MechanicsofHydraulicFracturing
Experiment,Model,andMonitoring
Editedby XiZhang
ChinaUniversityofGeosciences Wuhan,China
BishengWu TsinghuaUniversity Beijing,China
DiansenYang WuhanUniversity Wuhan,China
AndrewBunger UniversityofPittsburgh Pittsburgh,PA,USA
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Names:Zhang,Xi(Geologist),editor.
Title:Mechanicsofhydraulicfracturing:experiment,model,and monitoring/editedbyProfXiZhang,ChinaUniversityofGeosciences,Prof. BishengWu,TsinghuaUniversity,Prof.DiansenYang,Wuhanuniversity,Prof.AndrewBunger.
Description:Firstedition.|Hoboken,NJ,USA:Wiley,2023.|Includes bibliographicalreferencesandindex.
Identifiers:LCCN2022030857(print)|LCCN2022030858(ebook)|ISBN 9781119742340(hardback)|ISBN9781119742418(adobepdf)|ISBN 9781119742456(epub)|ISBN9781119742487(oBook)
Subjects:LCSH:Hydraulicfracturing.
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Contents
ListofContributors xiii
Foreword xv
Preface xvii
1HydraulicFractureGeometryfromMinebackMapping 1
R.G.Jeffrey
1.1Introduction 1
1.2SummaryofMappedFractureGeometries 1
1.2.1FracturesinCoal 1
1.2.1.1DHM-7Fracture 2
1.2.1.2DDH190Fracture 2
1.2.2FracturesinHardRock 5
1.2.2.1NorthparkesE48MappedFractures 5
1.2.3OtherMappedFractures 7
1.3ComparisonofMappedFractureGeometries 7
1.3.1DimensionlessParameters 7
1.4FractureGeometrySummary 8
1.5Conclusions 9 References 9
2MeasurementsoftheEvolutionoftheFluidLaginLaboratoryHydraulicFractureExperimentsinRocks 11 DongLiuandBriceLecampion
2.1Introduction 11
2.2MaterialsandMethods 12
2.2.1MaterialsandExperimentalSet-up 12
2.2.2Methods 12
2.2.3ExperimentalDesign 13
2.3Results 14
2.3.1MARB-005 – AHFGrowthwithaFluidLag 14
2.3.2MARB-007 – AHFGrowthduringandaftertheInjection 15
2.3.3GABB-002 – APoint-LoadLikeHFGrowth 16
2.4DiscussionsandConclusions 18
2.4.1ResolutionoftheFluidFrontLocation 18
2.4.2Quasi-BrittleEffects 18
2.4.3HydraulicFractureSurfaces 19
2.4.4Conclusions 21
DataAvailability 21
AppendixADeterminationoftheTimeofFractureInitiation 21 References 22
3MappingHydraulicFractureGrowthUsingTiltmeterMonitoringTechnique 25 Z.R.ChenandR.G.Jeffrey
3.1Introduction 25
3.2ForwardProblemFormulation 26
3.2.1ForwardModelDefinition 26
3.2.2ForwardModel 27
3.2.2.1PointSourceDislocationSingularityModel 28
3.2.2.2AGeneralDistributedDislocationModel 29
3.3BayesianInversionMethod 30
3.4FieldApplications 31
3.4.1InversionResultsUsingthePointSourceForwardModel 31
3.4.2InversionResultsUsingtheGeneralPlanarForwardModel 31
3.5Conclusions 34
Acknowledgments 34
References 34
4ExperimentalObservationsofHydraulicFracturing 37
GuangqingZhangandDaweiZhou
4.1Introduction 37
4.2ExperimentalSetuponLaboratory-Scale 37
4.3LaboratoryInvestigationofFluid-DrivenFracturesinVariousApplications 38
4.3.1HydraulicFracturinginOilandGasReservoirs 38
4.3.1.1BasicIssuesofBreakdownPressureandFractureGeometry 38
4.3.1.2MultipleHydraulicFractureGrowth 39
4.3.1.3InteractionsBetweenHydraulicFracturesandNaturalFractures 40
4.3.1.4FracturePropagationThroughtheLayeredFormation 41
4.3.1.5NonlinearFracturingintheDeepReservoir 42
4.3.1.6CyclicFracturing 43
4.3.2EnvironmentalFracturinginaShallowFormation 44
4.3.3HydraulicStimulationinEGS 44
4.4ConclusionsandFutureWork 45
References 46
5FirstFieldTrailandExperimentalStudiesonscCO2 Fracturing 51 HaiyanZhu,LeiTao,ShoucengTian,andHaizhuWang
5.1Introduction 51
5.2ReviewonscCO2 Fracturing 52
5.2.1ShaleandscCO2 Interaction 52
5.2.1.1MicroscalePhysicalChanges 52
5.2.1.2MicroscaleChemicalChanges 52
5.2.1.3MacroscaleMechanicalChanges 53
5.2.1.4ConclusionsontheExperimentsonShaleandscCO2 Interaction 54
5.2.2ExperimentsandNumericalSimulationsonscCO2 Fracturing 54
5.2.2.1ExperimentsonscCO2 Fracturing 54
5.2.2.2NumericalSimulationsonscCO2 Fracturing 57
5.3AFieldTrailonscCO2 FracturingofContinentalShaleinYanchangOilField 57
5.3.1scCO2 FracturingTechnology 57
5.3.2scCO2 FracturingFieldTest 58
5.3.2.1ReservoirPropertiesofTestWells 58
5.3.2.2FracturingProcessandOperationParameters 58
5.3.3FieldTestResultsandAnalysis 59
5.3.3.1MicroseismicMonitoringandInversionofFractureGeometry 59
5.3.3.2ProductionData 60
5.4ChallengesinscCO2 Fracturing 60
5.4.1scCO2 FracturingMechanismIsStillNotClear 60
5.4.2ChallengesinProppantsCarrying 60
5.4.3ChallengeonthePredictingandMonitoringCO2 Phase 61
5.4.4LackofSpecializedEquipmentforscCO2 Fracturing 61
5.5Conclusions 61
Acknowledgments 61
References 61
6AnUnstructuredMovingElementMeshforHydraulicFractureModeling 65 JohnNapierandEmmanuelDetournay
6.1Introduction 65
6.2DiscreteModelofaPlanarHydraulicFracture 65
6.2.1UnstructuredMesh 66
6.2.2DiscreteElasticityEquation 66
6.2.3DiscretizedLubricationEquationsforChannelElements 67
6.2.4TipElements 67
6.3Time-MarchingAlgorithm 67
6.3.1IterationLoops 68
6.3.2LocalFrontUpdate 68
6.3.3GenerationofaNewRingofTipElements 68
6.3.4CrackSurfaceRemeshing 69
6.3.5GeneralSolutionAlgorithmLogic 69
6.4NumericalSimulations:StressBarriers 70
6.4.1DescriptionofExperiment 70
6.4.2NumericalSimulations(noRemeshing) 70
6.4.3ComparisonwithExperimentalResultsandOtherSimulations 71
6.4.4IllustrationandAssessmentoftheElementRe-MeshingStrategy 71
6.5Conclusions 73 Acknowledgments 73 References 73
7StudyofHydraulicFractureInterferencewithaLatticeModel 75 C.Detournay,B.Damjanac,M.Torres,andY.Han
7.1Introduction 75
7.2 XSite CodeOverview 75
7.3NumericalStudiesofFractureInterference 75
7.3.1InteractionofaHydraulicFracturewithaNaturalFracture 76
7.3.2InteractionofTwoHydraulicFractures 76
7.3.2.1NumericalStudy 76
7.3.2.2InterpretationofResults 78
7.3.3InteractionofHydraulicFracturesinInjectionofMultipleClusters 79
7.3.4InteractionofHydraulicFracturesinFracturedMedium 81
7.3.5InteractionofHydraulicFracturesinZipper-StageInjection 83
7.4Afterword 83 References 85
8TheTippingPoint:HowTipAsymptoticsCanEnhanceNumericalModelingofHydraulicFractureEvolution 87 A.Peirce
8.1Introduction 87
8.2MathematicalModel 87
8.2.1Assumptions 87
8.2.2GoverningEquation 88
8.2.2.1Elasticity 88
8.2.2.2FluidTransport 88
8.2.2.3BoundaryandPropagationConditions 88
8.2.2.4TipAsymptotics,VertexSolutions,andGeneralizedAsymptotes 89
8.3Discretization,CoupledEquations,andtheMultiscaleILSASchemetoLocatetheFreeBoundary 91
8.3.1Discretization 91
8.3.1.1DisplacementDiscontinuityFormulationforPlanarFractures 91
8.3.2LocatingtheFreeBoundaryUsingtheImplicitLevelSetAlgorithm(ILSA) 92
8.4NumericalResults 95
8.4.1SymmetricStressBarrier: m-VertexSolutionvsExperimentandtheEffectofToughness 95
8.4.2AStressDrop:DistinctPropagationRegimesAlongthePeriphery 95
8.5Conclusions 95
8.6Acknowledgment 97 References 97
9Plasticity:AMechanismforHydraulicFractureHeightContainment 99 PanosPapanastasiou
9.1Introduction 99
9.2TheDependenceoftheEffectiveFractureToughnessonPropagationDirection 100
9.3EffectiveFractureToughnessvs.ClosureStress 101
9.4ANewBrittlenessIndexDefinesFractureContainment 102
9.5Conclusions 103 Acknowledgments 104 References 104
10TurbulentFlowEffectsonPropagationofRadialHydraulicFractureinPermeableRock 107 E.A.Kanin,D.I.Garagash,andA.A.Osiptsov
10.1Introduction 107
10.2ModelFormulation 108
10.2.1ProblemDefinition 108
10.2.2GoverningEquations 109
10.2.2.1CrackElasticity 109
10.2.2.2FluidFlow 109
10.2.2.3FracturePropagation 110
10.2.2.4BoundaryConditions 110
10.2.2.5GlobalFluidVolumeBalance 110
10.3SolutionApproach 111
10.4SolutionExamplesforTypicalFieldApplications 112
10.5LimitingPropagationRegimes 115
10.6NormalizationoftheGoverningEquations 118
10.7ProblemParameterSpaceAnalyses 119
10.7.1ZeroLeak-OffCase(ImpermeableRock) 120
10.7.2NonzeroLeak-OffCase(PermeableRock) 121
10.8Conclusions 122 Acknowledgments 124 References 125
11AnalysisofaConstantHeightHydraulicFracture 127 E.V.Dontsov
11.1Introduction 127
11.2GoverningEquations 128
11.3TipRegion 129
11.4VertexSolutions 132
11.4.1StorageViscosity 132
11.4.2Leak-offViscosity 133
11.4.3StorageToughness 133
11.4.4Leak-offToughness 133
11.5FullSolution 134
11.6ApplicationExamples 136
11.7Summary 137 References 137
12DiscreteElementModelingofHydraulicFracturing 141 MengliLiandFengshouZhang
12.1Introduction 141
12.2DiscreteElementModelingofHydraulicFracturing 142
12.3HydraulicFractureInteractingwithNaturalFractures 142
12.3.1HybridDiscrete-ContinuumMethod 143
12.3.2ModelCalibrationforaHydraulicFractureinIntactRock 144
12.3.3OrthogonalCrossing 145
12.3.3.1EffectsofStressRatioandFrictionofNaturalFractures 145
12.3.3.2EffectofStrength(Toughness)Contrast 147
12.3.3.3EffectofStiffness(Modulus)Contrast 149
12.3.4Non-OrthogonalCrossing 150
12.3.5FracturingComplexity 151
12.4DEMModelingofSupercriticalCarbonDioxideFracturing 153
12.4.1NewAlgorithmfortheToughness-DominatedRegime 153
12.4.2NumericalModelSetup 154
12.4.2.1ModelDescription 154
12.4.2.2ModelVerification 156
12.4.3HydraulicFracturinginIntactRockSample 157
12.4.4HydraulicFracturinginFracturedRockSample 161
12.5DEMModelingofFluidInjectionintoDenseGranularMedia 163
12.5.1BackgroundandExperimentalMotivation 163
12.5.2ModelSetup 165
12.5.3EffectoftheInjectionRate 166
12.5.4DimensionlessTimeScaling 168
12.5.5EnergyPartition 170
12.6Discussion 171
12.7Conclusions 171
References 172
13InteractionofaHydraulicFracturewithNaturalFracturesofLesserHeightandWeakBeddingInterfacesasa PossibleMechanismforFractureSwarms 177 XiaoweiWengandOlgaKresse
13.1Introduction 177
13.2PossibleMechanismsforFractureBifurcation 179
13.3InteractionofCloselySpacedParallelFractures 182
13.3.1FractureTipExtensioninOverlappedRegion 182
13.3.2InstabilityofCloselySpacedParallelHydraulicFractures – SharedInlet 183
13.3.3InstabilityofCloselySpacedParallelHydraulicFractures – SeparateInlets 184
13.4PossibleMechanismsforCreatingFractureSwarms 185
13.5Conclusions 188
References 189
14HydraulicFracturingMechanismsLeadingtoSelf-OrganizationWithinDykeSwarms 193 Andrew.P.Bunger,D.Gunaydin,S.T.Thiele,andA.R.Cruden
14.1Introduction 193
14.2SwarmMorphologyandFundamentalDrivers 193
14.3DykeSwarmModelandEnergetics 194
14.4Alignment 196
14.5Avoidance 197
14.6StressShadow 197
14.7StressPlugs 199
14.8Attraction 199
14.9EmergentSpacing 200
14.10SimulatingDykeSwarmAssembly 201
14.11Conclusions 202 Acknowledgments 203
References 203
15NumericalSimulationofThermalFracturingDuringHeatExtractionfromaClosed-LoopCirculationEnhanced GeothermalSystem 207
Z.Lei,BishengWu,andZ.Chen
15.1Introduction 207
15.2MathematicalFormulation 208
15.2.1ProblemDescription 208
15.2.2GoverningEquationsofCoupledThermoelasticModel 209
15.2.2.1FluidFlow 209
15.2.2.2RockDeformation 209
15.2.2.3FractureInitiationandPropagation 210
15.2.2.4ThermalTransportThroughFluidFlow 211
15.2.2.5HeatTransferinRockMatrix 211
15.2.3BoundaryandInitialConditions 211
15.3SolutionMethodologyandComputationalProcedures 211
15.3.1CoupledFluid-FractureSolver 211
15.3.1.1WeakFormandFEMDiscretization 211
15.3.1.2ExtendedFiniteElementApproximation 212
15.3.2CoupledFluid-ThermalSolver 213
15.3.3SolutionStrategy 213
15.4NumericalResults 214
15.4.1FluidFlowandProductionTemperature 214
15.4.2TemperatureDistributioninRockFormation 216
15.4.3FracturePropagation 216
15.4.3.1SingleFractureCase 216
15.4.3.2DoubleFractureCase 220
15.5Conclusions 221 References 221
16MultipleHydraulicFracturesGrowthfromaHighlyDeviatedWell:AXFEMStudy 225 YunZhouandDiansenYang
16.1Introduction 225
16.2ProblemFormulation 227
16.2.1GoverningEquations 228
16.2.1.1SolidDeformation 228
16.2.1.2FluidFlowinMatrix 229
16.2.1.3FluidFlowinFractures 229
16.2.1.4FlowRateDivisiontoMultipleFractures 229
16.2.1.5FracturePropagation 229
16.2.2WeakForms 230
16.3NumericalMethod 230
16.3.1XFEMApproximationof u(x, t)and p(x, t) 230
16.3.2SpatialandTimeDiscretization 231
16.3.3SolutionStrategy 231
16.3.3.1SolutionofHM-CoupledEquations 231
16.3.3.2SolutionofFlowRateDivision 231
16.4NumericalResults 232
16.4.1VerificationoftheModel 232
16.4.2Multi-ClusterHydraulicFracturinginHigh-AngleWell 233
16.4.2.1ModelSet-up 234
16.4.2.2OperationalParameters 235
16.4.2.3DeviationAngle 236
16.4.2.4FractureSpacing 239
16.4.2.5FracturePlacement 240
16.4.2.6FractureNumber 241
16.5Discussion 245
16.6Conclusions 245
Appendix16.ADimensionlessToughness κ 245
Appendix16.BDimensionlessParameter Gm246
Appendix16.CDimensionlessVariabilityCoefficient Cv 246 References 246
17HydraulicFracturing-InducedSliponaPermeableFault 251 Xi.Zhang,R.G.Jeffrey,andJ.Yang
17.1Introduction 251
17.2ModelSetup 252
17.3SummaryofModelingResults 254
17.3.1FullyClosedFractures 254
17.3.1.1ConstantFaultPermeability 254
17.3.1.2EnhancedFaultPermeability 254
17.3.1.3FaultPermeabilityReduction 255
17.3.2PartiallyOpenedFracture 256
17.3.2.1PlanarFault 256
17.3.2.2NonplanarFault 256
17.4RadiatedEnergy 256
17.5ConclusionsandFutureWork 258 Acknowledgment 259 References 259
Index 263
ListofContributors
AndrewP.Bunger
DepartmentofCivilandEnvironmentalEngineering, UniversityofPittsburgh,Pittsburgh,PA,USA and DepartmentofChemicalandPetroleumEngineering, UniversityofPittsburgh,Pittsburgh,PA,USA
Z.R.Chen
CSIROEnergy,ClaytonSouth,Victoria,Australia
A.R.Cruden
SchoolofEarthAtmosphere&Environment,Monash University,Clayton,Victoria,Australia
B.Damjanac
ItascaConsultingGroup,Minneapolis,MN,USA
EmmanuelDetournay
DepartmentofCivil,Environmental,and Geo-Engineering,UniversityofMinnesota,Minneapolis, MN,USA
C.Detournay
ItascaConsultingGroup,Minneapolis,MN,USA
E.V.Dontsov
ResFracCorporation,555BryantSt., #185PaloAlto,CA,USA
D.I.Garagash
DepartmentofCivilandResourceEngineering, DalhousieUniversity, Halifax,NovaScotia,Canada
D.Gunaydin
DepartmentofCivilandEnvironmentalEngineering, UniversityofPittsburgh,Pittsburgh,PA,USA
Y.Han
AramcoServicesCompany:AramcoResearchCenter, Houston,TX,USA
R.G.Jeffrey
SCTOperationsPty.Ltd.,Wollongong,NewSouthWales, Australia
E.A.Kanin
ProjectCenterforEnergyTransitionandESG, SkolkovoInstituteofScienceandTechnology(Skoltech), Moscow,RussianFederation
OlgaKresse Schlumberger,Houston,TX,USA
BriceLecampion
Geo-EnergyLaboratory,GaznatChaironGeo-Energy, EcolePolytechiqueFédéraledeLausanne,EPFL-ENACIIC-GEL,Lausanne,Switzerland
Z.Lei
DepartmentofHydraulicEngineering,Tsinghua University,Haidian,Beijing,China
MengliLi
KeyLaboratoryofGeotechnicalandUnderground EngineeringofMinistryofEducation,TongjiUniversity, Shanghai,China and
DepartmentofGeotechnicalEngineering,CollegeofCivil Engineering,TongjiUniversity,Shanghai,China
DongLiu
Geo-EnergyLaboratory,GaznatChaironGeo-Energy, EcolePolytechiqueFédéraledeLausanne,EPFL-ENACIIC-GEL,Lausanne,Switzerland
JohnNapier
DepartmentofMiningEngineering, UniversityofPretoria, Hatfield,Pretoria SouthAfrica
A.A.Osiptsov
ProjectCenterforEnergyTransitionandESG, SkolkovoInstituteofScienceandTechnology(Skoltech), Moscow,RussianFederation
PanosPapanastasiou DepartmentofCivilandEnvironmentalEngineering, UniversityofCyprus,Nicosia,Cyprus
A.Peirce DepartmentofMathematics,UniversityofBritish Columbia,Vancouver,BritishColumbia,Canada
LeiTao
StateKeyLaboratoryofOilandGasReservoirGeologyand Exploitation,ChengduUniversityofTechnology, Chengdu,China
S.T.Thiele
HelmholtzInstituteFreibergforResourceTechnology, Helmholtz-ZentrumDresden-Rossendorf,Freiberg, Germany
ShoucengTian
StateKeyLaboratoryofPetroleumResourcesand Prospecting,UniversityofPetroleum(Beijing), Beijing,China
M.Torres
ItascaConsultingGroup,Minneapolis,MN,USA
HaizhuWang
StateKeyLaboratoryofPetroleumResourcesand Prospecting,UniversityofPetroleum(Beijing), Beijing,China
XiaoweiWeng Retired,Schlumberger,Houston,TX,USA
BishengWu DepartmentofHydraulicEngineering,Tsinghua University,Beijing,China
DiansenYang SchoolofCivilEngineering,WuhanUniversity, Wuhan,China
J.Yang InstituteofRockandSoilMechanics,ChineseAcademyof Sciences,Wuhan,China
XiZhang FacultyofEngineering,ChinaUniversityofGeosciences, Wuhan,China and SCTOperationsPtyLtd,Wollongong,Australia
FengshouZhang
KeyLaboratoryofGeotechnicalandUnderground EngineeringofMinistryofEducation,TongjiUniversity, Shanghai,China and DepartmentofGeotechnicalEngineering,CollegeofCivil Engineering,TongjiUniversity,Shanghai,China
GuangqingZhang CollegeofPetroleumEngineering,ChinaUniversityof Petroleum-Beijing,Beijing,China
YunZhou
StateKeyLaboratoryofGeomechanicsandGeotechnical Engineering,InstituteofRockandSoilMechanics,Chinese AcademyofSciences,Wuhan,Hubei,China
DaweiZhou CollegeofPetroleumEngineering,ChinaUniversityof Petroleum-Beijing,Beijing,China
HaiyanZhu
StateKeyLaboratoryofOilandGasReservoirGeologyand Exploitation,ChengduUniversityofTechnology, Chengdu,China
Foreword
Forabout15yearsuntilhisretirementfromthe CommonwealthScientificandIndustrialResearch Organization(CSIRO)in2015,RobJeffreyledagroupof scientists,technicalofficers,students,andacademic visitors,allengagedinhydraulicfracturingresearch. UnderRob’sdirection,theCSIROHydraulicFracturing (CSIROHF)group,locatedinMelbourne,wasavibrant center,uniqueinthediversityofitsactivitiesthat involvedfieldtesting,laboratoryexperiments,theoretical modeling,andnumericalsimulations.Manyauthorsof thisbookhavebeenassociatedwiththeCSIROHF group,andtheircontributionsaretributestoRob’s inspiredleadership.
ApermanentfocusoftheCSIROHFgroupwastosupport thedevelopmentofhydraulicfracturingasameansto preconditionarockmassforcaveinducementofore bodiesandcoalminegoafs.Thisapplicationofhydraulic fracturingtominingwasRob’sbrainchild,aninvention foreshadowedbytheexperienceandexpertiseRobgained intheUnitedStateswhileworking,first,forthemining industryandlaterforthepetroleumindustryinresearch anddevelopment.Hispatentedtechniqueisnowbeing usedroutinelyinminesinseveralcountries,including Australia,Chile,andIndonesia.Preconditioningby hydraulicfracturinghasaprofoundeconomicimpacton hardrockminesrelyingonblockcavingandonlongwall coalmines,insomecasesensuringtheeconomicviability oftheseoperations.
OneofRob’soutstandingachievementsatCSIROwasthe establishmentofaworld-classlaboratorydedicatedto
hydraulicfracturingresearchwithalargepolyaxialcellas itscenterpiece.AparticularpointofprideforRobwasthe heavy-dutyequipmentusedtoconductfieldtests.Thelab wasthebirthplaceofinnovativeexperiments;somewere conductedinglassorpolymethylmethacrylate(PMMA)to enablevisualizationofthefractureevolutionand photometricmeasurementsoftheaperturefieldtotest computationalmodels,whileothersexploredthe interactionofahydraulicfracturewithdiscontinuities orwithafreesurface,tociteafew.Althoughexperiments inthefieldorinthelabareRob’spassion,hewas alwaysenthusiasticallysupportiveoftheoreticaland computationalwork.
SomemembersoftheformerCSIROHFgrouphave movedtoacademicpositionsinChina,Europe,and NorthAmerica,buttheyremainengagedinhydraulic fracturingresearch.Withthismigration,thetheoretical expertiseandknow-howforinnovativeexperimental workgainedintheCSIROlabhasbeenkeptaliveand furtherflourished.ThisisoneofRob’senduringlegacies. Onapersonalnote,IamindebtedtoRobforgivingme thewonderfulopportunitytoworkinhisgroup.Itwas initiatedattheARMAVailSymposiumin1999,when Robaskedme “wouldyoubeinterestedtospendsomeof yoursabbaticalinMelbourne …. ” Thisledtoalong-term collaborationandachancetodiscoverAustraliaon multipleoccasions.
EmmanuelDetournay,UniversityofMinnesota, Minneapolis,USA
Preface
Thisvolumeofcontributedchapterscomesfromthenetworkofresearchersworkingonmechanicsofhydraulic fracturingintheory,experiments,andapplications.Many ofthesecontributorscomprisedtheparticipantsinaseries ofHydraulicFracturingSummitsheldfrom2001to2015. Theseworkshopsprovidedavenueforlivelydebatesand effectiveexchangesofideas.ThesuccessoftheseSummits isattributedtoRobJeffreyandEmmanuelDetournay,who initiatedthisseriesofannualmeetingsandfosteredthe communityintheircollaborations.Theyrecognizedthescientificsignificanceofthisfast-growingfieldpriortothe surgeofunconventionaloilandgasreservoirstimulation, andthroughthisworkshopmentoredagenerationof hydraulicfracturingresearchers.
Thisvolumeisdevotedtocelebratingthe70thbirthdayof RobJeffrey,whowastheprogramleaderatCommonwealthScientificandIndustrialResearchOrganisation (CSIRO),Australia,whenthreeoftheeditorsworkedthere. Robisoneofthepioneersfortheapplicationofhydraulic fracturingincoalseamgasreservoirstimulationandthe foundationoftheknowledgeonhydraulicfracturegrowth innaturallyfracturedrocks.Duringhisfortyyearsworking inhydraulicfracturingfield,helaunchedmanyimportant researchdirections,beingamongthefirsttoworkinareas suchasfracturenetworkformation,novelfracturemonitoringmethods,mine-throughmappingoffull-scale hydraulicfractures,anduseofhydraulicfracturingtomodifyrockmasspropertiesforbothblockcavingandlongwall coalmining.Allofhisworkbroughttogethertheoretical andexperimentalmethodsinordertoaddressnewissues withrigorandpragmatism.InrecognitionofRob’soutstandingcontributionsbothtothefieldandtotheircareers, hisHydraulicFracturingSummitcolleaguesandothersare happytocontributethisbookthatisinhishonor.
Thisvolumecontains17chapters,coveringawiderange ofrelevantresearchtopicsandimportantapplicationareas. Theeditorswishtothankeverycontributorfortheirdiligenceandcarefulchoiceoftopics.Thechapterswillprovidethefirstcomprehensivestoryontheseexperimental andfieldmonitoringresults,includingcomparisonsofseveralnumericalmethodswithindustrialapplications.This bookwillbenefitawidespectrumofreaders,rangingfrom newcomersseekinganefficientorientationtothefieldto seasonedexpertslookingforauniquesetofreferences onsomeofthemostimportantissueswithinthehydraulic fracturingcommunityoverthepasttwodecades.
Theeditorsaregratefultothoseanonymousreviewersof thebookproposalfortheirpositivecommentsandconstructivesuggestionsandtothemanyvolunteerreviewers whospenttremendousamountoftimeandeffortinthe evaluationofeachchapter.WealsoowemuchtoDr.RituparnaBose,LaylaHarden,andStefanieVolkatWiley,who areawareoftherapid-growingknowledgeandsocial impactofhydraulicfracturing,andalongwithothereditorialteammemberssupportingthisbook.Withouttheir vision,enthusiasm,anddiligentefforts,thisbookwould nothavebeenpossible.
XiZhang,ChinaUniversityofGeosciences, Wuhan,China
BishengWu,TsinghuaUniversity, Beijing,China
DiansenYang,WuhanUniversity, Wuhan,China
AndrewBunger,UniversityofPittsburgh, Pittsburgh,PA,USA
HydraulicFractureGeometryfromMinebackMapping
R.G.Jeffrey
SCTOperationsPty.Ltd.,Wollongong,NewSouthWales,Australia
KEYPOINTS
• Detailsoffull-sizehydraulicfracturegeometryareonlyavailablefrommappingofminedfractures.
• Hydraulicfracturesmappedinnaturallyfracturedcoal,sandstone,andincrystallinerockshowmanysimilaritiesthatresultfrom interactionswithnaturalfracturesandshearstructures.
1.1Introduction
Hydraulicfractureshavebeenmappedduringminingina rangeofrocktypesandinavarietyofgeologicandinsitu stresssettings.Mappinghasoccurredforfracturesplaced intoclayandsoil[1],weldedtuff[2],coal([3–5]),andesite [6],andothercrystallineandmetamorphicrocksinporphyryorebodies[7–9].
Coalseamsaretypicallyfracturedforthepurposeof stimulatingproductionofseamgas(coalbedmethane), eitherforcommercialuseofthegasortoimprovedrainageofgasfromthecoalbeforemining.Approximately 50hydraulicfractureshavebeenminedandmappedin detailincoalseamsintheUnitedStatesandAustralia ([3,4,10,11];Jeffreyetal.1993).Incomparison,fractures mappedinothermaterials(soilandrock)totallessthan 20,with10oftheselocatedinporphyrycopperandgold orebodies([6,7,9];containingintrusivemonzonitesand metamorphosedvolcanicsediments.Thesefractureswere placedaspartofinvestigationsintofracturegeometry expectedtobeproducedbyhydraulicfracturesusedto preconditiontheorebodyinadvanceofmining.Thefracturesplacedintoweldedtuffwerepartofearlyresearch intohydraulicfracturegrowth[2]andthesoilfractures werepartofastudyoffracturingforwasteremediation[1].
Bycomparingthefracturegeometrymappedinthesedifferentnaturalmaterials,commonanddisparatefeaturesof thefracturesarehighlighted.Tohelpwiththecomparison,
dimensionlessgroupsthathavebeenshowntobeimportantinhydraulicfracturegrowtharecalculatedorestimatedforeachmappedfracture.Thereisanextensive bodyofworkusingexperimental,analytical,andnumerical methodstoinvestigateinteractionsofhydraulicfractures withbeddingandnaturalfracturesandfaults.Thispaper limitsitselftothemappedgeometryexposedbymining offull-sizehydraulicfracturesandpresentsacomparison offeaturesfoundincoal,sandstone,andstrongermetamorphicorigneousrocks.
1.2SummaryofMappedFracture Geometries
Selectedhydraulicfracturesthatwereplacedincoal,sandstone,andhardrockaredescribedinthissection.Foreach fracturethetreatmentparameters,rockproperties,andin situpropertiesarelisted.
1.2.1FracturesinCoal
Twofracturesplacedintocoalseamswillbedescribed,one locatedinAustraliaasdescribedbyJeffreyetal.(1993)and onelocatedintheUnitedStates[10].Thefracturedescriptionsincludedetailsofthetreatmentandsitecharacterization,includingrockmechanicalpropertiesandinsitu stressdata.
MechanicsofHydraulicFracturing:Experiment,Model,andMonitoring,FirstEdition. EditedbyXiZhang,BishengWu,DiansenYang,andAndrewBunger. ©2023JohnWiley&Sons,Inc.Published2023byJohnWiley&Sons,Inc.
1.2.1.1DHM-7Fracture
Aspartofaprogramtobetterunderstandhydraulicfracturestimulationofcoal,wellDHM-7wasfracturedusing lineargelwithsandproppant[10].DHM-7wasdrilled andcompletedopenholethroughtheBlueCreekcoalseam inAlabama.ThewellwaslocatedovertheOakGroveMine intheWarriorBasin.Table1.1summarizestheparameters ofthesiteandthetreatmentparameters.Itisdifficultto measure σH incoalbecausethecleatandnaturalfractures makeovercoringimpracticalandthedeterminationoffractureinitiationisdifficultwhenusingamicrofracstresstest. Avaluefor σH thatislargerthantheverticalstresshasbeen selectedbecausetheminebackmappingrevealednodevelopmentofproppedfracturesatrightanglestothedirection ofthemainverticalfracturebranch.
Roomandpillarminingexposedthefractureinthecoal ribaroundthesidesoftwopillarsandalongthecoalrib nearestthewell(Figure1.1).Averticalandhorizontal proppedfracture(T-shaped)wasfoundwiththevertical partconsistingofanumberofparallelstrands.Thevertical fractureextendedformorethan30mtothenorthofthe well.Ahorizontalproppedfracture,extendingovertheverticalfracture,waslocatedatthecoal-roofrockinterfaceand wasmappedindetail(seeFigure1.2).Thefracturewasnot minedtothesouth,butbasedontheareaandfracture widthsmapped,theproppedfracturemappedonthenorth sideofwellDHM-7wasestimatedtocontainapproximately 75%ofthesandproppantinjected[12].
ThetreatmentpumpedintoDHM-7usedanoncrosslinkedguar-basedgelfluidthatwasinjectedatanaveragerateof8.3barrels/min(0.022m3/s).Athickresin-coated sandsystemwaspushedintothenear-wellpartofthe
Table1.1 DHM-7.Coal,well,andtreatmentparameters.
ParameterValueUnitsDescription
σH >8.1MPaMaximumhorizontalstress
σh 6.2MPaMinimumhorizontalstress
σv 8.1MPaVerticalstress
Po <3MPaPorepressure(estimated)
k 1.2mdPermeability,millidarcy
E 4000MPaYoung’smodulusofcoal
ν 0.3Poisson’sratioofcoal
Q 0.022m3/sInjectionrate
μ 25×10 9 MPasApparentfluidviscosity, at170s 1
Depth331.3mDepthtotopofBlueCreek seam
r 0.108mWellboreradius
fractureonthemorningfollowingthemaintreatment.This resincoatedsandwasusedtotestitsabilitytostabilizethe wellboreregion.Itisdesignedtoretain80%ofthesand’spermeabilityaftercuring.Muchoftheverticalfractureexposed nearestthewellbore,atlocationAinFigure1.1,wasfilled withthisresin-coatedsand.Samplesofthisproppedfracture wereexcavatedandtakenfromthemineforlateranalysis anddisplay.Noresin-coatedsandwasfoundatthenext exposureatlocationB.Mappingofthehorizontalfracture wasdoneatthedetaillevelrepresentedinFigure1.2along allproppedexposures.
ThemappedgeometryinDHM-7andothercasespresentedbelowrelyontheproppanttomarkthefracture path.Thehydraulicfracturetypicallyextendsbeyondthe proppant,especiallywhenlessviscousfluidsareused. Hydraulicfracturescannotingeneralbefoundormapped iftheydonotcontainproppant.Thedistributionofthe proppant,especiallyinahorizontalfracture,dependson boththefracturewidthandthefluidvelocityfield.Proppanttransportinhorizontalfracturesisanareaofstudy thathasreceivedlittleattention,primarilybecausehorizontalfracturegrowthisthoughttobearareoccurrence atdepthsgreaterthanapproximately300m.Thehydraulic fracturesdescribedbelowthatwereplacedintotheorebody atNorthparkesatadepthof580mwerehorizontal.Tshapedfracturesarecommoninstimulationofcoaland bettermodelsthatcandealwithhorizontalfracturegrowth andtheassociatedproppanttransportproblemwouldbe welcome.
1.2.1.2DDH190Fracture
AnuncasedHQ-sizecoredborehole(DDH190)wasdrilled throughtheGermanCreekcoalatCentralCollieryin Queenslandandwashydraulicallyfracturedusingaborate crosslinkedhydroxypropylguargelfluid.Thesitewascharacterizedbyundertakingwelltesting,stressmeasurement, coretesting,andfracturetestingbeforethemainfracture treatment.Table1.2summarizesthesiteparametersrelevanttothetreatmentasgivenbyJeffreyetal.[5].
Mappingofthefractureduringandafterdevelopmentof theroadwaysinthisareaoftheminerevealedavertical fractureinthecoalthatextendedintotheroofrock (Figure1.3).Thefracturetraceintheroofrock (Figure1.3a)wasprimarilyasinglefracture,butinteractionswithnaturalfracturesresultedintheformationof someoffsetsandshortparallelbranches.Theverticalfracturetraceinthecoalatthenorthsideof13cut-through (Figure1.3b)wastypicalofotherverticalsectionsmapped atthissite,consistingofasinglefracturethatinteracted withbeddedandshearedcoal.The150mm-thickmid-seam shearzone(mssz)runsthroughmuchoftheGermanCreek seamandiscomposedofshearedcoal,withparticles
North
DHM-7
Oak Grove Mine
Alabama
Mine roadway Sand Propped Horizontal Fracture
coal pillar
Mine roadway
25 Mapping station with average sand thickness* for 1 foot section of horizontal fracture. * Sand thickness reported in hundredths of inches. Vertical fracture at coal rib
Figure1.1 AplanviewoftheproppedhydraulicfracturemappedattheDHM-7siteisshowninthetopdrawingwhilethree mappedverticalfracturesectionsexposedatthecoalribareshowninthelowerdrawing.Mappingofthehorizontalfracture occurredalongtheribsoftheroadwayswherethefracturewasexposedneartheroof(Source: Boyeretal.[12]/GasResearchInstitute).
Depth193.5mDepthtotopofGermanCreekseam r 0.048mWellboreradius 4 1HydraulicFractureGeometryfromMinebackMapping
Figure1.2 Sevenlinearfeetofhorizontalfracturemappedalongthenorthsideofthecoalpillar(Source: Boyeretal.[12]/GasResearch Institute).ThelocationofthisportionofthehorizontalfractureisindicatedinFigure1.1andlabeledasstation1through6.The entirehorizontalfracturewasmappedatthislevelofdetailandthenumbersinFigure1.1alongthepillarboundaryindicatetheaverage proppedwidthinhundredthsofinches(e.g.30represents0.30in.).
Table1.2 DDH190siteparameters.
ParameterValueUnitsDescription
σH >4MPaMaxhorizontalstress,inroof
σh 2.5 1.9
Minhorizontalstressincoal Minhorizontalstressinroof
σv 4.5MPaVerticalstress
Po 1.08MPaPorepressure
k 4.2mdPermeability,millidarcy
E 2000 25000 MPa MPa Young’smodulus,coal Young’smodulus,roofrock
ν 0.35 0.13 Poisson’sratio,coal Poisson’sratio,roofrock Q 0.0025m3/sInjectionrate
μ 610×10 9 MPasApparentfluidviscosityat170s 1
Figure1.3 Lookingdownonproppedfracturetrace(a)insandstoneroofrockat13cut-through.Verticalsection(b)showing proppedfractureexposedonnorthcoalribof13cut-throughnearboreholeDDH190(
Source: Jeffreyetal.[5]/CoalbedMethane AssociationofAlabama). 1.2SummaryofMappedFractureGeometries
rangingfromclaysizetoafewcentimetersinsize.Themssz issofterandweakerthanthecoalaboveandbelowit.This hydraulicfractureandothersmappedinthiscoalseam oftendevelopedanoffsetacrossthemssz.
ThefractureatDDH190extendedintothelowerstress roofrockwithonly12%oftheproppantinjectedestimated tobeaccountedforbytheproppedfractureinthecoal seam.Themappingclearlyshowsthetraceofthepropped hydraulicfractureintheroofandinthecoal,butdoesnot revealifthefracturewasgrowingprimarilylaterallyorverticallyatthesectionsmapped.Modelingofthisfracture suggestsupwardgrowthof7mintotheroofrockatthe borehole[13].
1.2.2FracturesinHardRock
Hydraulicfracturingisusedinminingtoinducecavingand topreconditionrockforcaving[7].Morerecently,preconditioninghasbeenusedinareasofhighstressasameansof reducingthepotentialfortheoccurrenceoflarge,damagingseismicevents[14].Atotalofninefractureshavebeen minedandmappedatfourmetalliferousminesitesin
Australia[7,9,15]andChile[6].Thefracturesdescribed byJeffreyetal.[9]willbecomparedtothefracturesplaced intocoalseams.
1.2.2.1NorthparkesE48MappedFractures
Sixhydraulicfractureswereplacedaheadofatunnelat580 mdepthattheE48Northparkesmineaspartofaminethroughexperiment[9].Thefractureswereproppedwith coloredplasticandsandandweremonitoredbymicroseismicandtiltmeterarrays.Themappedhydraulicfracturesat Northparkesconsistofnearlyhorizontalsegmentswithoffsetsatintervalsalongthemproducedasthefracturegrew intoandalongdippingveins,naturalfractures,andshear zones.Fracturebranchesandsub-parallelproppedsections werealsomapped,makingup10–15%ofthetotalfracture extent.Therockmassatthesiteisnaturallyfractured,containingapproximatelyfivenaturalfracturespermeter. Additionaldetailsofthesite,fracturing,andmine-back canbefoundinJeffreyetal.[9].Table1.3listssiteandrock parameters.ThestressdirectionsgiveninTable1.3are basedonovercoringdatameasurednearthefracturesite.
Table1.3 SiteandrockparametersforNorthparkesE48.
ParameterValueUnitsDescription
σH 40MPaMaxhorizontalstress,2900,80 dip
σh 22MPaMinhorizontalstress,220,110 dip
σv 15MPaVerticalstress,1650,760 dip
Po <1MPaPorepressure
k 0.005mdPermeability,millidarcy
E 50000MPaYoung’smodulus
ν 0.2Poisson’sratio
Q 0.0075m3/sInjectionrate
μ 610×10 9 1×10 9 MPas MPas
Apparentcrosslinkedgelviscosityat170s 1 Viscosityofwater
Depth580mDepthbelowsurface
r 0.048mWellboreradius
Figure1.4 HydraulicfracturesmappedalongatunnelattheNorthparkesE48mine(Source: Jeffreyetal.[9]/SocietyofPetroleum Engineers).Theinitiationpointofeachfractureisindicatedalongtheboreholewhichisdrawnwithapurpleline.
Theminimumstresswasnearlyverticalwiththemaximum stressorientednearlyhorizontalanddirectedapproximatelyeast–west.
ThehydraulicfracturesweremappedalongtheE48D102 tunnelasitwasdriven.Thefractureswerehorizontalwith stepsalongtheirpathoftenformingwheretheyinteracted withnaturalfracturesandshearstructures.Theseoffsets werelargeenoughtoincreasetheaveragedipbecausethe fracturetracefollowingastair-steppattern.Figure1.4shows anoverviewofthefivefracturesthatwereproppedwith coloredplasticandmappedalongthesidesofthetunnel.
Fractures6and7wereplacedusingcrosslinkedguargel whilefractures5,8,and9wereplacedusingwater.Thecolorsofthepointsmappedalongeachfractureshownin Figure1.4correspondtothecolorsoftheplasticproppant
used.Thegridlinesshownaretheminecoordinatesin meters.Theinitiationpointofeachfractureisindicated byablacksquaresymbolandtheboreholeisshownby thelineconnectingthesesymbols.Figure1.5containsa moredetailedsketchofFracture8,whichcontainedthelargestoffsetorstepmappedinanyofthefracturesatthissite. Theinjectioninterval,whichwasintheboreholeE48D102, isshown.Thisintervalislocatedapproximately2.5moutof theplaneofthefracturetraceshownbecausetheborehole wasdrilledalongthecenterlineofthetunnel.Fracture growthislikelytohavebeensemi-radialfromtheinjection intervalandshouldnotbevisualizedasoccurringpurely alongthefracturetrace.Fracture7containedanoffsetsimilarinsizetotheoneshowninFigure1.5andaseriesof smalleroffsets(butconsistingof200–400mmsteps)were
8, looking south
Scale 2m
Figure1.5 Themappedtraceoffracture8,exposedalongthe southsideofthetunnel,attheNorthparkesE48mine-backsite (Source: Jeffreyetal.[15]).
mappedatoneexposureofFracture5.Therefore,atthissite thecrosslinkedgeldidnotproducemoreplanarfractures comparedtothewaterdrivenfractures.
Detailsofthefracturegeometrywerenotcollectedexcept alongthesidesofthetunnelandoccasionallyacrossthe tunnelfaceandback.Thisisacommondifficultyexperiencedwhenmappinghydraulicfracturesinacommercial mine.Thetunnellingorcoalextractionoperationstake precedenceovermapping.InthecaseoftheE48site,the tunnelisextendedinapproximatelyfour-meterintervals usingadrill-blast-muck-supportcycle.Mappingis restrictedtooccurbetweenthemuckandsupportorsupportanddrillsteps.
1.2.3OtherMappedFractures
Hydraulicfractureshavebeenplacedintoarangeofrock andsoilmaterialsfollowedbyminingandmapping.Warpinskietal.[16]andWarpinskiandTeufel[2]describe hydraulicfracturesplacedintovolcanictuffandMurdoch [1]describefracturesplacedintoclayandsoilatshallow depth.Detailsofthesefracturescanbefoundinthepapers cited.
Table1.4 Dimensionlessgroupsusedincomparisons.
1.3ComparisonofMappedFracture Geometries
Nondimensionalparametersareusefulinhelpingtodeterminethetypeoffracturegrowthtoexpectindifferentrocks andatdifferentsites.Forexample,thedimensionlesstoughnessordimensionlessviscositycanbeusedtodetermineif thetreatmentwascarriedoutinthefracturetoughnessor fluidviscositydominatedregime[17].Viscositydominated growthleadstomoreplanarfracturegeometries[18].
Thefracturewidthrestrictionsresultingfromoffsetsand branchesalongthefracturepatharepotentialsitesfor proppantbridgingandthenarrowwidthattheselocations resultinahigherinjectionpressureandslowerfracture growthcomparedtomoreplanarfractureconditions[19]. Thegrowthofcloselyspacedfractureshasbeenshownto bewelldescribedbyseveraldimensionlessparameters [8,20].Resultsfromnumericalcalculationsofthepathof anewfractureplacednexttoanexistingfracturewerepresentedbyBungeretal.[20]andwerecomparedtominebackandlaboratoryexperimentsinBungeretal.[8].The fracturesmappedanddescribedabovewillbecompared witheachotherandarefurthercategorizedbycalculating valuesforthedimensionlessviscosityanddifferentialstress parameters.
1.3.1DimensionlessParameters
Thenondimensionalparametersusedarelistedin Table1.4.Thephysicalparametersthatareusedindefining thenondimensionalparametersarelistedinTable1.1.The nondimensionalviscosityisusedtodeterminewhetherthe fracturegrowthisdominatedbyrockfracturetoughnessor byfrictionallossesassociatedwithviscousfluidflowinthe fracturechannel[17].Thedimensionlessstressiscommonlyusedinstudiesofhydraulicfracturesinteracting withotherhydraulicfracturesorcrossingnaturalfractures [8,18,19].Twovaluesofdimensionlessdifferentialstress aredefinedbecauseahydraulicfracturegrowingwithan
Fracture
Table1.5 Nondimensionalparametersforthreesites.
Site Md13 d23 NorthparkesF87.71.70.5
orientationsuchthatitopensagainst σ3 willbesubjectto arangeofdifferentialstresses,dependingonthelocation ofthepointbeingconsideredwithrespecttotheother twoprincipalstresses.Theseparametersaretherefore usedhereasreferencevaluesinthecomparisonsofoverallfracturegrowthbehaviorasdeterminedbymine-back mapping.
Thevaluesofthethreedimensionlessparametersateach ofthethreemappingsitesarelistedinTable1.5.
1.4FractureGeometrySummary
The M valuelistedfortheNorthparkessiteappliestofracturescreatedusingwater,whichwasthecaseforFracture 8,whilethetreatmentinDHM-7usedlineargelanda crosslinkedgelwasusedinDDH190.The M valuesbelow 0.25representafractureregimethatistoughnessdominatedwhilean M valueofoneorgreaterisviscositydominated.Allofthefracturesdescribedherefallintothe viscositydominatedregime.Thefracturegrowthregime forthepartofthefracturethatextendedintotheroofrock atDDH190wasstronglyviscositydominated.
Naturalfracturesandshearzoneswerecrossedbythe hydraulicfracturesatallthreesites.Thedownwardgrowth ofthefractureattheDDH190sitewasbluntedbyasoftclay atthecoal-floorrockinterface.
Fracture8atNorthparkesentereda45 dippingnatural fractureandgrewalongitforapproximately2.5mbefore exitingdowndip.Theexitpointcoincideswithadditionalcalcitemineralizationinthenaturalfracture.This
sectionofthestructurewouldbestrongerinshear,resultinginhighershear-generatedtensilestressandwepropose thatthisallowedtensilefracturesorwingfractures[21]to formatthatlocation,allowingthehydraulicfractureto escapethenaturalfracture.
Table1.6providesasummaryofthefeaturesthatare presentinthefracturesatthethreesitesfortwodifferent resolutions.Theresolutionisdefinedasthesmallestfeature onthehydraulicfracturethatisincludedindescribingthe fracturegeometry.InTable1.6,nonplanarmeansthemain fracturechannelisnotinasingleplanewhilebranching meansthefracturetreatmenthasproppedseveralfractures overpartsofitsextentthatmaybeparalleloratanangleto themainfracture.
TheproppedfractureattheDHM-7sitewasT-shaped, witheachsegmentplanarwhenviewedwitharesolution of1m.Whenviewedinmoredetail(witharesolutionof 0.01m)offsetsandbranchesareapparentasshownin Figures1.1and1.2.AT-shapedfractureisconsideredto beabranchedfracture,becausethehorizontalandvertical segmentsarebothconsideredasbranchesofthefracture channel.InTable1.6,theDHM-7fractureisconsidered tobenonplanarandbranchedatthecoarserresolution. T-shapedfracturesrepresentacasewherethehydraulic fractureisdivertedintothehorizontaldirectionratherthan crossingthehorizontalfeatureandcontinuingtogrowvertically.HorizontalfracturesassociatedwithT-shapedfractureshavebeenmappedtoextend10–100sofmeters withoutdivertingbackintothecoalorupintotheroof. Themechanicsofthisobservedfeatureofthesefractures hasnotbeencompletelyexplained.
ThequalitativedescriptioninTable1.6showsthatthe classificationregardingplanarity,branches,andoffsets dependsonthescaleoftheobservation.Whenamore detailedviewisavailable,thefracturesatallthesitesshow offset,nonplanar,andbranchedgrowth.Themainfactor thatcontrolsthedevelopmentofthesefeaturesinahydraulicEfractureisthepresenceofnaturalfracturesorbedding interfaces.Thefracturegrowthregimeandthedeviatoric stress,atleastintherangerepresentedbythethreesites reviewedhere,donotseemtoresultinastrongchange
SiteOffsetsNonplanarBranchingOffsetsNonplanarBranching
NorthparkesYesYesNoYesYesYes
DHM-7NoYesYesYesYesYes
DDH190coalNoNoNoYesYesYes
DDH190roofNoNoNoYesYesYes
Table1.6 Fracturegeometryversusresolutionofobservation.
inthefracturegeometry,atleastasexpressedbymapping ofitsproppedextent.Networklikefracturedevelopment wouldbeexpectedasthedeviatoricstressisreducedtoward zero,butwedonothavemappedfracturesrepresenting thatstresscondition.
1.5Conclusions
Threesitesatwhichhydraulicfractureshavebeenmined andmappedaredescribed.Measuredparametersaregiven foreachsitethatarethenusedtofindthedimensionless
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