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Road,Received7December2015Receivedinrevisedform31January2016Accepted6April2016Availableonline11April2016Keywords:SilurianLongmaxiFormationinsoutheasternSichuanBasin,China.Therelationshipbetweenanisotropiclelandperpendiculartobedding,becauseofthepresenceofbed-ding.Shalepermeabilityalongbeddingdirectionisusuallymagnitudeshigherthanthatatperpendiculartobeddingdirection,withsomeliteratureresultsvariedbetween3.1and6800[2,4,8,9].Permeabilityanisotropyisalsofoundbetweenparalleltobeddingtounderstandcharacter-permeabever,thecombinationofindividualplugsmaynothelptounder-standthetruepermeabilityanisotropyofhighlyheterogeneousrocks,suchasshale[22,24],becausethepore/microfracturesystemineachplugisdifferent.Panetal.[22]recentlyreportedanewani-sotropicpermeabilitymeasurementmethodforrocks,usingcubicsampleswitha3D-printedmembraneinatriaxialcell.Thesystemcanbeusedtomeasurepermeabilityinthreedirectionsbyreori-entingthesamecubicsampleinthetriaxialcelleachtime.Because⇑Correspondingauthor.E-mailaddress:Zhejun.Pan@csiro.au(Z.Pan).Fuel180(2016)106–115Contentslistsavailablemustexceed100ndtobecommerciallyviable[7].Shalereservoirpermeabilityisalsostronglyanisotropicbetweendirectionsparal-shaleusuallyuseplugsdrilledhorizontallyorverticallyfromrecoveredcores,thentestedintriaxialrigs[2–4,8,9,18,23].How-fromsub-nanodarcies(nd)[1–4]totensofmicrodarcies(ld)[5,6],makingitdifficultforgastofloworbeproducedwithoutstimula-tion.EmpiricalexperienceinmanyNorthAmericanshalegasbasinssuggeststhatthematrixpermeabilityofshalegasreservoirsreservoirsimulation[22].Therefore,itisimportanttheanisotropicpermeabilityandthepore/microfractureisticsoftheshalesample.Laboratorymeasurementsoftheanisotropichttp://dx.doi.org/10.1016/j.fuel.2016.04.0290016-2361/CrownCopyrightC2112016PublishedbyElsevierLtd.Allrightsreserved.ilityofShaleisafine-grained,sedimentaryrockthatisoftenconsid-eredascaprockandanaturalbarriertothemigrationofoilandgas.Forshalegasthatisgeneratedandstoredinplace,shaleactsasboththesourcerockandthereservoir.Thepermeabilityofshalegasreservoirsiskeyforgasproduction,andisextremelylowcom-paredwithconventionalgasreservoirs.Shalepermeabilityrangesrienceddifferenttectonicmovements,andthereforemayhaveformeddirectionalpore/microfracturesystemsintheshale.Geo-logicalfactorscontrollinganisotropicpermeabilitymainlyincludeshalefabric[4,10–12],porosityandlithology[13–18],poresizedistribution[19],thermalmaturity[5]andmicrofractures[20,21].Theimpactofanisotropicpermeabilityongasproduction(includingfreeandadsorbedgas)hasbeendemonstratedthroughGasshaleAnisotropicpermeabilityCTSEMMethane1.Introductionpermeabilityandfracturestructureascharacterisedbymicro-computedtomography(CT)andscanningelectronmicroscope(SEM)Mapswasalsoinvestigated.Atconfiningpressureof3MPa,permeabilitypar-alleltobeddingofthethreeshalesvariesbetween37.6and3042.4nanodarcies(nd),whilepermeabilityperpendiculartobeddingvariesbetween3.6and17.3nd,usingheliumandmethane.Thepermeabilityanisotropyratiobetweentheparallelandperpendiculartobeddingdirectionsvariesfrom5.2to510.5forthethreesamplesusingthetwogases.Moreover,permeabilityintwoparalleltobeddingdirectionsalsoshowsstronganisotropy.TheCTandSEMMapsresultssuggestthatmicrofracturesarecriticaltopermeabilityintheparalleltobeddingdirection;theyalsoleadtoahigheranisotropyratiobetweentheparallelandperpendiculartobeddingdirections,whilereducingtheanisotropyratiobetweentheparalleltobeddingdirections.Theheliumpermeabilityis1.4to3.3timesthatofmethanepermeability,whiletheheliumtomethanepermeabilityratiodecreaseswiththesample’stotalorganiccarboncon-tent:anobservationthatrequiresfurtherinvestigation.CrownCopyrightC2112016PublishedbyElsevierLtd.Allrightsreserved.directions,becausedifferenthorizontaldirectionsmayhaveexpe-Articlehistory:WeinvestigatedtheanisotropicpermeabilityinthreedirectionsofthreecubicshalesamplesfromLowerFullLengthArticleExperimentalstudyofanisotropicgaspermeabilitywithfracturestructureofLongmaxiShales,YongMaa,b,ZhejunPanb,⇑,NingningZhonga,LukeD.aStateKeyLaboratoryofPetroleumResources&Prospecting,ChinaUniversityofPetroleumbCSIROEnergyFlagship,PrivateBag10,ClaytonSouth,VIC3169,AustraliacChinaHuadianGreenEnergyCo.,Ltd,HuadianIndustryPark,EastAutomobileMuseumarticleinfoabstractFueljournalhomepage:www.elseanditsrelationshipSichuanBasin,ChinaConnellb,DavidI.Downb,WenlieLina,YiZhangc(Beijing),FuxueRoadNo.18,Changping,Beijing102200,ChinaFengtaiDistrict,Beijing100160,ChinaatScienceDirectvier.com/locate/fuelapproachcancorrelatepermeabilityanisotropywiththepore/permeabilitydifferenceindifferentdirectionsandbetweendiffer-180entgases,andtheirrelationshipwithmicrofracturestructure.2.Experimentalmethod2.1.SampledescriptionThreeshalesampleswerecoredfromashalegasexplorationwellintheLowerSilurianLongmaxiFormationinChongqingCityinsoutheasternSichuanBasin,whichisclosetoChina’smainshalegasexplorationanddevelopmentarea[33,34].TheLongmaxiShaleisover-maturatedwithanaverageequivalentvitrinitereflectancevalue(EqVRo)of3.99%oftheLongmaxiShalesamplesrecoveredfromthesamewell[35].ThebottomsectionoftheLongmaxiFor-mationismadeupofblackshalesdepositedinthedeepshelf;affectedbyfallingsealevels,thelithofacieslaterchangedtolami-natedsiltyshalesinthemiddleandshallowshelf[36].Besides,striationsarewidelyfoundinthebottomsectionoftheLongmaxiFormation[34],whichshowstheinterlayergliding.Asamaindetachmentlayer,theSilurianformationiswidelydevelopedintheeasternSichuanBasin[37].ThedepthoftheLongmaxiShaleinthiswellisabout700–750m:shallowerthanthatinJiaoshibaarea,whichisabout2600mdeep.Thetotalorganiccarbon(TOC)contentoftheLong-maxiShalegraduallydecreasesfromthebottomtothetop[36].Thethreeshalesamplesstudiedinthisworkarecoarsesiltyshales,finesiltyshalesandblackshalesfromdepthsof705.4,748.3and754.2m,respectively(Fig.1).TheTOCcontentsofthesesamplesare0.95%,3.72%and8.49%,respectively.Thesampledescriptions,includingmineralcompositions,aresummarisedinTable1.2.2.Samplepreparationmicrofracturesystem.Methodsthatstudythegeologicalcontrolofshaleanisotropicpermeabilityareusuallybasedon2Dobservations,includingthin-sectionandscanningelectronmicroscope(SEM)imaging,andporestructurecharacterisation,includinglowpressureN2adsorptionandhighpressuremercuryintrusion.Especially,theSEMobservationshavehighresolutionoftheshalepore/microfrac-turestructure[25,26].However,thesemethodscannoteitherdetectordescribetheinternalpore/microfracturestructurethatisimportanttopermeability.MicroX-raycomputedtomography(CT)maybeagoodnon-invasive,non-destructivemethodfor3Dimagingofinternalstructure(includingporosityandmineraldis-tribution)ofrockssuchassandstoneandshale[23,27–32].Never-theless,theirvoxelresolutioncanonlyreach1lm[30],whichisinsufficienttocharacterisethemicrofracturessmallerthan1lm,thecombinationofmicroCTandSEMobservationmightbeause-fulmethodtostudytheanisotropicpermeabilityinshale.Recently,highgasyieldsrangingfrom3C2103to500C2103-m3/dayperwellhavebeenachievedintheLongmaxiFormationinJiaoshibaarea,southeasternSichuanBasin,China[33,34].TheLongmaxiShalehasthereforebecomethefocaltargetofshalegasexplorationanddevelopmentinChina.However,fewstudiesofthisshale’sanisotropicpermeabilityhavebeenconducted.Inthiswork,weusedthemethodofPanetal.[22]tomeasuretheanisotropicpermeabilityofthreecubicshalesamplesfromtheLongmaxiFormationinthreeorientationsusingbothheliumandmethane,andusedCTcombinedwithSEMobservationstocharac-terisetheirmicrofracturestructures.Wediscusstheanisotropicallthemeasurementscanbeperformedonthesamesample,thisY.Maetal./FuelThecubicsamplesorcubeswerecutslowlyfromtherecoveredcoresusingadiamondwiresaw,andairwasusedtocoolthewiresaw.Waterwasavoidedduringthecuttingaswatermayreactwithshaleandinducefractures.Theprocedureforcuttingthecubehasbeendescribedinourpreviouswork[38].Sinceitisdifficulttocreateparallelsurfacesandequalsidedimensionsusingthewiresaw[22],thesampleswerethenlappedinagrindingmachinetomakefinalcubeswithlengthsof20.8±0.1,19.8±0.1and22.6±0.1mmforsamplesY4-60,Y4-98andY4-103,respectively.Thecubicsamplesweregrindedintermittentlyandslowlytoavoidoverheatingthesamplesandpossiblecreationofmicrofractures.OffcutsofsampleswerecrashedforTOCandmineralcontentanal-yses.Beforepermeabilitymeasurements,thesamplesweredriedinavacuumoven.Itispreferabletodrysamplesatlowertemper-aturesforalongertimethanatahightemperatureforashortertime,sincecracksormineralalterationcanoccurathightemper-atures,particularlyforsampleswithahighamountofclayminer-als[31].Therefore,thesamplesweredriedat35C176Cundervacuumuntiltheirmassremainedconstantforatleast24h.Permeabilitymeasurementexperimentswerealsoperformedat35C176C.Afterpermeabilitymeasurements,thecubicshalesampleswereobservedusingtheXradiaMicroXCT-400atChinaUniversityofPetroleum(Beijing)tocharacterisethemicrofractures(seeSec-tion2.4fordetailedparametersofthemicro-CT).Moreover,sec-tionscutinthedirectionperpendiculartobeddingfromtheoriginalrecoveredcoresweremilledbyargon-ion-beamaftermechanicalpolishingforSEMobservation.2.3.PermeabilitymeasurementAnisotropicpermeabilitymeasurementswereperformedusingcubicsampleswitha3D-printedmembraneinatriaxialperme-abilityrig,asdescribedindetailinPanetal.[22].Ashortdescrip-tionoftherigandcalculationmethodisprovidedbelow.TheschematicplotofthetriaxialrigisshowninFig.2.Therigincludesatemperature-controlledcabinetthatcontrolstemperaturewithin±0.2C176C.TheISCOgasinjectionandconfiningpumpswereusedtocontrolgasandconfiningpressures.Upstreamanddownstreamtubing,eachwithavolumeofabout7ml,wereusedasgascylin-ders.Thesmalltubingvolumemakesthepressurechangesmoresensitive,sothatmeasurementsforrockswithultra-lowperme-abilityarelesstimeconsuming.Duringthepermeabilitymeasure-ment,confiningpressurewasfirstapplied,andthenthesystemandsamplewerevacuumed.Then,theupstreamcylinderwaschargedwithgas,whichflowsthroughthesampletothedown-streamcylinder.Thepressuresintheupstreamanddownstreamcylinderswererecordedovertimeandusedinthepermeabilitycalculation.Thepressuredecaycurveacrossthesamplecanbedescribedas[22]:ðPuC0PdÞðPu;0C0Pd;0Þ¼eC0atð1ÞwherePuC0Pdisthepressuredifferencebetweentheupstreamanddownstreamcylinders,Pu;0C0Pd;0isthepressuredifferencebetweentheupstreamanddownstreamcylindersattheinitialstage,tistime,andaisdescribedbelow[20]:a¼kAðPu;0þPd;0Þ2lL1Vuþ1VdC18C19ð2Þwherekispermeability,Listhesamplelength,Aisthesamplecross-sectionarea,andVuandVdaretheupstreamanddownstreamcylindervolumes,respectively.TheaboveequationsarederivedfromthetransientmethodofBraceetal.[39],whichiswidelyusedfortightrockpermeability(2016)106–115107measurements[3,4,22,40].Althoughsteadystatemethodisabletobeappliedinpermeabilitymeasurementforlowpermeabilityrocksusingwater[41],thetransientmethodisstillpreferredwhenFig.2.Schematicplotofthetriaxialrigforpermeabilitymeasurement(fromPanetal.[22]).Fig.1.Beddingtextureofthreeshalesamples(scaleisthesameforallsamples).Left,YC4-60,coarsesiltyshale,TOCcontent0.95%;middle,YC4-98,finesiltyshale,TOCcontent3.72%;right,YC4-103,blackshale,TOCcontent8.49%.Table1SampleinformationandmineralcompositionsoftheLongmaxigasshales.SampleIDDepth(m)TOC(%)Mineralcomposition(%)QuartzK-feldsparPlagioclaseCalciteDolomiteSideritePyriteTotalclayChloriteIlliteI/SaY4-60705.40.95354.812.93.616.701.525.51.0210.7113.77Y4-98748.33.7244.82.18.94.541.45.828.55.1311.411.97Y4-103754.28.4949.71.67.313.70.91.734.14.43312.95816.71aIllite/smectite.108Y.Maetal./Fuel180(2016)106–115setup;thereforeEqs.(1)and(2)canbeusedtocalculateperme-17.3nd.Thisshowsthatpermeabilityisstronglyanisotropicbetweendifferentdirectionsforshales,especiallybetweenhori-Afteranisotropicpermeabilitywasmeasuredusinghelium,the180abilityfromthepressuredecaymeasurementwithouttherequire-mentfortheupstreamanddownstreampressuretoequilibrate.Thisisextremelyusefulforlow-permeabilitymeasurements,becausecalculationscanbeperformedusingrelativelyearlypres-suredecaydatafromthemeasurement[22]tosavetime.Bothheliumandmethaneareusedinthisstudy.Forallthemeasurements,theconfiningpressurewas3.0MPaandgasatabout1.5MPawasinjectedfromtheupstream.Notethatshalepermeabilitydecreaseswithincreasingconfiningpressureandgaspressure[2,3],ourmeasurementsdonotrepresenttheperme-abilityunderreservoirconditionsastheconfiningandgaspres-suresintheexperimentsarelowerthanthoseinthereservoir.However,theexperimentsarecarriedoutatsameconditionsthereforethepermeabilityanisotropycomparisonismeaningfulamongdifferentgasesandsamples.Methaneisregardedasanidealgasinthiswork,becausethepressureusedisupto1.5MPa.However,usingEqs.(1)and(2)formethaneathigherpressuresrequiresvalidation.2.4.CTmeasurementandinterpretationtechniqueAfterpermeabilitymeasurements,thethreecubicshalesam-pleswereobservedusingtheXradiaMicroXCT-400tocharacterisetheircomplexporestructures.Thishigh-resolutioninstrumentemploysanX-raydetectorwithsub-micronresolutioncombinedwithamicrofocusX-raysourcewhichcanproducepixelsizesfrom0.5lmto50lm.However,duetothelargesamplesize(withdiagonallengthofabout28mm),imageresolutionbymicroX-rayCTcanonlyreachabout13lm.Althoughthisresolutionisinsuffi-cienttodetectporesintheshalematrix,itisvalidforobservingmicrofractureslargerthan13lm.TheCTdatawasimportedtoAvizoC210Fire8.0imagingsoftwaretogeneratea3Drenderingoftheshale.ThemicrofractureisblackintheCTimage,andcanbeextractedbysettingthresholdsonthegreyscale.2.5.SEMandSEMMapsimagingAftertheargon-ion-beammilling,verticalsectionedsamplescoatedwithcarbonwereinsertedintoanFEIHeliosNanoLabTM650DualBeamFIB-SEMTMforimagingandSEMMaps.TheSEMMapsisanimagemosaicmethodofnumerousSEMimages,whichcanprovidealargefieldviewofsampleswithhighresolution(sameresolutionsettingofanindividualimage)thatiseffectivetoobservethemicrofracturesdistribution.Imagesweretakenusingbackscatteredelectronat5kVaccelerationvoltageandaworkingdistanceof4mm.3.Results3.1.ShaleanisotropicpermeabilityusingheliumAllpermeabilitymeasurementswereconductedat3MPacon-finingpressure.Heliumatabout1.5MPawaschargedtotheupstreamtubingafterthetubingsystemandthesamplewereusinggas.ThekeyassumptionsintheoriginalBracemethodderivationstatethatcompressibilityissmallandconstant,andliq-uidwasusedtosatisfytheseassumptions[39].Ghanizadehetal.[3]demonstratedthatifthegasisideal,ananalyticalsolutionsim-ilartotheoriginalBraceanalyticalsolutionstillexistsforanypres-suredifference.Panetal.[22]appliedsamevolumeoftheupstreamanddownstreamgascylindersintheexperimentalY.Maetal./Fuelplacedunderavacuum.Fig.3ashowstheupstreamanddown-streampressurechangeforthetwohorizontaldirections(x-directionandy-direction)andtheverticaldirectionforsamplesameexperimentalconditionsandcalculationmethodwereappliedusingmethane.Theresults,whicharesummarisedinTable2,showthathorizontalpermeabilityusingmethanevariesfrom37.6to918.9nd,accountingforone-thirdtothree-quartersofthatmeasuredbyhelium.Theverticalpermeabilityofthethreesamplesvariesfrom3.6to7.2nd,accountingforabouthalfofthepermeabilitymeasuredbyhelium.Theseresultsareconsistentwithpreviousstudyoforganic-richshales[2,42–44]andindicatethatthetypeofgasaffectsthegaseffectivepermeability.3.3.SEMandSEMMapsDetailedSEMobservationsoftheLongmaxiShalesamplesshowthatmicrofracturesbetweendifferentmineralgrainedges(Fig.4a)andinsidetheclay
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