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permefracturesupportedwithproYulingTana,b,c,ZhejunPanc,*,LukeD.ConnellcaStateKeyLaboratoryofGeomechanicsandGeotechnicalChinabUniversityofChineseAcademyofSciences,Beijing100cCSIROEnergy,PrivateBag10,ClaytonSouth,VIC3169,dSchoolofMechanicalandChemicalEngineering,TheUniveCollegeofGeoscienceandSurveyingEngineering,ChinafDepartmentofCivilEngineering,MonashUniversity,Clayton,andtheamountofldirections.differedfromdistributioninthegivenporepressure,willsignifi-.AlthoughthepswithabsolutepbetweenbyacubiclawAllrightsreserved.1.IntroductionWiththedepletionofconventionalgasreservoirsinmanypartsoftheworldandincreasingdemandfornaturalgas,shalegashasrecentlyattractedgreatinterest.Currently,afewcountrieshavesuccessfullycommercialisedshalegasproduction,andmanyothercountiesareactivelyexploringthisoption(McGladeetal.,2013;Howarthetal.,2011).Forinstance,shalegasproductionintheUnitedStatesincreasedfrom4%ofthecountry'stotaldomesticdrygasproductionin2005toabout50%in2015,andisexpectedtoincreaseto69%by2040(EIA,2016).Incontrasttoconventionalgas*Correspondingauthor.ContentslistsavailableScienceJournalofNaturalGasScienceandEngineering44(2017)250e264E-mailaddress:Zhejun.Pan@csiro.au(Z.Pan).fractureunderthesameexperimentalconditions,withboththeproppantsizeproppantsaddedaffectingthisincrease.ThepermeabilitywasanisotropicintwohorizontaThedirectionandratioofpermeabilityanisotropyoftheproppant-supportedfracturethoseofthenaturalfracture,dependingontheamountofproppantsaddedandtheirfracture.Themodelindicatedthatpermeabilitydecreasedwitheffectivestressataandwithporepressureatagiveneffectivestress.Italsosuggestedthataddingproppantscantlychangetheabsolutepermeabilitybutnottheinitialfracturecompressibilitypermeabilityhadstronganisotropy,theinitialfracturecompressibilityrelationshipermeabilitywereindependentofflowdirectionandfollowedthesametrend.TherelationshitheKlinkenbergconstantandabsolutepermeabilitywasfoundtobewelldescribedfunction.©2017ElsevierB.V.CompressibilityUnconventionalgascompressibility,Klinkenbergcoefficientandabsolutepermeability.Thepermeabilitiesofproppedfrac-tureswerefoundtobeafewhundredorevenafewthousandtimeshigherthanthoseofthenaturalarticleinfoArticlehistory:Received6February2017Receivedinrevisedform18April2017Accepted18April2017Availableonline25April2017Keywords:ShalegasAnisotropicpermeabilityFractureProppanthttp://dx.doi.org/10.1016/j.jngse.2017.04.0201875-5100/©2017ElsevierB.V.Allrightsreserved.ppantJishanLiua,d,YantingWuc,e,AsadulHaquef,Engineering,InstituteofRockandSoilMechanics,ChineseAcademyofSciences,Wuhan430071,049,ChinaAustraliaersityofWesternAustralia,35StirlingHighway,WA6009,AustraliaUniversityofMiningandTechnology(Beijing),Beijing100083,ChinaVIC3168,AustraliaabstractShalegasisanimportantunconventionalnaturalgasresource,butshalehasextremelylowpermeability.Productionofshalegascanbeimprovedbyusingproppantsforhydraulicfracturingandmaintainingfractureconductivity,andabetterunderstandingofthepermeabilityanditsanisotropyofproppant-supportedfractureswouldbeusefulinoptimisinggasproduction.Thispaperdescribedexperimentsonshalepermeabilityanditsanisotropywithrespecttogaspressure,effectivestressandgastypeforanaturalfracturesupportedwithtwosizesofproppant.AcubicsamplefromtheSilurianLongmaxifor-mationintheSichuanBasin,China,wasusedinthestudy;thetestingdirectionofthesamplewasaltered,andbothhelium(non-sorbing)andmethane(sorbing)weretested.MicroscopicX-raycompu-terisedtomography(m-CT)scanningwasusedtorevealtheproppantdistributionandfractureshape.Finally,ananalyticalmodelwasappliedtodescribethepermeabilitywithrespecttoporepressureandeffectivestressandtheresultswereusedtodeterminetherelationshipsbetweeninitialfractureExperimentalstudyofabilityanditsanisotropyforshaleJournalofNaturalGasjournalhomepage:www.elsevier.coatScienceDirectandEngineeringm/locate/jngsereservoirs,shalereservoirshaveextremelylowpermeability;therefore,hydraulicfracturingisneededtoproducethegasatacommercialscale(EstradaandBhamidimarri,2016;Sobhaniaraghetal.,2016).Proppantssupportthehydraulicfracturesandarecriticaltomaintainfractureconductivityandallowgastoflowthroughtotheproductionwell(Keshavarzaetal.,2016;Wenetal.,2016).Inadditiontolowpermeability,gasshalesoftenhavestrongpermeabilityanisotropy(i.e.,thepermeabilityisdirectionallydependent);thisanisotropyisoneofthekeyfactorsincontrollinggasflowandgasproductionrates.Recently,theanisotropicpermeabilityofshalehasbeenthesubjectofextensivestudy(Ghanizadehetal.,2014a,b;Maetal.,2016;Panetal.,2015;Wangetal.,2016).Thepermeabilityanisotropyofshaleisparticularlystronginthedirectionsparallelandperpendiculartothebedding.Thisanisotropyisduetoseveralfactors,themostimportantofwhichisstratigraphiclayering(Bhandarietal.,2015;Vegaetal.,2014).Othermajorfactorsaffectingtheanisotropyaremineralfoliation,discontinuitiesandstratificationintheshale(Choetal.,2012),andtheorientationofmineralsandporesorcrackscausedbyrockdepositionanddiagenesis(Chenetal.,1999;Georgietal.,2002).Differenttypesofporesinshaleaffectthepermeabilityanisotropy(DewhurstandAplin,1998;Fredrichetal.,1993;Wangetal.,2016).Usingmercuryintrusionporosimetry,Boltonetal.(2000)foundthatsignificantanisotropywasduetoparallelmicrofractures.Theroughtopographyoffractures(MeheustandSchmittbuhl,2001;ThompsonandBrown,1991)andthesheardisplacement(Auradouetal.,2005;Yeoetal.,1998)canalsocauseanisotropicpermeability.Gasshaleshavealsoshownstrongpermeabilityanisotropyinthedirectionsparalleltothebedding(e.g.,Maetal.,2016;Panetal.,2015),perhapsbecauseofthedirectionaltectonicstresstheyexperiencedduringtheburialandupliftprocesses,whichmayhaveformeddirectionalfracturesys-tems(Maetal.,2016).Previousexperimentalworkonshalepermeabilityanditsanisotropyhasgenerallyfocusedontheoriginalreservoirrocks;therehavebeenfewstudiesonproppedfracturepermeability.However,becauseshalegasproductionisheavilydependentonhydraulicfractures(e.g.,PanandConnell,2015),thepermeabilityanditsanisotropyofproppant-supportedhydraulicfracturesareimportant.Somerecentstudieshaveexaminedtheeffectoftheproppantongas-flowcapacityinfracturesforunconventionalreservoirrocks.Forinstance,Kumaretal.(2015),workingoncoalandusingCO2,comparedgaspermeabilitiesforbothnon-proppedandproppedfractures,andfoundthatthepermeabilityshoweda“U-shaped”trendwithincreasinggaspressureatconstantconfiningstress.Houetal.(2017)bothmeasuredanddevelopedanalyticalmodelstocalculatetheflowcapacityinshalewithdiscontinuousproppantplacement.Theyfoundproppantdistri-butionplayedasignificantroleinimprovingstimulatedwellperformance.Experimentalstudieshaveshownthatpermeabilityisalsostronglysensitivetogaspressureandtotheeffectivestressappliedontheshale(e.g.,Ghanizadehetal.,2014a,b;Panetal.,2015).Modellinghasbeenusedtobetterunderstandtherelationshipbetweenshalepermeabilityandgaspressureandstress.Relation-shipsthathavebeenmodelledincludeabsolutepermeabilityundervariablestressconditions(Weietal.,2016),acorrelationbetweenshalepermeabilityandeffectivestress(Chenetal.,2015)andtheapparentpermeabilityinshaleandstressdependence(WasakiandAkkutlu,2014).Intermsofmodellingpermeabilitybehaviourforhydraulicfacturessupportedwithproppant,Chenetal.(2017)developedapermeabilitymodelasafunctionofeffectivestress.Toanalysetheflowinfracturessupportedwithproppant,Yanetal.Y.Tanetal./JournalofNaturalGasScience(2015)combinedtheDarcy-Brinkmanequationwithhomogenisationtheoryandfiniteelementnumericalsimulationtoanalysetheflowinfracturessupportedwithproppant.MollanouriShamsietal.(2016)calculatedthewidthandpermeabilityoffracturewithproppantatvariousstressstates,andshowedthatthehighergradedproppantpackwasmorebeneficialthanthelowergradedpackformaintainingfractureconductivity.Althoughtheeffectofproppantongasflowinfractureshasbeenstudied,therehasbeenlittleworkontheanisotropyofpermeabilityinfractureswithproppant.Furtherworkisrequiredtobetterunderstandthepermeabilitybehaviouranditsanisotropyofhydraulicfractures,aswellastheirrelationshipswithporepressureandeffectivestress.Suchinformationisakeyaspectoftheevaluationofshalereser-voirsandtheanalysisofgasproduction.Inthiswork,westudiedshalepermeabilityanditsanisotropywithrespecttoporepressureandstressfornon-proppedandproppedfractureswithtwosizesofproppant.WeusedacubicsamplecutfromtheshalecorerecoveredfromtheSilurianLong-maxiformationoftheSichuanBasin(China'smostproductiveshale).Anisotropicpermeabilitywasmeasuredbyalteringthetestingdirectionofthecubicsampleusingbothhelium(He)andmethane(CH4).MicroscopicX-raycomputerisedtomographyscanning(m-CT)wasusedtodemonstratethedistributionandamountofproppantinthefracture.Also,ananalyticalmodelwasappliedtodescribetheexperimentaldataandtherelationshipsbetweentheinitialfracturecompressibility,Klinkenbergcoefficientandabsolutepermeability.2.Experimental2.1.SampledescriptionandpreparationAcubicsamplecutfromashalecorerecoveredfromtheSilurianLongmaxiformationfromtheSichuanBasin,China,wasusedintheexperiment.Thecoresamplewasrecoveredfromadepthofabout2600m.Table1summarisesthetotalorganiccontentandminer-alogyresultsoftheoffcutsofthissampleusingX-raydiffraction(XRD).ThecubicsamplewaspreparedfollowingtheproceduredescribedinWanetal.(2015).Thesamplewasabout21mmineachdirection,andwascuttoalignwiththebeddingdirection,usingadiamondwiresaw.Itwasthenbrokenintotwopartsalongaweakplaneatthebeddingdirectionundertension,toformafracturethatsimulatedanaturalfracture.Fig.1showsthesampleandthefractureusingm-CTscanning.Permeabilitywasmeasuredforthefracturedsamplewithandwithoutsupportingproppantalonghorizontaldirections1and2.ThedetailedexperimentalprogramofworkisdescribedbelowinSection2.3.Toeliminatetheinfluenceofmoisture,thesamplewasdriedinavacuumedovenat70C14Cfortwodaysbeforeeachseriesofpermeabilitymeasurement,andwasweighedeveryfewhoursuntilitsweightremainedunchanged.2.2.ApparatusandpermeabilitymeasurementTheschematicoftherigshowninFig.2andpermeabilityanisotropymeasurementmethodhavebeendescribedindetailinPanetal.(2015)andtheyareappliedinthisworkandarebrieflydescribedhere.Afterobtainingthecubicsample,a3D-printedmembranewaspreparedtoaccordwiththesample'sdimensions,usingphotopolymerwithanoutsidediameterof1.5inches(3.81cm).Thesamplewasplacedinthemembraneandthenintoastandardrubbersleeve,whichwastheninstalledinatri-axialcellformeasurementofpermeability.TherigwasinsideatemperaturecontrolledcabinetwhichmaintainedconstanttemperatureandallandEngineering44(2017)250e264251themeasurementswereperformedat34.5C14C.AccurateconfiningTable1TotalorganiccontentandmineralogyoftheSilurianLongmaxishalesample.TOC(%)Mineralcomposition(weight%)QuartzIllite/MuscoviteIllite-smectiteClinochloreMicroclineAlbitePyriteAnkeriteCalcite2.14238.9411.5113.929.916.409.414.402.003.50Fig.1.TheSilurianLongmaxishalesamplewithafracture.Fig.2.Thetri-axialapparatusforpermeabilitymeasurement(fromPanetal.,2015).Y.Tanetal./JournalofNaturalGasScienceandEngineering44(2017)250e264252pressurewassuppliedbyanISCOpump.Duetotheextremelylowpermeabilityofshale,permeabilitywasmeasuredusingthetran-sientmethodofBraceetal.(1968).Duringthemeasurement,gaswasinjectedintotheupstreamcylinder;itthenflewthroughthesampletothedownstreamcylinder.Thegaspressuredifferencebetweenthetwoendsofthesamplewasmeasuredtocalculatethepermeabilityusingthefollowingequation(Panetal.,2015):ðPuC0PdÞC0Pu;0C0Pd;0C1¼eC0at(1)wherePuC0PdandPu;0C0Pd;0arethepressuredifferencebetweentheupstreamanddownstreamcylindersattimetandinitialtime,respectively.aisexpressedas(Panetal.,2015):a¼kAC0Pu;0þPd;0C12mLC181Vuþ1VdC19(2)wherekispermeability,Listhelengthofthecubicsample,Aisthecross-sectionalareaofthecubicsample,andVuandVdarethevolumesoftheupstreamanddownstreamcylinders,respectively.Duringtheexperiments,thepermeabilitymeasuredvariedfromabout1narnodarcy(nd)tomorethan1millidarcy(md)whichwillbedemonstratedintheresultssection.Theupstreamanddown-streamgascylindervolumeswereabout6mlforpermeabilitylessthan10microdarcy(md)andabout27mlforpermeabilityabove10mdsothattheexperimenttimewasreasonableforeachperme-abilitymeasurement.2.3.ExperimentalprogramofworkToexplorethepermeabilityanditsanisotropyforthefracturewithandwithoutproppant,fourdifferentexperimentswereconducted:C15Case1:Thepermeabilityoftheoriginalfracture,alongthreeprincipaldirectionswasmeasured.C15Case2:Thepermeabilityofthefractureproppedwithproppantof0.1mminaveragediameter,alongtwohorizontaldirections,wasmeasured.C15Case3:Thepermeabilityofthefractureafterremovalofthepreviousproppant,alongtwohorizontaldirections,wasmeasured.C15Case4:Thepermeabilityofthefractureproppedwithproppantof0.539mminaveragediameter,alongtwohorizontaldi-rections,wasmeasured.Bothtypesofproppantsusedinthisworkwereglassbeadswithauniformsphericalshapeandparticlesize.Allpermeabilityex-perimentswereperformedat34.5C14Cwithconfiningpressureupto9MPaandaveragedgaspressureuptoabout2.6MPa.BothHeandCH4wereusedtostudythepermeabilityasthepropertiesofthetwogasesaredifferent:Heisconsideredtobeanon-adsorbinggasforshale,whereasCH4isconsideredtobeadsorbing.Theshalesamplewasbrokenintotwopieces(Fig.1).Theproppantwasaddedtothefracturesurfaceofonepiece,thenthetwopiecesofthecubewerepressedtogetherandattachedtoeachotherbya5mmwidestickytape,perpendiculartothefracture.Thecubewithproppantwasthenplacedinthe3D-printedmembrane.Thetapeontheflowdirectionwascarefullyremovedandafilterpaperwasplacedbetweenthesampleandtheplaten,beforetheywereinstalledintherubbersleeveandthenintherig.Confiningpressurewasappliedonthesampletosettletheproppantinthefracture.MeasurementswererepeatedtotestthehowwelltheY.Tanetal./JournalofNaturalGasScienceproppantshadbeenplacedandsettled.2.4.X-raym-CTmeasurementBecauseaddingproppanttothefracturewaschallengingandthedistributionoftheproppantwasunknown,X-raym-CTwasusedtodetectthelocationsoftheproppantsandtheopeningofthefracture.m-CTscansweretakenaftertheCase2andCase4exper-iments(inwhichthesamplefracturewassupportedbyproppant).Thesamplewiththe3Dmembranewasscannedtoensurethattheproppantswerestillinposition.ThemachineusedforX-raym-CTwasaZeissXradiaXRM520versa,establishedforimagingGeo-materials(XMFIG)attheDepartmentofCivilEngineeringofMonashUniversity(AlMahbubandHaque,2016),theresolutionofwhichcanreachabout12mmforthesampleusedinthiswork.3.ResultsFollowingtheexperimentalprogramofworkoutlinedabove,thepermeabilityresultsarepresentedintablesbelowaccordingtotheirmeasurementsequence.3.1.Permeabilityresults:Case1Table2presentsthepermeabilitiesinthreeprincipaldirectionsofthecubicsamplewiththeoriginalfracture.Itcanbeseenthatpermeabilitydecreasedstronglywitheffectivestressatagivenporepressure.TherelationshipbetweenpermeabilityandeffectivestressisdiscussedinSection3.5.Table2alsoshowsthatthepermeabilitydecreasedwithporepressureatagiveneffectivestress;thiswillbediscussedinthediscussionsection.Thepermeabilitywasbetweenafewhundredndandafewthousandndforthemeasurementconditionandgasesusedhereforthetwohorizontaldirections.Althoughwedidnotmeasurethepermeabilityofthesamplebeforeitwasbroken,thehorizontalpermeabilitiesfoundforotherLongmaxishaleswereinasimilarndrange(Maetal.,2016).Thesefindingsindicatethattheperme-abilityoftheopenfracturedoesnotshowmuchimprovementunderstress.Strongpermeabilityanisotropywasfoundi
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