatmighthavebeenappliedbeforedeconvolution.Ourmigrationmethodisaprecisewave-equationreworkingofthediffractionstack(Kirchhoff)method(Milleretal1987;Burridgeetal,1998).Itisparticularlywell-suitedtotarget-specificapplicationssinceimagepointscanbehandledindependentlyandcontributionsfromindividualsourcesandreceiverscanbecalculatedindependentlysubjectonlytopriorknowledgeofthegeometryandvelocitymodel.Inpractice,thismeansthatindividualshotgathers(orsubgathersusingdistinctreceiversubsets)canbecalculatedseparatelyandcombinedtoformanimage(or,forcomparison,severalsubimages)asapostprocessingstep.Themethodcanbeimplementedveryefficientlywhenthemodelistranslationinvariant(1D).AnisotropyandP-Sscatteringcanbeincorporatedwithnosignificantextraprocessingcost.Allmigrationsweremadewiththesamebackgroundmodelandadip-limitedgeneralizedRadontransform(GRT)migrationoperator,butthepreprocessingofthegeophonedatawasdoneindependently.AlltheDASdatausedwasfromtheSMfibre.Figure6showsmigratedimagesmadefromtheentire2Dvibroseiswalkawayline.ThemiddletwopanelsweremadewithchannelscoveringthesameintervalinthewellforbothDASandgeophones.TheleftmostmigrationpanelwasmadeusingthefullextentoftheDASdata.Evidentlyitcoversabiggeraperture.Intheoverlap,theDASimagesseemtomatcheachotherabitbetterthantheymatchthegeophoneimage.Wesuspectthatthisdifferencestemsfromdifferencesinpreprocessing.TherightmostpanelismadeusingonlytheDASchannelsbetween350mand930m.

Figure6MigratedDASVSPimagesfor2Dvibroseiswalkawaydata.

Figure7comparesDASimagesmadewithdynamitedatafromthefullaperturewithdatafromtheupper27%oftheDASchannels.AsseeninFigure5,theSNRisbestinthiszone.Ithasbettercementintheannulusandalowerformationimpedance.

Figure7MigratedDASVSPdatafromexplosivesourcepoints.(left)DASdatafrom350-2510mdepths.(right)DASdatafrom350-930mdepths.

Figure82Dslicesfrom3DmigratedDASVSP.

Figure8shows2Dslicesfromafull3DprocessingoftheSMdynamitesurvey.Itwasmadeusing664shotsand69receiverchannelsat4mspacingbetween650mand930m.Thehorizontaldepthsliceshownisattheleveloftheplannedinjection.

SummaryandConclusionsAseriesoftestsofDASVSPwereconductedaspartoftheAquistoreCCSmonitoringproject.WedemonstratetheimprovementinSNRofDASdatafromweightedstacking.WeshowexampleofdataacquiredfromMMandSMfibresdeployedwithidenticalcouplinganddemonstratecomparableSNRandsensitivity.ThisisanimportantconclusionduetothelargeinstalledbaseofboreholeMMfibres(usedfortemperaturesensing)whichcannowbeconsideredforVSPacquisition.AcomparisonofDASdata(convertedfromstrain-ratetoparticlevelocity)toco-locatedgeophonesindicatesthattheDASdataisconsistentwithgeophoneresponse.Aprecisequantitativecomparisonwaslackingthegeophoneanalog-to-digitalcalibration,butasingleempiricalDAS-geophonecalibrationfactormatchedalldata.VariabilityinwellcementingwasobservedtoimpactDASdataquality.ADASVSPdatamigrationprocessingflowisdescribedwithresultscomparedtoequivalentgeophoneVSPmigration.Asexpected,thelongerDASarrayhaslargerlateralimagingdistance.WealsoshowthatasubsetofDASchannelswithbettercouplingcangiveresultssimilartothefullchannelset.Finally,a3DmigratedimagevolumefortheAquistoresiteispresented.TheAquistoreprojectteamhasdecidedthattheDASVSPdataissufficienttouseforcontinuedmonitoringofsubsurfaceCO2storage.

AcknowledgementsWewouldliketothankKyleWorthofthePTRCformanagement,technicalinputandsupportoftheAquistoreproject.FundingforLBNLwasprovidedthroughtheCarbonStorageProgram,U.S.DOE,AssistantSecretaryforFossilEnergy,OfficeofCleanCoalandCarbonManagementthroughtheNationalEnergyTechnologyLaboratory,of the U.S. Department of Energy, under contract No. DE-AC02-05CH11231. AdditionalfundingforLBNLfromChevron.

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