SPE-173326-MS

SPE-173326-MS9

Figure10—Widthprofileoffracturenetworksattheendofinjectionforthreecaseswithdifferentdifferentialstresses(clusterspacing?100ft,slickwater,60bpm,relativeanglebetweenNFandHF?45o,a?2,NFspacing?55ft).

suppressedbyhighdifferentialstresswhenhydraulicfracturesdivertalongthenaturalfracturesforthedifferentdifferentialstresses.Fracturespropagatinginareservoirwithhighdifferentialstresshaveshallowreservoirpenetrationandwidefractureaperture(Figure10),becausefracturepropagationisgreatlysloweddownwhenintersectingwithnaturalfracturesintheformationwithhighdifferentialstress.

10SPE-173326-MS

Figure11—RosediagramsshowingfracturetrendsforassociatedthreecasesinFigure10.

TheorientationhistogramsoffracturegeometryareshowninFigure11byrosediagrams.Underisotropicstressconditions,hydraulicfractureshavethetendencytocontinuegrowingalongthedirectionofnaturalfracturesafterbreakingoutandhaveverysmallchancetogrowintheiroriginaldirection.Underanisotropicstressconditions,thechanceofgrowingintheoriginaldirectionincreasesanddependsonthemagnitudeofdifferentialstress.Figure11(b)showsthatfracturespropagatealongnaturalfractures,andthenturnbacktotheoriginaldirectionaftercomingoutfromthetipsofnaturalfractures.Asdifferentialstressincreases,Figure11(c)indicatesthathydraulicfracturesmainlypropagatealongtheiroriginaldirection.Thecomplexityoffracturegeometryismitigatedbyhighdifferentialstress.Theeffectsofnaturalfracturepatterns

Thenaturalfracturepatternswithdifferentorientations,spacing,andlengthscansignificantlyimpacttheshapeoffracturegeometry.Inourmodel,asetofnaturalfractureswiththesameorientationandpowerlawlengthdistributionwascreated.Fourfracturespropagatingsimultaneouslyintheformationswithdifferentnaturalfractureorientationsweresimulatedtoinvestigatetheeffectsoftheorientation.

HydraulicfracturestendtocrossnaturalfractureswhentherelativeanglebetweenHFandNFis90o,asshowninFigure12(a).Naturalfractureswithlargerelativeangletohydraulicfractureshavelesschancetoalterpropagationtrajectoryofthehydraulicfracturesbecausehydraulicfractureshavegreaterpossibilitytocrossthenaturalfractures(Figure2).Astherelativeangledecreases,thechanceofhydraulicfracturescrossingnaturalfracturesdecreases,whichimpliesthatmorenaturalfracturesareopened.Figure12(b)and(c)showmorecomplexfracturegeometriesasaresultfromhydraulicfracturesdeflectingintonaturalfractures.

Theresults(Figure13)ofnaturalfracturepatternswithdifferentnaturalfracturelengthsandspacingshowaprogressioninhydraulicfracturepatternsfromapatternwithapreferentialfracturetrendinthedirectionofnaturalfractures(Figure13(a)),toapatternwiththethrough-goingfracturesroughlygrowingalongtheiroriginaldirection(Figure13(d)).Thelengthofnaturalfracturesarecontrolledbytheexponentaofpowerlaw.Therangeofaisfrom1to3basedonthegeologicalobservationfromoutcrops.Morelargefracturescanbegeneratedwitha?1thana?3,asshowninFigure13.Decreasingthecomponentatendstoincreasethelikelihoodtocreatelongdiversionlengthalongthenaturalfractures,whichenhancesthepossibilitytocontroltrajectoryofhydraulicfractures.ComparingFigure13(a)and(c),itisclearthatthepreferentialtrendsofoverallfracturetrajectoryaredifferent,eventhoughthedirectionofnaturalfracturesis45oforbothcases.Thepreferentialfracturetrendincase(a)witha?1isalmostparalleltothenaturalfractures.Increasingthecomponentacausesplentyofnaturalfractureswithshortlength,whichincreasesvariationsofwidthandorientationalongthefractures,asshowninFigure13(c)and(d).Thevariationsinfracturepathmightenhancetheflowresistanceandpreventproppantfrom

SPE-173326-MS11

Figure12—WidthprofileoffracturenetworksattheendofpumpingforthreecaseswithdifferentrelativeanglebetweenHFandNF(clusterspacing?100ft,slickwater,60bpm,DS?100psi,a?2,NFspacing?55ft).

flowingthroughfracturenetworks.Naturalfracturespacingalsohasgreatinfluenceonfracturegeometry.Naturalfracturepatternswithspacing28ftand50ftwerecreated(Figure13).Thedecreaseinnaturalfracturespacingisadirectresultofmoreandmorenaturalfractureslikelyencounteredandactivatedbythehydraulicfractures,whichcouldtriggermoremicroseismicactivities.

12SPE-173326-MS

Figure13—Widthprofileoffracturenetworks(leftside)andthechangeofthemaximumshearstresswithhypotheticalmicroseismiceventpatterns(rightside)attheendofinjectionforfourcaseswithdifferentnaturalfracturepatterns(clusterspacing?100ft,slickwater,60bpm,DS?100psi,relativeanglebetweenNFandHF?45o,‘*’:microseismicactivity).

Conclusions

Thispaperstudiedcomplexfracturenetworksinducedbymultiplehydraulicfracturespropagatinginnaturallyfracturedreservoirs.Naturalfracturesaffectthehydraulicfracturepropagationthroughthemechanicalpropertiesofthenaturalfracturesandtheirspatialdistribution.Whenintersectingwithanaturalfracture,thehydraulicfracturepropagationdirectionisaffectedbythemechanicalpropertiesofthenaturalfracture,relativeanglebetweenthenaturalandhydraulicfracture,anddifferentialstress.Naturalfracturepatternsweregeneratedbyastochasticrealizationmethod.Perforationclusterspacing,differentialstressaswellasnaturalfracturepatternshaveagreatinfluenceonfracturegeometry.Totaleffectivefracturelengthincreasesasdecreasingperforationclusterspacing,butstartdecreasingwhenthespacingreachesacriticalvalue.Closeclusterspacingcausesimmaturedevelopmentoftheinteriorfractures.Highdifferentialstresstendstoincreaseinjectionpressureoffracturingtreatmentsinnaturallyfracturedreservoirsandreducescomplexityoffracturegeometry.Besides,naturalfracturepatternsplayasignificantroleincontrollingpreferentialtrendofoverallfracturepath.Thepreferentialtrendofhydraulicfracturesismorelikelytobeparalleltonaturalfractureswithsmallrelativeangletohydraulic