碳酸盐岩石物理问题.docx
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碳酸盐岩石物理问题.docx
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碳酸盐岩石物理问题
CarbonateRockPhysicsIssues
EnruLiu,MichaelA.Payne,ShiyuXu,GregorBaechle,ChristopherE.Harris/ExxonMobilUpstreamResearchCopyright2009,InternationalPetroleumTechnologyConference
ThispaperwaspreparedforpresentationattheInternationalPetroleumTechnologyConferenceheldinDoha,Qatar,7–9December2009.
ThispaperwasselectedforpresentationbyanIPTCProgrammeCommitteefollowingreviewofinformationcontainedinanabstractsubmittedbytheauthor(s).Contentsofthepaper,aspresented,havenotbeenreviewedbytheInternationalPetroleumTechnologyConferenceandaresubjecttocorrectionbytheauthor(s).Thematerial,aspresented,doesnotnecessarilyreflectanypositionoftheInternationalPetroleumTechnologyConference,itsofficers,ormembers.PaperspresentedatIPTCaresubjecttopublicationreviewbySponsorSocietyCommitteesofIPTC.Electronicreproduction,distribution,orstorageofanypartofthispaperforcommercialpurposeswithoutthewrittenconsentoftheInternationalPetroleumTechnologyConferenceisprhoehribitend.bPyermissiotntorepperroduacseirnprintisresrtirictLeidbtoriaanaIbstra,ct.of.notxm8ore3th3a,n3i0c0awrordos;,illuXstr5ation-sm83a6y,no.t.be,cfoapxie+d.The-9abstra4ctm.ustcontainconspicuousacknowledgmentofAbstract
Summary
Limestoneanddolomitereservoirsaccountforapproximately50%ofoilandgasproductionworldwide,yetseismicresponsesincarbonaterocksarepoorlyunderstood.Developmentofacarbonaterockphysicsmodelisextremelydifficultbecauseporesystemsaremorecomplexincarbonatesthantheyareinclastics.Carbonatescanhaveavarietyofporetypes,suchasmoldic,vuggy,interparticle,andintraparticle.Thecomplexporesystemcreatessignificantscatterintheporosity-velocityrelationship,asindicatedinexperimentalresults(e.g.,AnselmettiandEberli,2001).Poreshapeappearstobethedominantfactorincarbonaterockphysics.Moldic,intraframe,andvuggyporestendtoberoundedandmaketherockstronger(faster)thanwhentheporesareinterparticle.Micropores(e.g.,microcracks)tendtobeflatandmaketherockweaker.Toeffectivelycharacterizecarbonatereservoirrocks,itiscriticaltodeveloparockphysicsmodelcapableofhandlingdifferentporetypes.
Inadditiontoporetypesandporeshapes,otherfactorsneedtobeincludedinaphysics-basedrockmodel.Somespecificadditionalfactorsarelithologyandgrainshapes,multiphasefluidsandwettingeffects,rock-fluidinteractions(poro-elasticity),stresseffects,anisotropy,heterogeneityandscaleeffects,chemicalchangestotheframework,andcorrectionsforenvironmentaleffectsduetologgingconditions.Anyrockphysicsmodelshouldbecalibratedandvalidatedwithcontrolledlaboratoryexperiments,fieldmeasurements,andcomputationalrockphysics.Inthispaper,wedemonstratetheimpactandvalidationofseveralofthesefactors.
CarbonateRockPhysicsModel
Wepreferananalyticalrockphysicsmodelratherthanempiricalonesduetoboththepredictivepowerthatitprovideswithouthavingtoacquiresomuchanalogdataanditsphysics-basednature.Empiricalrockphysicsmodelsarewidelyusedduetotheirsimplicityinfittingarelationshipbetweenparameters.Theadvantageofananalyticalmodelisthatoncetheparametersdescribingthephysicalcontrollingfactorsaredetermined,themodelcanbeappliedanywherethatthecontrollingparameterscanbeestimated.Thisachievesbetterpredictionsanddeeperunderstandingofthesubsurfacethanempiricalmodelswithlessdata.Nonlinearphysics-basedinclusionmodels(e.g.,KusterandToksoz,1974)areattractivebecausetheyhandlevariousfactorsthatimpactseismicresponseinaninternallyconsistentmanner.
WehaveextendedtheXu-White(1995)modeltocarbonaterocks(XuandPayne,2009).Ourmethodconsistsoffoursteps:
(1)mix(e.g.,theReuss-Voigt-Hillaverage)themineralstocreateasolidmatrix;
(2)addporestothematrixintwostepsusingtheDifferentialEffectiveMedium(DEM)process(XuandWhite,1996)andKuster-Toksoz(1974)theorytoaccountforthemechanicalinteractionbetweenthepores;themicro-poreswithboundwater(e.g.,claypores)areaddedfirstandwillbeincludedinthesolidmaterialforfluidsubstitution,followedbyallotherporesincludingwater-wetmicro-poresandempty(ordry)non-bound-waterpores,whichwillusedtocalculatethedryframeelasticproperties;(3)mixthewaterwhichisnotboundtomicro-poreswiththehydrocarbonsusinganappropriatefluidmixinglaw;and(4)useGassmann’sequationstoaddthefluidmixtureintotheporesystemtoyieldthefinaleffectiveelasticpropertiesforthesaturatedrock.Theexistingmodelispartitionedintoclay-relatedpores,interparticlepores,microcracks,andstiffpores.Wekeepclayporesinthemodeltomakeitapplicableinamixedcarbonate-clasticenvironment.Theintroductionofmicrocracksandstiffporesmakesiteasytostudythepore-typeeffectonseismicresponseincarbonaterocks.
PhysicalEffects
Itiscriticaltoproperlyaccountforphysicaleffectsintherockphysicsmodeltogeneratetheproperseismicresponse.Thisisparticularlytruesinceweusetherockphysicsmodeltoinvertgeophysicalmeasurementsforsubsurfacepropertiessuchasporosity,permeability,lithology,poretype,stressstate,andothersthatdescribehydrocarbonvolumesandproductionrates.Sinceproducibilityinheterogeneouscarbonatesettingsisverydependentontheporetype,itisimportanttohavemethodstopredicttheporetypefrominversions.Cross-plotsofvelocityversusdensityoftendemonstratesignificantscatter.However,ifonesegmentstheplotintothedifferentporetypes(Figure1),thenthescatterforeachpopulationissignificantlyreduced.Inaddition,eachpopulationcanbemodeledseparately.Ourcarbonaterockphysicsmodelrepresentseachporetypebyaspecificaspectratio(shortaxistolongaxis).FollowingKumarandHan(2005),weassumethattheinterparticleporeisthemostcommonporetypeincarbonaterocksand,therefore,givesareferenceporosity-velocitytrend(Figure2).Sampleswithhighervelocitiesthanthereferencetrendcanbemodeledasamixtureofinterparticleandrounded(stiff)pores.Thosewithslowervelocitiescanbemodeledasamixtureofmicrocracksandinterparticlepores.Wecandefineatemplateofcurvesfromouranalyticalrockphysicsmodelthatrepresentsporetype.Inthismanner,wecanpredicttheporetype(idealizedporeshape,e.g.,roundedorcrack-like)byinversion.
Duetothebrittlenatureofcarbonates,fracturesaregenerallyprevalentinthem.Fracturescreatesignificantanisotropy,asdemonstratedinfigure3.Seismicdatawereacquiredoverasurfacewithaprevalentfracturedirectionasindicated.Analysisofthesedatayielded19%anisotropyinadirectionconsistentwiththatofthefracturesystem.Thesefracturesystemsactasconduitsforfluidflowandcanconsiderablyenhancehydrocarbonproductioninlow-porositycarbonates.Althoughmappingthosefracturesystemsiscriticalforimprovingproduction,modelingtheeffectoffracturesontheseismicresponsecanbedifficult.WecaneasilyaccountforanisotropyusingDEMtheory(Xuetal.,2006)andtheanisotropicGassmannequationstodofluidsubstitution.Sinceourrockphysicsmodelhandlesanisotropyuptoorthorhombicsymmetry,itcansimulatebothfractureanisotropyandshaleanisotropyconsistently.XuandPayne(2009)describethefeaturesofthismodelasappliedtofractures.Itgenerallyreliesondistinguishingporetypes.Microcracksrepresentthemostcompliantcomponentinrocks,sotheyareextremelysensitivetostress.Thepreferredorientationdistributionwillgoverntheanisotropicbehavioroftherock.Otherporesaregenerallyinsensitivetostressandhavenopreferredorientation.
Whenvariationsoccuroverscaleslargerthanporesbutlessthanawavelength,differentmixinglawsneedtobeinvoked.Thesemeso-scaleeffectsareparticularlyimportantinheterogeneoussettingslikecarbonates.Figure4showslaboratorymeasurementsconductedbyPurdueUniversityundercontracttoExxonMobilthatdemonstratethesignificantchangesinvelocitythatcanoccuroververyshortdistances.Thecomplex,multi-scaleporesystemcreatesissuesastowhenGassmanntheorymaybeappliedincarbonatesandindicatesthatmeso-scaleeffectsmustbeincorporatedintorockphysicsmodels.
FluidSubstitutioninCarbonateRocks
GassmannfluidsubstitutionisapowerfultoolforAVOanalysisand4-Dinterpretation,butGassmann’stheoryrequiresmanyassumptions.Theseassumptionstendtobeviolatedmoreeasilyinrocksampleswithmulti-poretypes(XuandPayne,2009).Ourrockphysicsmodeltreatsthevariousporetypesdifferently.Fluidsareaddedseparatelyintothesystemwithinthedifferentporesusingtheinclusion-basedtheory.Gassmanfluidsubstitutionisonlyappliedinthoseporeswhereitisappropriatetodoso.ThisapplicationgeneratesthecorrectresponseinamixedsystemconsistingofGassmann-consistentmacroporesandnon-Gassmann-consistentmicrocrackswithunrelaxedporefluid.
Conclusions
Theoptimalproductionofreservoirsrequiresanaccurateandpreciseassessmentofthehydrocarbonvolumesandoftheratesatwhichthosereservoirscanbeproduced.Thisassessmentisparticularlyimportantincarbonates,whichtypicallyyieldonlyabout30%oftheiroilduetothecomplexityofthemineralogyandporesystems.Goodrockphysicsmodelscanallowgeophysiciststoconvertfromgeophysicalmeasurementstothedesiredengineeringandgeologicrockproperties.Severalexamplesareshowntodemonstratetheimpactthatporetypes,fluidsubstitution,scaling,anisotropy,andenvironmentaleffec
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