Dedication
Thisbookisdedicatedtoallthosewho taught,mentored,andinspiredme.Also, tothosewhoworkedalongside,and learnedfromme,andalsochallengedme. Butaboveall,thisbookisdedicatedto thosewhomademebelievethatReachis alwaysfartherthanGrasp. KMS
IdedicatethisbooktoProf.Dr.Devang Khakhar(Ex-Director,Departmentof ChemicalEngineering,IITBombay). SM
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Preface xvii
Acknowledgments xix
1.Poresandporespace
1.1Theporespaceofrocks 1
1.1.1Theporespaceofgranularrocks1
1.1.2Theporespaceofcarbonaterocks anexampleof theporosityclassification4
1.1.3Theporespaceofcoals4
1.1.4Porespaceofshalereservoirrocks6
1.1.5Theporespaceoftightsedimentaryrocks12
1.1.6Theporespaceofnonsedimentaryrocks13
1.2Classificationofporesbysize 13
1.3Poresandporethroats 14
1.4Logsandporespace 16
1.4.1Poreattributesusedinpetrophysicalmodelsofpore networks17
1.4.2Porethroatmodels18
1.4.3Apopularandsuccessfulmodelofporespace thecapillarytubebundleformodelingflowofan incompressiblefluidthroughporousrocks20
1.4.4Agenerictreatmentoftransportofanincompressible fluidthroughamediumsuchasaporousrock20
1.4.5Modelingtheorientationofporesinspace20
1.5Thefractalmodelofporespace 21
1.6Useoflogdata 22
1.6.1Understandingthedegreeofconnectivitybetween twoporesizeclassesusingNMRlogdata23
1.6.2Insightsfromhigh-resolutionresistivityimagingtool dataintotheporesizeheterogeneity24
26
28
2.Inversionoflogdatatothegrossattributesofpore space
2.1Estimationofthebulkporosityoflaminatedformations usingdeterministicapproach 31
2.1.1Computationofeffectiveporosityusingshallow resistivity,neutroncapturegammarayspectrometry, formationbulkdensity,andneutronporositydata32
2.1.2Computationofeffectiveporosityusingshallow resistivity,gammaray,formationbulkdensity,and neutronporositydata38
2.1.3Totalporosity40
2.1.4Thedifferentwaysclay/shaleismanifestwithin clasticrocks44
2.1.5TheThomas Stieberapproach45
2.2Stochasticinversionoflogdataforlaminatedformation 55
2.2.1Basicresponseequationsthatleadtotheforward model55
2.2.2Theforwardmodel65
2.2.3Essentialconstraints66
2.2.4Additionaloutputscomputed68
2.2.5Horizontalpermeabilityandverticalpermeability68
2.2.6Usageofhigh-resolutiondata69
2.3Evaluationofmicroporosity
2.4Evaluationofblocky(nonlaminated)reservoirs
3.Poreattributesofconventionalreservoirs
3.1Theporespaceofintergranularrocks 117
3.2Attributesofporespace 117
3.2.1Poreshape,poresize,andporethroatsize118
3.2.2Poresasthebuildingblocksofporespace118
3.2.3Geometryoftheporebody119
3.2.4Sizeofapore119
3.2.5Theconceptofporeclass120
3.2.6Surfaceareatovolumeratio120
3.2.7Thecharacteristiclengthscaleofporespace121
3.2.8Hydraulicradiusmeasureoftheporespace121
3.2.9Theporeshapefactor122
3.2.10T2 logmean122
3.3Distributionofincrementalporosityoverporeradius 124
3.3.1Computationof CP(r)usingNMRdata124
3.3.2Distributionofincrementalporosityoverpore throatradius125
3.3.3Ratioofporesizetoporethroatsize125
3.3.4Harddataonthedistributionofporethroatsize overincrementalporosity ðCPT ðR ÞÞ 127
3.3.5Obtaining CPT(R)frommercuryinjectiondata128
3.3.6Obtaining CPT(R)fromlogdata129
3.4Computationof CPT(R)fromtheNMRdata 131
3.4.1Thelinearconversionworkflow131
3.4.2Nonlinearconversionworkflow132
3.5PoreshapefactorthroughintegratingNMRandMICP (mercuryintrusiondata) 134
3.5.1Frequencydistributionofporeradius134
3.6Asimplevisualizationofconstrictionanditseffectonthe grosspermeabilityofporespace 135
3.6.1Modelpredictionofpermeability141
3.6.2Timur Coatespermeabilitypredictorfromthe perspectiveofconstriction143
3.7Fractalattributesofporespace 144
3.7.1Thefractalmodeloftheporespace,basedona pore porethroatassemblagevisualizationofthe physicalporespace145
3.7.2Afractalmodeloftheporespace147
3.7.3Permeabilityfromtheperspectiveofthefractal modeloftheporespace148
3.7.4Cumulativeporevolume149
3.7.5Representativehydraulictortuosityandcumulated surfaceareatocumulatedvolumeofthecapillaries150
3.8Electricalformationfactorfromtheperspectiveofthe fractalmodeloftheporespace 155
4.Porespaceattributesofnonconventionalreservoirs
4.1CBMreservoirs 161
4.1.1Thecomponentsofthespaceoccupiedbyfluids incoals161
4.1.2Characterizationoftheporespaceofcoals cleats andfracturesthatarenotcleats162
4.1.3Characterizationoftheporespaceofcoalsusing NMRdata170
4.1.4Permeabilityofcoal measurement173
4.2Shalereservoirs 177
4.2.1Poresizeencounteredwithinshalereservoirs178
4.2.2Differentiationofporeclassesforshalereservoirs179
4.2.3MultidimensionalinversionofNMRechodata usingmaximumentropyprinciple183
4.2.4Presentationoftheresultsofinversion188
4.2.5Porositypartition188
4.2.6Themethodofdiffusionediting191
4.2.7ThemethodofLaplaceInversionwith regularization195
4.2.8D-T2 plots(morefamiliarlyknownas D-T2 maps) forwardmodels197
4.2.9D-T2 mapsandotherplotsrelatedtotheresults ofechodatainversion fieldexamples203
4.2.10Partitioningoftotalgasintofreeandadsorbed gascomponentsusingonlyNMRdata207
4.3Characterizationoffracturedreservoirs
5.Logmeasurementscommonlyusedforfinding thebulkporosityofconventionalreservoirs
5.1Porespaceattributesofconventionalreservoirs
5.2Measurementofbulkporosity
5.2.1Measurementofformationdensityforbulkporosity227
6.Logmeasurementsessentialforcharacterizingthe porespaceofunconventionalreservoirs
6.1Measurementoftotalporosityusingnuclearmagnetic resonance 345
6.1.1NMRtheory345
6.1.2Porespaceattributesandtherelaxationof transversemagnetization367
6.1.3Totalporosityandthebinporosities377
6.1.4Estimationoftotalporositydirectlyfromtheecho data379
6.1.5Obtainingtotalporosityusingtheformation densityandNMRdata380
6.2NMRandtheporosityofCBMreservoirs 381
6.2.1Coalpores381
6.2.2The T2 relaxationspectraofcoals382
6.2.3Totalporosityandgasvolume385
6.2.4Porosityavailablewithintherockforholding freegas385
6.2.5Cleatvolumeperunitrockvolume385
6.3Porosityofshalegas/shaleoilreservoirs 385
6.3.1Estimationofeffectiveporosity389
6.4Estimationofelementalconcentrationinrocks 390
6.4.1Neutroncapturegammaspectrometry390
6.4.2Inelasticgammaspectrometry395
6.4.3Computationoftheaveragedensityofsolidpartof theformation398
6.4.4Theacquisitionofinelasticandcapturegammaray spectra399
6.5CharacterizingtheporespaceofCBMreservoirsusing imagedata 403
6.5.1Cleat/fractureapertureandvolume,andmatrix porosityofCBMreservoirs403
6.6Characterizingtheporespaceofshalereservoirsusing imagedata 413
6.6.1Shalepores413
6.6.2Delineationoffractureswithinshalereservoirs usingimagedata414
6.6.3Boreholeelectricimagesanddistributionof organicmatter414
6.7Generationofhigh-resolutionelectricalimagesofthe boreholewall 415
6.7.1Sensors415
6.7.2Positionofeachsensorinspace416
6.7.3Dataacquisition417
6.7.4Processflowforcreatingboreholeimagesfrom thebuttoncurrentmaps418
7.Characterizingporesandgrainsusinglogs
7.1Porefacies
7.1.1Poresizedistribution447
7.1.2Poreshapesfromlogs455
8.Archie’scementationexponent
8.1Introduction
8.2PredictionofthevalueofArchiecementation exponent“m”usingeffectivemediumtheories 500
8.3ApproachesusedinmodellingArchie’s m parameter GeneralRemarks
8.4Theapproachforcomputing“m”usingsingle-frequency dielectricdataandusingArchie’sequation 501
8.5Theapproachforcomputing“m”usingmultifrequency dielectricdataandusingArchie’sequation 501
8.6Theapproachforcomputing“m”fromgrainattributes obtainedthroughmultifrequencydielectricdatainversion 503
8.6.1InversionofArchie“m”fromsingle-frequency dielectricdata503
8.6.2InversionofArchie“m”frommultifrequency dielectricdata511
8.6.3Workflowforthegenerationofthedispersion modelofdielectricpermittivity ε 522
8.6.4Thedispersionmodelforrockconductivity524
8.6.5Applicabilityofthemodeltoclayeyrocks527
8.6.6Theroleofcontributiontocomplexpermittivity, comingfromunconnectedpores528
8.6.7ApproachestoestimationofArchie“m”basedon differentialeffectivemediumtheory530 Appendix1 551 Appendix2 differentialeffectivemediumtheoryforaligned inclusionscase 556
Appendix3 559
Approachestomodel“m”:incaseofShalyrocksusingthe Bergmanspectraldensityrepresentationoftheeffective permittivityofabinarymixture 564 Appendix4 581
AnapproachtoArchie“m”throughNMRdataanalysis 589 PercolationtheoriesandArchie“m”factor 592
Appendix5:logarithmicmixinglawforeffectivepermittivityofa mixture 594
Approachestoestimate“m”throughfractalmodelofpore space 602
9.Permeabilityofunimodalporesystem
9.1Introduction 613
9.2Responseoflocalpressurefield,localfluidvelocityfield, andaveragefluidvelocityfieldtochangesindriving pressure 614
9.3Asimplemodelofporespacepresented 616
9.4Flowthroughacapillary 617
9.4.1Howporespaceattributesinfluencepermeability620
9.5Surfacearea,or,representativeporedimensionorcharacteristiclengthscaledrivenapproachestopermeability 621
9.5.1The“bundleofcapillarytubes”modelofpore space621
9.5.2Forwardmodelofpermeabilityintermsofpore spaceattributes626
9.5.3Thestreamtubemodelofflowthroughaporous mediumwhosegrain-andpore-arrangementofa macrolevelvolumesegment,isisotropic,and thus,macroscopicpermeabilityfieldisascalar628
9.6Depictionofporespaceinthemodel 628
9.6.1Streamtubes628
9.7Elementalstreamtube 629
9.8Integralrepresentationofmacroscopicpermeability (orsimply“permeability”),andtheconceptof microscopicpermeabilitydensityorlocalpermeability density 630
9.9Integralrepresentationofpermeability 631
9.10Averagepermeabilityfield ks ,alsoreferredto,asthe “effectivepermeabilityfactor” 631
9.11Integralrepresentationofaveragepermeabilityfield (effectivepermeabilityfactor) ks andrelationbetween permeability k andeffectivepermeabilityfactor ks 632
9.12Theelementalstreamtubepermeabilityfactorfield 633
9.12.1Explicitrepresentationofpermeabilityfactorof astreamline(elementalstreamtube)intermsof someofitsattributes634
9.12.2Decompositionofpermeabilityintomacroscopic porespaceattributes,namely,hydraulictortuosity, hydraulicconstrictionfactor,andhydraulicpore radius636
9.13Insightsfromthesimplestpossibleporespacemodeland theroleplayedby“hydraulicradius”inpermeability modeling derivationofageneralizedKozeny-Carman equation 638
9.14Conceptofhydraulicradius 638
9.14.1Howmicroscopicstreamlineattributesand macroscopicporespaceattributesarerelated inaporousmedium640
9.14.2HydraulicConstrictionfactor(C s )ofconnected porespace,whichisthemacroscopic counterpartofstreamlineattribute C ðS Þ 640
9.15GeneralizedKozeny-Carmanequation 641
9.15.1ConventionalformofKozeny-Carmanequation predictingpermeability643
9.15.2Wellknownequationsforpermeabilityprediction fromlogmeasurements.Theequationsdiscussed belowareforwaterwetrockonly646
9.16FromKozeny-Carmanequation,toVanBaaren’s equation 663
9.17TheRGPZequation
10.Permeabilityandelectricalconductivityofrocks hostingmultimodalporesystemsandfractures
10.1Poresizenomenclature
10.1.1Micropores,mesopores,andmacropores737
10.2PoreclassificationusingNMR T2 distribution 738
10.2.1RelationbetweenporesizesandNMR T2: thefastdiffusionlimit738
10.2.2Limitsofvalidityofa T2 threshold-basedporosity partition742
10.2.3Model-basedporositypartitionforthecaseof rockshavingintragranularporosity742
10.2.4Casewherevugsarenotsparselydistributed intherock746
10.3Poreclassificationusingwellboreimages 747
10.3.1Preconditioningofdata747
10.3.2Dippicking747
10.3.3Closingthedatagap747
10.3.4Extractionoffracturesegments753
10.3.5Matrixextraction758
10.3.6Theproblemofcomputingtheporevolume contributionofheterogeneities761
10.3.7Anefficientmethodologyforextracting heterogeneities762
Appendix1 770
10.4Porositypartitionusingacousticlogs 772
10.4.1Challengesofporositypartitionusingacoustic logs772
10.4.2Aworkflowofporositypartitionusingacoustic logs773
Appendix2 778
10.5Electricalconductivityofanunfracturedcomposite hostingdualporosity 788
Modeling s2 forfullywatersaturatedcomponent2case789
Evaluationof s theelectricalconductivityofacomposite hostingdualporosity792 Caseofpartialsaturation793
10.6Permeabilityofanunfracturedcompositehostingdual porosity 794 Baserockpermeability794 Permeabilityofthecompositerock796
10.7Electricalconductivityoffracturedrocks 798 Partialsaturationcase799
10.8Thevariablecementationexponentmethodofcomputing watersaturation 801 Caseofconnectedporespacehavingtortuosityunity801 Porevolumes801
RationalebehindEquation(10.89)802 LevelbylevelevaluationoftheArchiecementation exponentoftheformation804 Discussion804 Computationofwatersaturation807
Preface
Thisbookexplainshowlogmeasurementsexplainporespace.Theideaisto familiarizeindustrypersons,studentsofgeophysics,petrophysics,andpetroleumengineering,whostudywellloggingatthegraduatelevel,andthose embarkingonresearchintopicsrelatedtopetrophysics.Westartfromthe basicconceptsthatleadtomodelsfortheimportantmicroscopicand macroscopicporespaceattributes.Thisbook,whilehavingthisstatedgoal,is alsoaimedatresearchers,withtwoobjectives:(i)tomakeitlesstimeconsumingforaresearchertoimbibepublishedmaterials;(ii)tofamiliarize theresearcherwiththeprinciplesbehindsomeoftheinstrumentationinvolved indataacquisitionandprocessing.Thefocusthroughoutthisbookhasbeento restricttotheimportanttechniques,andkeepthediscussiongeneric.Itis assumedthatareaderhasabackgroundofclassicalphysics,quantummechanics,vectoranalysisincludingvectorcalculus,andabasicknowledgeof tensors.Itispresumedthatthereaderhassomegroundinginthenuclear magneticresonance basedtechniquesforelicitingporespaceattributes. Basicknowledgeofnuclearphysicsisalsoassumed.Familiaritywiththe techniquesofmorphologicalimageprocessing,effectivemediumtheories,and logdatainversionforformationattributescanbehelpfulinfollowingsomeof thechapterswithease.
Thelistofbooksgivenbelowisrecommendedforappreciatingsomeofthe topicscoveredinthisbook.
Coates,G.R.,Xiao,L.Z.,Prammer,M.G.,2000.NMRLoggingPrinciplesand Applications,GulfPublishingCompany,Houston.
Dunn,K.-J.,Bergman,D.J.,LaTorraca,G.A.(ed.),2002.NuclearMagnetic Resonance:PetrophysicalandLoggingApplications,PergamonPress,New York.
Ellis,D.V.,Singer,J.M.,2008,WellLoggingforEarthScientists,seconded, Springer.
Griffiths,D.J.,2013,IntroductiontoElectrodynamics,fourthed,Pearson.
Tuck,C.C,1999,EffectivemediumTheory:PrinciplesandApplications, ClarendonPress.
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Poresandporespace
1.1Theporespaceofrocks
1.1.1Theporespaceofgranularrocks
Inorganicgranularrocksareformedduetothecompactionandlithificationof sedimentwhosegrainsareclastic(e.g., MukherjeeandKumar2018)and alsoincludeooliticrocks(e.g., DasguptaandMukherjee2020).Organic granularrocksareformedduetothecompactionandlithificationofgrains composedoftheskeletalremains,fecalfragments,radiolarii,algalfragments andcoralfragments,andotherremnantsconnectedwithmarinefloraand fauna.
Thesedimentarygrainsgivingrisetoinorganicgranularrocksdonothave internalvoidspaces.Ontheotherhand,thegrainsofthesedimentwhichgives risetoorganicgranularrockscanhaveinternalvoidspace.Thereasonforthis isthefactthatagrain,inthecontextoforganicgranularrocks,canitselfbean aggregateofsmallerunits,e.g.,fecalfragments.Itcanalsohavedissolution porespace,as,forexample,inthecasewhereagrainisaskeletalfragmentor aggregatesofskeletalfragments.
Rockssuchasbound-stonescanhaveporespace,whichisintra-aswell asinterfragmentary.Theterm‘fragment’herewouldrefertoaunitofthe bound-stone,whichistheresultofthebindingofthesefragments.
Porespaceisdefinedbyporesthataretheelementaryunits,whose aggregateistheporespace.Usually,sedimentswhichcompactandlithifyinto granularrockshaveasizedistributionoftheirgrains.Asaresult,wehavea grainsizedistributionandcorrespondingly,aporesizedistribution.
Oftenthegrainsizecanbedividedintotwoclasses.Thesizeclassofthe lowerrange,say < 40microns,overlapswithwhatisgenerallyreferredtoas ‘matrix.’Whenthelargergrainsfloatwithinthematrix,theydonotcontribute totheporespace.Suchrockswherethematrixgrainsaretheframeworkgrains arecalledasmud-supportedrocks.Theoppositeisthecasewherethelarger grainscomprisetheframeworkandthematrixisdistributedaspartings,or lenses,orasapore-fillingcoatingoftheframeworkgrains.Suchrocksare calledasgrain-supportedrocks.
Itisnotunusualtofindrocks,whicharepartlygrainsupportedandpartly mudsupported.Thesearetexturallyimmaturerocksfoundincertain depositionalsettingssuchasinfans.
Ingeneral,thenatureofthedistributionofthegrainsizeofarockdefines itstexturalmaturity.Besidestheintergranularandintragranularporespace, microporesariseinsideovergrowthsorcement(Fig.1.1). Fig.1.1 showsthe typicalporetypesthatarisewithinsandstones.
Onefurthercategoryofporespaceexists thedissolutionporespace.It arisesduetothedissolutionofgrainsduringneogenesisordiagenesis. Examplesofthisaremoldicporosity,vugularporosity,solutionchannels,and leachedzones.Thephenomenonofgrain-dissolutiondilatestheoriginalpores, andporesoccludedbyneogeneticcalciteprecipitation,bysubsequentdissolutionthatspreadsaspatches.Thesecanbedescribedasleached-poreporosity patches.Dissolutionporespacealsoarisesduetothepressuresolutionphenomenongivingrisetostylo-lamination.Thestylo-laminationsareanastomosedandalsoareassociatedwithtensiongashes, whicharisefromthe necessityofstress-release.Theporefabricencounteredisoftentheresult ofmultiplecyclesofporositycreationanddestructioninthecourseof cementation,neogenesis,anddiagenesis.
Bythesametoken,thevolumeoftheintergranularporespacepostlithificationisoftenreducedowingtotheocclusionofporesduetoovergrowths,secondarycementation,grainalteration,andauthigenesis,during diagenesis.Dolomitization,whichisaspecialcaseofgrainalteration,results intheincreaseinthesizeofthecalcitegrainswithoutanysignificantalteration ofthemorphologyofthegrain-to-graincontacts.Thisusuallyresultsinthe
FIGURE1.1 Idealizedrepresentationofdifferentporetypessuchasintergranular,dissolution, fracture,andmicropores,thatcanarisewithinsandstones. Adaptedfrom Pittman(1979). 2 UnderstandingPoreSpacethroughLogMeasurements
enhancementoftheporevolume.Inthecasesmentionedabove,secondary cementation,spardepositioninlimestones,authigenesisofclaymineralson thegrainsurfaces,andstructuresthatstraddletheporebodyreducethepore volume.Examplefortheformeristheauthigenesisand/orgenerationof montomorilloniteasgraincoats/alterationoffinemicagrainswhichwere originallypartofthematrix,andtheauthigenesisofchloriteoritsgenesisas thealterationproductofmaficminerals.Anexampleofthelatteristhe authigenesisofilliteasthreadlikestructureswhichstraddlethepore-body space.
Theporespacethatisintimatelyconnectedwithgraincontactsisthe gatewaythatconnectstwocontiguouspores.Secondarycementationand authigenesis,whichoccurduringdiagenesis,oftenconstrictthisimportant connectingporevolume(theconnectingporevolumeiscalledasporethroat). Thisinturnhampersthedegreeofconnectivityoftheporesforfluidtransport andtherebyhinderstheporespacetotransportfluid.Thequantitativemeasure oftheabilityoffluidflowiscalledaspermeability.Thus,diageneticprocesses asmentionedabovereducepermeability.
Mud-supportedrocksfallintoaclassbythemselves.Mud-supported rockshaveclay-sizeandsilt-sizeparticles.Theporeswithinthemudare largelyflatpores.Theporestructurecomplicateswhenamud-supportedrock hostscarbonaceousmatterincludingbitumenandkerogen.Bitumenhosts poresysteminsomecasesbutpostitsformation.Kerogen,however,hosts multi-nanometer-sizedpores.Theporespacewithinkerogeniscalledas ‘kerogenporespace.’Kerogenfloatsinsidetheclaymatrix,inamudsupportedrockcontainingcarbonaceousorganicmatter.Kerogenporesare alsocommonlyreferredtoasorganicpores.Inamud-supportedrockhosting organicmatter,wehaveinorganicpores,whichareporespresentwithinthe matrixnotoccupiedbyorganicmatterandkerogenporeswithintheorganic matter,inadditiontoporeswithinbitumen,ifpresent.Additionally, submicron-sizedinter-kerogenporesarisingwithintheintersticesbetween microvolumesofkerogenmaterialcanalsoexist.Kerogenporeshost adsorbedmethane.
1.1.1.1Porespaceofsomegranularrocksnotfallingwithinthe abovecategories
1.1.1.1.1Intercrystallineporespace
Cementationduringneogenesisanddiagenesisofcarbonaterockscanresult insignificantporevolumeoccupiedbycalcitecrystals.Thesecrystal assemblagessometimeshostsubmicrontonanometer-sizedpores,whichare knownasintercrystallinepores.Suchporescanalsodevelopduringthe postdepositionaldolomitizationprocessinthecaseofsomecarbonates. Intercrystallineporesareflatandhaveaverylowdegreeofconnectedness.
4 UnderstandingPoreSpacethroughLogMeasurements
1.1.2Theporespaceofcarbonaterocks anexampleofthe porosityclassification
Thedifferentvarietiesofporespacediscussedaboveareexemplifiedin carbonaterocks.ChoquetteandPrayclassification(ChoquetteandPray1970) isuseful(Fig.1.2).
Appearanceofdifferentporetypesincarbonatesisgivenat Fig.1.3 asan example.
1.1.3Theporespaceofcoals
Coalsaremadeupofmaceralsandfinelydistributedsiltandclay,madeupof inorganicmineralsincludingcalciteinsomecases,inadditiontoclayandiron
FIGURE1.2 ChoquetteandPrayclassificationoflimestoneporosity. Reproducedfrom ChoquetteandPray(1970)
FIGURE1.3 Panel(A)isanexampleofanintergrainporosity.Panel(B)isanexampleofmoldic porosity.Panel(C)representsintragrain-dissolutionporosity.Panel(D)isanexampleofintrafossil porosity.Panel(E)presentsmicropores-hostedporosity.Panel(F)showsintergrain,graindissolutionporosity.Panel(G)isanexampleofvuggyporosity.Panel(H)isanexampleofa stylolitehostingsomeporosity.Panel(I)presentsporosityarisingfromamicrofracture.Allthe panelsareSEMphotographs. Reproducedfrom Luetal.(2021).
