Acoustic emission characteristics in hydraulic fracturing of stratified rocks: A laboratory study

10 Pages·2020·1.947 MB·English

by  Zhizhong Jiang, Quangui Li, Qianting Hua, Yunpei Liang, Yangcheng Xu, Le Liu, Xiaobing Wu, Xuelong Li, Xiaoguang Wang, Liangping Hu, Faping Ling

PowderTechnology371(2020)267–276
ContentslistsavailableatScienceDirect
Powder Technology
journal homepage: www.elsevier.com/locate/powtec
Acoustic emission characteristics in hydraulic fracturing of stratified
rocks: A laboratory study
ZhizhongJianga,b,QuanguiLia,b,c,⁎,QiantingHua,b,YunpeiLianga,b,YangchengXua,b,LeLiua,b,XiaobingWua,b,
XuelongLia,b,c,XiaoguangWanga,b,LiangpingHua,b,FapingLinga,b
aStateKeyLaboratoryofCoalMineDisasterDynamicsandControl,ChongqingUniversity,Chongqing400044,China
bSchoolofResourcesandSafetyEngineering,ChongqingUniversity,Chongqing400044,China
cStateKeyLaboratoryCultivationBaseforGasGeologyandGasControl(HenanPolytechnicUniversity),China
a r t i c l e i n f o a b s t r a c t
Articlehistory: Acousticemission(AE)isapopulartechniquetomonitortheprocessofrockfailureduringhydraulicfracturing
Received30November2019 forunconventionalnaturalgasproduction.Itcontainsabundantinformationthatwillbeusefultostudyin-depth
Receivedinrevisedform1April2020 thenatureofhydraulicfracturing.Inthisstudy,wefocusedontheAEcount,energy,peakfrequency,crackclas-
Accepted14May2020 sification,andlocationrecordedfromfourrockspecimenssubjectedtoaspecifictriaxialstresscondition.We
Availableonline20May2020
foundthemulti-frequency-responsephenomenonofAE,andproposedthemulti-frequency-responseindexto
indicatethemomentofthemacrohydrauliccrackformation.Furthermore,itwasfoundthatthepowerlawdis-
Keywords:
tributionindexofAEenergyofnon-stratifiedspecimenwasbiggerthanthatofstratifiedspecimensduringhy-
Acousticemission
Hydraulicfracturing draulicfracturing.Thetensilecrackdominatedinallhydraulicfracturingtests.Ourresultsareofsignificance
Coalbedmethane forunderstandinghydraulicfracturinginstratifiedrocks.
Peakfrequency ©2020ElsevierB.V.Allrightsreserved.
1.Introduction AE characteristics,such ascount,energy, frequency,location,
havebeenextensivelystudied,amongwhichtheAEcountfeature
Stratifiedrocksarewidelydistributedinunconventionalnaturalgas isthemoststudied.Ingeneral,themoretheAEcountthegreater
productionsources,suchasshalesandcoalbeds[1–3].Inordertoen- theenergyofAEevents,andthelargerthescaleofcoal/rockfrac-
hancetheirpermeability,andthusincreasegasproduction,manytech- tures.AEcountcharacteristicsdemonstrategoodKaisereffectina
niquesareemployed,amongwhichhydraulicfracturingiscommonly homogeneousmedium[14],andFelicityeffectinaheterogeneous
applied[4–8].Hydraulicfracturinginducesfailureandfractureofrock medium[25].AEcountoftengoesthroughseveral“suddenincrease
mass,whichresultsinacousticemission(AE)[9–13].AE,whichhas andcalm”stagesduringloadingtestsofcoalandrock(heteroge-
beenasubjectofresearchonrockfailuresincethe1950s[14],isbasi- neousmedium)[22].Hydraulicfracturingofcoalandrockisame-
callyanelasticwave,whichissimilartomicroseismingeophysics.AE chanicalprocess.AEcountpresentsanobviousincreasewhencoal
eventsareassociatedwiththeinitiationandpropagationofhydraulic orrockisdestroyedbywaterinjection[12].Inaddition,inhighstress
cracks[15,16].Globally,scientistsandengineersaretryingtounder- regions,AEcountinducedbyfailureofcoalorrockmassismorethan
standtheintrinsicmechanismofrockmassfracturefromAEcharacter- thatinlowstressregions[15,26–28].Ishidaetal.[9]foundthatthe
istics,aswellastorecognizetheseAEcharacteristicsduringhydraulic AEcountincreaseddramaticallyandthecracksexpandedrapidly
fracturing,asrockswithdifferentphysicalproperties[17–20]orunder with the sudden drop in pressure after the injection of high-
differentstressconditions[21,22]willproduceAEwaveswithvarying pressurewaterintogranite.Lietal.[29]reportedthattheAEcount
characteristicsduringthedamageprocess.Furthermore,theAEcharac- duringthestageofbreakdownwasfarmorethanthatduringother
teristicsvaryduringdifferentstagesofhydraulicfracturing[15].There- stages. This phenomenon was also reported in other studies
fore, by studying AE characteristics, the failure law of hydraulic [15,30–32].AEwaveformcharacteristicsarestudiedtoidentifyfail-
fracturingcanbeindirectlyrevealed,withsufficientinvestigationof uremodes,suchastensionfailure,shearfailure,ortheircombina-
AEcharacteristicsinthelaboratoryexpectedtoimprovethemicroseis- tion. There are two methods to identify failure modes: moment
micmonitoringofhydraulicfracturingduringfieldapplications[23,24]. tensoranalysis[33]andanalyzingtherelationshipbetweenRAand
AF,whichisobtainedfromAEdata[34,35].AEfrequencyisanimpor-
tantparameterofAEsignal,whichcanindirectlyreflectthematerial
⁎ Correspondingauthorat:StateKeyLaboratoryofCoalMineDisasterDynamicsand
properties.Whilestudyingcompositefiberdamage,AEfrequency
Control,ChongqingUniversity,Chongqing400044,China.
E-mailaddress:[email protected](Q.Li). presents good zoning characteristics with different materials
https://doi.org/10.1016/j.powtec.2020.05.050
0032-5910/©2020ElsevierB.V.Allrightsreserved.
268 Z.Jiangetal./PowderTechnology371(2020)267–276
[36,37].AEfrequencydistributionisregularinrockuniaxialcom- ofthePCI-2workstation(PhysicalAcousticsCorporation,USA)and
pressionfailure[37–39].Forexample,thefrequencybandcanreflect RS54Bsensors(BeijingSoftlandTimesScientific&TechnologyCo.Ltd.,
thefailuretypeofAEsource.DifferentrockmaterialsemitAEsofdif- China).ParameterssetfortheAEacquisitionworkstationandsensors
ferentfrequencieswhentheyareloadedtofailure[22].Yaoetal.[40] arelistedinTables1and2.ThePDT,HDT,andHLTinTable1arethe
foundthattheenergyofhigh-frequencyAEissmallerthanthatof peakdefinitiontime,hitdefinitiontime,andhitlockouttime,respec-
low-frequencyAEwhenrockfailureoccursbyuniaxialcompression. tively.ThecorrectsettingofPDTwillensurecorrectidentificationof
Caietal.[41]foundthathigh-frequencyAEcorrespondstosmall- risetimeandpeakamplitudedetection.ThecorrectsettingofHDT
scalecracks,whereaslow-frequencyAEcorrespondstolarge-scale willensurethatanAEsignalinthedatastructureisreflectedinthesys-
cracksinthenumericalsimulationofundergroundcavernexcava- temastheoneandonlyonehit.ThecorrectsettingofHLTwillavoid
tion.Uniaxialcompressionexperimentshaveshownthatfrequency falsedetectionofsignalattenuationandimprovethespeedofdata
of AE signals following coal failure is lower than that of rock acquisition.
[15,17].Thesestudiesprovideaprimaryreferencefortheinvestiga- ThetriaxialstressenvironmentandplacementofAEsensorsareil-
tionofAEsignalfrequencyduringhydraulicfracturing. lustratedinFig.2.Thevalueoftheprincipalstressissetasσ =5.0
v
Still, AE characteristic and evolution are rarely investigated in MPa,σ =7.0MPa,andσ =4.5MPa,whichishalftheratioofdiggings
H h
stratifiedrockssuchascoalbedseams.Mechanismoffractureinitia- inthesouthwestofChina.Sensorswereevenlydistributedonthefour
tion and propagation are complex in stratified coalbed reservoirs surfacesofthespecimen.
withdifferentproperties.Itisdifficulttoperformhydraulicfracturing
inthesestratifiedrocks[42].Dingetal.[43]havestudiedhydraulic
fracturinginastratifiedformation,whichwasmadeofsimilarmate- 2.2.Specimenpreparation
rials.TheyalsoanalyzedtheAEcountwiththehydraulicfracturing
Theexperimentalmaterialsweregypsumpowder,Portlandcement,
pressure.However,theydidnotanalyzetheAEfrequencyandenergy,
andthecrackclassification,whicharecrucialtounderstandmecha- andcrushedstone(diameter≤8mm).Thematerialproportionswith
different uniaxial compressive strengths were obtained as demon-
nismsoffracturepropagation.Inthispaper,weaimtoobtainrelation-
stratedpreviously[44,45].Theuniaxialcompressivestrengthofrock
shipsbetweenmultipleAEcharacteristicsandinjectionpressureof
hydraulicfracturing.Wewillanalyzetheinfluenceofstratifiedforma- was14.34MPa,whereasthatofcoalwas1.98MPa.
Fourspecimens corresponding to different engineering condi-
tiononfracturingpressureandAE.Tothisend,thepiezoelectricce-
tionswereprepared(Fig.3a).Thefourspecimenswereallmade
ramicAEsensorsareusedtorecordtheAEsignalsgeneratedduring
with a 300 × 300 × 300-mm3 mold, and numbered as S1, S2, S3,
hydraulicfracturingoffourkindsofspecimensunderatruetriaxial
and S4 (Fig. 3 b). Among them, S1 simulates a single coal seam;
stressconditions.
S2simulatesasinglerockseam;andS3andS4arestratifiedsam-
pleswithtwo coalseamsandthreerockseams.Allinterfacesare
2.Materialsandmethod naturallyformedwithoutanyspecialtreatment.Allrockseamsin
S3andS4were50-mmthick.BothcoalseamsinS3hadathickness
2.1.Experimentalequipment of 75 mm. The upper coal seam in S4 was 50-mm thick,whereas
the lower was 100 mm. A 20-mm diameter borehole was drilled
Aschematicdiagramofthetruetriaxialhydraulicfracturingexperi- in themiddleof each specimen.The length of the boreholein S1
mentalsystemusedinthisworkispresentedinFig.1.Thesystem and S2 was 155 mm, whereas in S3 and S4 it was 255 mm. A
mainlyconsistsofatruetriaxialstressloadingsubsystem,anAEand metalpipewasinsertedintotheboreholeandawateroutletwas
pressuremonitoringsubsystem,andapumpsubsystem.Thetruetriax- arrangedatthebottomofthemetalpipe.Theexternalandinternal
ialsubsystemcouldsimulateinsitustressbyloadingindependentlyin diametersofthemetal pipewere15and8mm,respectively.The
threemutuallyperpendiculardirections,witheachofthemaximum externalwallofthemetalpipewasmaderoughtoincreasefriction
pressurebeingabletoreach70MPa.Themaximumpumpratewas withtheboreholewallandpreventslidingduringfracturing.Epoxy
100ml/min. resinwasusedtosealtheholetoensurethemetalpipeistightly
Aneight-channelhigh-frequencyAEinstrumentwasusedtocollect attached to the hole wall. The sealing lengths of S1 and S2 were
theAEsignalsduringhydraulicfracturing.TheAEinstrumentconsisted 145mm,andthoseofS3andS4were50mm.
Loading frame
AE
sensor
Water
pump
e
ol
h
HF fier
Oil pump Jack Specimen mpli
a
e
AE sensor r
P
Oil pump
AE collec(cid:2)ng instrument
Jack
Fig.1.Truetriaxialhydraulicfracturingandacousticemissionmonitoringsystem.Thereisthetwo-dimensiondiagram,threejacksareactuallyemployedfortruetriaxialstress.
Z.Jiangetal./PowderTechnology371(2020)267–276 269
Table1 Table3
ParametersofthePCI-2AEsystem. Sensorscoordinates.
Preamp Frequency Sampling PDT HDT HLT Signal Sensors x y z Sensors x y z
gain(dB) range(kHz) frequency (μs) (μs) (μs) threshold No. (mm) (mm) (mm) No. (mm) (mm) (mm)
(103s−1) (dB)
1 0 75 225 5 300 75 75
40 100–400 10,000 50 200 300 45 2 0 225 75 6 300 225 225
3 75 75 0 7 225 75 300
4 225 225 0 8 75 225 300
Table2
Parametersofthesensor. simulatingthestressconditionsexistinginthefieldspecifiedinFig.2.
Scale Working Hit Peaksensitivity Frequencyrange Enoughred-dyedwaterwaspreparedforinjectiontoinvestigatethe
(mm) temperature(°C) limit (10−6Pa) (kHz) cracks.
8×21.9 −20–130 10,000 −650 100–400 Finally,thewaterpumpandAEsystemwereturnedonsimulta-
neouslytoinjectwaterintotheboreholeandcollectAEsignals.The
pumpratewassetas100ml/min.Aftertheinjectedwaterflowedout
ofthespecimensurface,thewaterpumpwasclosed,markingthecom-
2.3.Experimentalprocedure pletionoftheAEsignalsacquisitionprogram.
Firstly,thepreparedspecimenwasputintotheloadingframe.AE 3.Resultsanddiscussion
sensorswereinstalledasillustratedinFig.2.Theceramiccontactsur-
faceoftheAEsensorwasinclosecontactwiththespecimensurface Eachspecimen wassuccessfullytreatedbyhydraulicfracturing,
throughVaseline(couplingagent).AfterconnectingthelineofAEin- whichresultedinobviouscracks.AsillustratedinFig.4,therewere
strument,AEparametersspecifiedinTable1andTable3wereset, markedtracksoffluid(red-dyedwater)flowingoutofthefractured
andthestabilityofAEsystemwastestedtoensuretheexperimentef- specimensandcracksinthesectionsacrossthespecimens.Thedirec-
fects.Meanwhile,thefracturingpipelinewasconnected,withitsgas tionsofthecracksvariedbetweenthefourspecimens.Thedirection
tightnesstestedinadvance. ofcrackscanbecategorizedaseitherhorizontalorvertical.Thehorizon-
Secondly,stresseswereindependentlyappliedtothespecimenin taltrendwasseeninS1andS4,whereastheverticaltrendwasseenin
threedirectionsusingflatjackswith0.01MPapersecondincreasing, S2andS3.Thesedifferencesmightbecausedbytheirregularityofthe
Fig.2.TriaxialstressconditionandAEsensorslayoutdiagram.
Fig.3.Specimensfortest.(a)specimenimage,(b)profilemapofinteriorstructureofspecimens,brownpresentsepoxyresin(sealinglength),redpresentsinjectionfluid,arrowspresents
interfacebetweeninjectionfluidandspecimen.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)
270 Z.Jiangetal./PowderTechnology371(2020)267–276
Fig.4.Crackformationafterhydraulicfracturing.
specimensurface,whichledtoasymmetricalloadingthatresultedin thespecimencube.Thespecimenbrokedownat475s,atwhichpoint
thedeviationofprincipalstressorientation.Inaddition,thesediffer- the injection pressure dropped again, beyond which it never rose
encesmightbecausedbytheloadstressdifferenceamongthethreeor- again. The breakdown pressure of S2 was 6.39 MPa, and pressure
thogonaldirection.Smallstressdifferenceinthistestsisapttoincrease remainedatabout5.71MPa(89%ofbreakdownpressure)intheend.
theuncertaintyofdirectionofcrack'spropagation.Somecrackbranches ThemaximumAEcumulativecountswere1963.Itisworthnoting
werealsonotedinthestratifiedspecimensS3andS4,indicatingthat thatthepressurehadfluctuatedforover385sfrom90sto475s,
morecracknetworksformedinthestratifiedspecimenthaninthe whichwasextremelylongerthanthatobservedinS1.Thislongerpe-
non-stratifiedspecimen. riodofpressurefluctuationsuggeststhatthecrackproducedpropa-
gated gradually rather than instantaneously. This result is also
3.1.AEcount supportedbytheoveralltrendofgradualincreaseintheAEcount.
In thetest involving S3,the breakdown pressurewas4.39MPa
DamageandfractureofrockmassinduceAEactivity.Inthewave- (Fig.5).Theinjectionpressuredecreasedlinearlyforabout35safter
formofAEsignal,thenumberoftimestheamplitudecrossesthethresh- themomentofbreakdown.Therateofdescentwithinthe35swasrel-
oldvalueiscalled“ringingcount”,whichisalsocalled“AEcount”.The ativelylessthanthoseofS1andS2,whichpresentedasharpdrop.These
moretheAEcount,themoreseverethedamageinsidetherockmass. resultsindicatethatalthoughsomecracksmighthaveformed,causing
InthetestinvolvingS1,theinjectionpressurereacheditsmaximum thefluidtoleak,thesewerestilltoosmalltoallowamassiveflow.Inad-
valueof9.32MPaat270sandthendroppedimmediately(Fig.5),indi- dition,followingbreakdown,AEactivitywasdetectedbyonlyC2and
catingthatthespecimenwasbrokendown.Attheprecisemomentof C6.At710s,alltheAEcountofallchannelsincreasedenormously(C5
breakdown,AEcountsofC1,C5,andC6rosesteeply,especiallyforC6 wasbroken),withaconsiderablefallinpressure.Whencameto710s,
whichincreasedfrom56to836;bycontrast,allremainingchannels injectionfluidof1183.3ml(equalto4.4%volumeofS3specimen)
respondedpoorly.Thisdifferencebetweenthechannelsmightbedue hadbeeninjectedintoS3specimen.Whiletheporosityofallspecimens
topriorloadingoftruetriaxialstress,whichcouldchangetheinterface waslessthan1%.Basedontheseresults,weinferredthatasnaplikely
connectingthespecimenwiththeAEsensor.AfterbreakingdownS1, occurredinthespecimen,whichwouldcausethefluidtoleakoutof
theinjectionpressuredroppedandthenroseagain;however,itcontin- the specimen. The injection pressure finally remained around 3.14
uedtofluctuateforover200s.Duringthepressurefluctuationperiod, MPauntilthepumpwasturnedoff.AmongallAEchannels,theevolu-
theAEcountofC1increasedsimultaneously,suggestingthatcrack tionofC6agreedwellwiththeinjectionpressurehistory.
wasproducedandcontinuedtopropagateandenlargethroughthe InthetestinvolvingS4,breakdowndidnotoccuruntil1276s(peak
specimencube.Meanwhile,theAEcountofC5andC6,whichwassen- pressure,8.04MPa;Fig.5).Beforebreakdown,perturbationofinjection
sitiveatthemomentofbreakdown,hardlyincreased,implyingthat pressurewasverydrasticandremainedthesameforabout236s.Dur-
thecontactofthesechannelswiththespecimenwasswitchedoffdue ing the period, the AE cumulative counts simultaneously grew the
tothedisplacementandmovementofS1duringbreakdown.Atthe fastest.Thisresultsuggestedthat236swasrequiredformacrocrackini-
endoftheinjectionprocessofS1,thepressureremainedatabout4.19 tiationandpenetrationthroughS4.Beforedrasticperturbationofinjec-
MPa (45% of breakdown pressure) and the AE cumulative counts tionpressure,anS-stylechangeinpressurewasnoted.Initially,thefluid
reached1255. wasinjectedtofillthevolumeoftheinjectionhole,whichcausedthe
InthetestinvolvingS2,bysynthesizinginjectionpressurecurveand pressuretogrowslowly.Oncetheinjectionholewasfull,thepressure
AEcumulativecountsevolution,S2wasfoundtohaveexperienceda grewapproximatelylinearity.Whenthepressuregrowthtrendshifted
relativelyintensefailureat348s(marketedinFig.5).Atthispoint, tononlinearity,fluidleakagebegan,andtheslopeofthepressurecurve
thepressurereached5.46MPaanddroppedimmediatelyto4.48MPa, decreased,indicatingmicrocrackswereformingbeforebreakdown,
andtheAEcountofC1andC4presentedanobviousincrease.However, andthesemicrocrackscouldcontinuouslyletfluiddiffusioninthe
thepressurecontinuedtoincreaseandbecamehigherbeyondthis specimen.
point;meanwhile,theAEactivityhadnotcalmeddown.Thissuggested SimilartoIshidaetal.[29,34],theAEcountsbeforeandaftera
thatalthoughacrackmighthaveformedinside,itdidnotyetpenetrate sharpdropinpressureduringhydraulicfracturingwerefarmore
Z.Jiangetal./PowderTechnology371(2020)267–276 271
Fig.5.InjectionpressurehistoryandAEcountaccumulationofallchannels.
than at other times in our experiments, especially in S1 and S3 theinjectionpressurehistorythroughoutexceptC1andC4.Thus,in
(Fig.5).Bycontrast,wefindthattheinjectionpressuresometimes thefollowingsections,datafromC1andC4weremainlyanalyzed.
experiences a long, slow fluctuation process (Fig. 5), and the AE
count increases rapidly. This is due to the fact that microcracks 3.2.AEenergy
havebeenproducedinthespecimenallthetime;however,they
didnotpenetratetheoutersideofthespecimen.Oncethecrack TheAEenergystatisticsofthefourtestaresummarizedinTable4,in
propagationbreaksthroughtheoutersurfaceofthespecimen,the whichS2hasthehighestenergyvalueofmean,sum,andmaximum,
pressurewilldecreasedramatically.Itisworthaffirmingthatthe whereasS1hasthelowestvalues,whichagreedwellwiththestrength
moreintensethepressurefluctuation,thegreaterthescaleandthe ofthespecimens.ThestrengthofS3andS4wascalculatedbyanequiv-
longerthelengthofthecracksformed,althoughthecracksdidnot alentapproach[46].Inaddition,bycomparingthefourspecimens,it
penetrate the specimen. Therefore, the faster the increase in AE wasfoundthattheAEenergywaslargerifrockstratumwasbroken.
count, the larger the crack size. The sudden rapid growth of AE Therefore,themagnitudeofAEenergyislikelytoindicatethetypeof
countsindicatesthatlarge-scalecracksarebeinggenerated. rupturedstratum.
Unexpectedly,theresponseofeachAEsensorisdifferentinthefour TheAEenergyofbrittlematerialsisknowntoobeythepower
tests.TheAEcumulativecountswerenotequalateachtimepoint;in law distribution [47]. To investigate the power law distribution
fact,theyvariedsignificantly.Besides,fewchannelsagreedwellwith index, we analyzed the energy data in double logarithmic
Table4
AEenergystatistics.
Specimen Strength Mean StandardDeviation Sum Minimum Median Maximum
(Mpa) (aJ) (aJ) (aJ) (aJ) (aJ) (aJ)
S1 1.98 690.61 1013.31 147,099.55 14.61 318.16 6937.00
S2 14.34 2120.58 18,084.43 897,007.20 6.59 420.66 367,434.00
S3 5.61 1017.20 1639.33 545,219.76 23.42 472.41 16,399.00
S4 2.78 935.02 1313.84 476,860.84 6.55 415.35 9017.00
272 Z.Jiangetal./PowderTechnology371(2020)267–276
Theprobabilityplot[48]isagraphicaltechniquetoassesswhether
ornotadatasetfollowsagivendistributionsuchasthelog-normal.
Thedataareplottedagainstatheoreticaldistributioninsuchaway
thatthepointsshouldformapproximatelyastraightline(cf.therefer-
encelineinFig.8).Departuresfromthereferencelineindicatedepar-
turesfromthespecifieddistribution.AsFig.8illustrates,thefirstfew
pointsandthelastfewpointsshowsomedeparturefromthereference
line(identifiedbelowthereferenceline).Nevertheless,thetrendofper-
centilecoincidedwiththereferencelineonthewhole,whichindicated
thattheAEenergywasinaccordancewiththelog-normaldistribution
inthefourspecimenstested.
3.3.AEfrequency
Thepeakfrequencyisthekeyparameterwhichissubsequentlyac-
quiredfromtheAEdetection,indicatingthevibrationfrequencyofma-
terial damage and fracture. The peak frequency is calculated
correspondingtothemaximumofFFT(fastFouriertransform)ofAE
amplitude.Sometimesthepeakfrequencyisalsocalledthemainfre-
quency, which refers to the main frequency of the AE sensor. The
mainfrequenciesofthesensorsusedinthisexperimentwere100–
Fig.6.Fitofpowerlawdistributionindex.
400 kHz; therefore, the AE peak frequency was filtered out to this
range(Fig.9).Toinvestigatetheevolutionofpeakfrequencyassociated
withtheinjectionpressure,peakfrequencies,pressures,andenergies
coordinates and fitted these with linearity as shown in Fig. 6. wereplotted(Fig.9).
Theselinesfittedwellwithenergyvalue,whichrangedfromhun- TheAEeventswithhigherenergywereconsideredtoidentifythe
dreds to thousands. The opposite number of slope of the fitting failurestagesofhydraulicfractureinthisexperiment.Thepeakfrequen-
line was equivalent to the power law distribution index. As ciesoftheseeventswithhigherenergywerecatalogedandclassified
shown in Fig. 6, the power law distribution index of non- intotreesbands(Fig.10).Whencombinedthesepeakfrequenciescor-
stratifiedspecimen(S1and S2) was biggerthan thatofstratified respondtothefailurestagesofhydraulicfracture.TheeventsinbandA
specimens(S3andS4).Thissuggestedthatthestratifiedspecimen werecorrespondtothestageofintensepressurefluctuation,eventsin
was more unstable than the non- stratified specimen. Further- bandBtothestageofpressurefluctuationafterthepeakpressure,
more,theslopeofS1 wasequaltothatofS2,which indicatesno andeventsinbandCtoallstages.
influenceofthestrengthonthepowerlawdistributionindex. Furthermore,asshowninFig.9,arelativelywiderangeofpeakfre-
Toinvestigatethedistributionofallenergydata,wecalculatedthe quencywasnotedbetween100and300Hzwhentheinjectionpressure
probabilityofenergyandplottedtheprobabilityhistogram(Fig.7). fluctuatedsharplyandthelow-frequencyeventsincreasedatthesame
Wefoundthatthedistributionobeyedlog-normaldistributionfitted time.Forthetimebeing,thisphenomenonofpeakfrequencyrange
curvesinFig.7. broadening will be termed “multifrequency response (MFR).” MFR
Fig.7.Energyprobabilitydistribution.Thethincurveabovebarsisthefittedstandardlog-normaldistributioncurve.
Z.Jiangetal./PowderTechnology371(2020)267–276 273
Fig.8.Log-normalprobabilityplotofenergy.Thereferencelinerepresentspercentofabsolutelog-normalprobability.Thecloserdatapoint(percent)istothereferenceline,themoreit
agreeswiththelog-normaldistribution,viceversa.
characteristicsareassociatedwiththesizeoffrequencyrange,AEnum- vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
bmeorr,eanthdeAAEEdeevnesnittysainndtimtheesscmalaell.eTrhtehelafrrgeeqrutehnecyfredqifufeernecnycerabnegtew,ethene StdevðlÞ¼uutPni¼−11(cid:3)li−li(cid:4)2 ð2Þ
them,themoresufficienttheMFR.TheMFRindex(MFRI)wasassumed i n−2
torepresentthevaluepositivelycorrelatedwiththedegreeofMFR.
ItisassumedthattherearenAEeventsint~t+Δtduringhydraulic whereStdev(li)denotesthestandarddeviationofli.Thesmallerthe
fracturing,andtheirfrequenciesaref (i=1,2,3,…,n).Thefrequency Stdev(li),themoreevenlytheAEeventsaredistributedinLf,andthe
i largertheL,themoresufficientthedegreeofMFR.
bandwidthofMFRisthedifferencebetweenmaxf andminf int~t+ f
Δt.Therefore,thatthefrequencybandwidthcanbei writtenais Therefore,theMFRIcanbeexpressedas
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
Lf ¼ThmeafrxeðqfuiÞe−ncmyidniðfffeiÞrencebetweenadjacenteventsisl =f −ð1fÞ, MFRIðtÞ¼uutðn−2Þ½Pmni¼a−1x1ð(cid:3)fliiÞ−−lim(cid:4)2inðfiÞ(cid:2)2 ð3Þ
i i+1 i
(i=1,2,3,…,n−1),andthestandarddeviationofl is
i
Fig.9.AEpeakfrequencycharacteristicsduringthehydraulicfracturingprocess.
274 Z.Jiangetal./PowderTechnology371(2020)267–276
Fig.10.AEpeakfrequencyrangedeviationaccordingtoAEeventdata.
Usingformula(3),MFRIateachtimefromttot+Δtcanbecalcu- inferthatmacrofracturewasproducedonceinS1,thriceinS2,and
lated.Mathematically,ittakesΔt→0togettheMFRIoftimet.AsΔt twiceinS3andS4.Therefore,theMFRImethodcanbeanefficient
isassociatedwiththeAEeventrate,bysettingt=1s,theresults approachtoidentifytheoccurrenceofmacrofracture.
werecalculated(Fig.11).
FromanMFRIviewpoint(Fig.11),itisclearthatarelationship 3.4.Crackclassification
existsbetweenAEpeakfrequencyandinjectionpressureevolution.
MFRIisprominentwithadropininjectionpressure.Withtheap- Crackmechanismisanimportantfeatureofmaterialfailure.InAE
pearance of large MFRI, AE activities increase extensively and testing,theAF–RAmethodiswidelyusedtofastclassifycrackmodelin-
macrofracturesareformed.Thisresultisconsistentwiththeprinci- cludingtensilecrackandshearcrack[50],whereAFisAEcountdivided
pleofvibrationmechanics[49],becausetheformationofcracksis byduration,andRAisrisetimedividedbyamplitude.Thus,itisimpor-
the result of the convergence and penetration of multiple tantthatthethresholdsetisaccurateandappropriatetoobtainthetrue
microcracks.Beforethemicrocrackspenetratedthespecimen,the risetimeandduration.IgnoringthedeviationfromAEsystem,wecould
rock mass yielded locally in many places, forming microvoids. findthattensilecrackcomprisedover94%ofthecracksinalltests,with
Priortothepenetrationofvoids,therockmasswasrelativelystable, theremainingbeingshearcracks(Fig.12).Thedominationoftensile
thestrengthwasgoodandconsistenteverywhere,andthepeakfre- cracksdemonstratesthatmostfactureswhichcontributedtoinjection
quencyofAEwasrelativelysingle.However,whenthevoidsstartto pressureactedasthetensilestressinsidethespecimens.Thisresult
penetrate,cracksstarttoform,andnumerousmicrovoidwallsare was well in accordance with the tensile theory [51] of hydraulic
generatedinstantaneously.Therockmassdeformationincreased fracture.
subsequently,whichcausedrockmassinstabilityanddeteriorated Unfortunately,theresultofAElocationinthisfourtestswasnotvery
itsstrength.Therefore,theAEfrequencybecameunstable,low- encouraging.AsshowninFig.13,whatlocationacquiredintheprofile
frequency events occurred, and MFRI increased. Thus, we could depart invariably from the crack marked red induced by hydraulic
Fig.11.MFRIduringhydraulicfracturing.
Z.Jiangetal./PowderTechnology371(2020)267–276 275
Fig.12.CrackclassificationbytheAF–RAmethod.
summarizedasfollows:1)low-frequencyAEeventsincreasedandthe
phenomenon of MFR occurred when the pressure curve fluctuated
sharply. The MFRI was positively correlated with the formation of
macrocracksinhydraulicfracturing.2)asuddenrapidgrowthofAE
countsindicatedthegenerationoflarge-scalecracks.TheAEcountin-
creasedsharplywhenthepressurecurvefluctuatedsharply;bycon-
trast, the AE count increased steadily when the pressure curve
fluctuatedslightly.3)AEenergywasmoresensitivetoformationcondi-
tions. AE energy tend to be larger if the rock stratum was broken.
Concerningthemagnitudes,AEenergywasinaccordancewithlog-
normaldistributioninthisfour-sampletest.TheAEenergyvaluevaried
fromhundredstothousandsandfittedwellwithpowerlawdistribu-
tion;thepowerlawdistributionindexofnon-stratifiedspecimenwas
biggerthanthatofstratifiedspecimensduringhydraulicfracturing,sug-
gestingthatthestratifiedspecimenwasmoreunstablethanthenon-
stratified specimen. 4) The tensile crack, classified by the AF–RA
method,dominatedinalltests,indicatingthatmostfactureswhichcon-
tributed to injection pressure acted as the tensile stress inside
specimens.
Overall,hydraulicfracturingofcoalbedmethaneisacomplexpro-
cess,someencouragedperspectivesweresummarizedwithafewtypi-
Fig.13.AElocation,withS2asanexample.Tofitwiththecrack,theeventslocatedinthe
calexperimentsprovidedinthispaper.Theonlydrawbackisthatnotall
crosssectionwereextracted.
AEsensorscouldcompletelyrecordthecriticalsignalsoffracturing,and
theAElocationhaderrorsduetoheterogeneityofspecimens.Toobtain
moreinformationaboutAEsignalcharacteristicsduringhydraulicfrac-
fracturingtreatment.Thisinconsistencymaybeduetotheanisotropyof turingincoalseamgasreservoirs,moreexperimentalstudiesshouldbe
thespecimens[52],namely,AEvelocitywasnotaconstantineverydi- carriedoutusingareliablelocationmethod.
rectionandalsoineverymomentoftheinjectionperiod.Furthermore,
first-arrivaldetections,whichrequiremanualcorrection,wereallde-
pendentonautomationoftheAEsystem,resultingininevitablelocation DeclarationofCompetingInterest
errors.Thisdiscrepancypresentedsomechallengesforevaluationofhy-
draulicfracturing.Therefore,itisnecessarytocomeupwithanewand Theauthorsdeclarethattheyhavenoknowncompetingfinancial
reliablelocationmethod. interestsorpersonalrelationshipsthatcouldhaveappearedtoinflu-
encetheworkreportedinthispaper.
4.Conclusion
Acknowledgement
Hydraulicfracturingunderfourcoalseamconditionswassimulated
inatruetriaxialstressenvironment,andtheirAEdatawerecollected, ThispaperwassupportedbytheNationalMajorScienceandTech-
analyzed, and discussed in depth. Some meaningful insights are nology Projects of China (No. 2016ZX05045004, No.
276 Z.Jiangetal./PowderTechnology371(2020)267–276
2016ZX05043005),theNationalNaturalScienceFoundationofChina [24] Z.Z.Jiang,Q.G.Li,Q.T.Hu,J.F.Chen,X.L.Li,X.G.Wang,etal.,Undergroundmicroseis-
micmonitoringofahydraulicfracturingoperationforCBMreservoirsinacoal
(GrantNo.51604045),theStateKeyLaboratoryCultivationBasefor mine,EnergySci.Eng.7(3)(2019)986–999.
GasGeologyandGasControl(HenanPolytechnicUniversity),Natural [25] M.Fuwa,A.R.Bunsell,B.Harris,AcousticEmissionandFatigueofReinforcedPlastics,
ScienceFoundationofChongqing,China(cstc2019jcyj-bsh0041).We 197477–79.
[26] M.Shimada,A.Cho,H.Yukutake,Fracturestrengthofdrysilicaterocksathighcon-
thankSWANEditorialServices(www.swaneditorial.com)forediting
finingpressuresandactivityofacoustic-emission,Tectonophysics96(1–2)(1983)
thispaper. 159–172.
[27] E.C.Donaldson,W.Alam,N.Begum,Rockmechanicsoffracturing-hydraulicfrac-
References turingexplained-Chapter3,HydraulicFracturingExplained2013,pp.47–76.
[28] D.Tiab,E.C.Donaldson,Chapter9-effectofstressonreservoirrockproperties,in:D.
[1] hGi.gDhulyvestarua,tiJfi.Fe.dShroacok,sA,Imnto.dJ.ifiReodcksiMngelcehp.Mlanine.o3f5w(6e)ak(n19es9s8)th8e0o7r–y8f1o3r.thefailureof Tiniagb,,BEu.rCli.nDgotonnal2d0so0n4,(pEpd.s5.)5,4P–e6tr7o0p.hysics,SecondeditionGulfProfessionalPublish-
[29] N.Li,S.C.Zhang,Y.S.Zou,X.F.Ma,S.Wu,Y.N.Zhang,Experimentalanalysisofhy-
[2] P.Wang,M.Cai,F.Ren,C.Li,T.Yang,Theoreticalinvestigationofdeformationchar-
draulicfracturegrowthandacousticemissionresponseinalayeredformation,
aGcetoesrciis.tJi.c2s1of(2s)tr(a2t0ifi1e7d)2ro1c3k–s22co2n.sideringgeometricandmechanicalvariability, RockMech.Rock.Eng.51(4)(2018)1047–1062.
[30] S.Stanchits,A.Surdi,P.Gathogo,E.Edelman,R.Suarez-Rivera,Onsetofhydraulic
[3] N.A.Do,D.Dias,V.D.Dinh,T.T.Tran,V.C.Dao,V.D.Dao,etal.,Behaviorofnoncircular
fractureinitiationmonitoredbyacousticemissionandvolumetricdeformation
tunnelsexcavatedinstratifiedrockmasses–caseofundergroundcoalmines,J. measurements,RockMech.Rock.Eng.47(5)(2014)1521–1532.
RockMech.Geophys.Eng.11(1)(2019)99–110.
[31] G.Kwiatek,K.Plenkers,P.Martinez-Garzon,M.Leonhardt,A.Zang,G.Dresen,New
[4] Y.Tan,Z.Pan,X.-T.Feng,D.Zhang,L.D.Connell,S.Li,Laboratorycharacterisationof
insightsintofractureprocessthroughin-situacousticemissionmonitoringduring
fracturecompressibilityforcoalandshalegasreservoirrocks:areview,Int.J.Coal
FatiguehydraulicfractureexperimentinAspohardrocklaboratory,Process.Eng.
Geol.204(2019)1–17. 191(2017)618–622.
[5] A.Rezaei,B.Dindoruk,M.Y.Soliman,Onparametersaffectingthepropagationofhy-
[32] X.F.Ma,N.Li,C.B.Yin,Y.C.Li,Y.S.Zou,S.Wu,etal.,Hydraulicfracturepropagation
draulicfracturesfrominfillwells,J.Pet.Sci.Eng.182(2019)106255.
geometryandacousticemissioninterpretation:AcasestudyofSilurianLongmaxi
[6] J. Cai, Z. Zhang, W. Wei, D. Guo, S. Li, P. Zhao, The critical factors for FormationshaleinSichuanBasin,SWChina,Pet.Explor.Dev.44(6)(2017)
permeability-formationfactorrelationinreservoirrocks:pore-throatratio,tortuos- 1030–1037.
ityandconnectivity,Energy188(2019)116051. [33] K.Ohno,M.Ohtsu,Crackclassificationinconcretebasedonacousticemission,
[7] W.Yan,H.Ge,J.Wang,D.Wang,F.Meng,J.Chen,etal.,Experimentalstudyofthe Constr.Build.Mater.24(12)(2010)2339–2346.
frictionpropertiesandcompressiveshearfailurebehaviorsofgasshaleunderthe [34] T.Ishida,Acousticemissionmonitoringofhydraulicfracturinginlaboratoryand
influenceoffluids,J.Nat.GasSci.Eng.33(2016)153–161. field,Constr.Build.Mater.15(5–6)(2001)283–295.
[8] S.Sharma,V.Agrawal,R.N.Akondi,Roleofbiogeochemistryinefficientshaleoiland [35] D.G.Aggelis,Classificationofcrackingmodeinconcretebyacousticemission
gasproduction,Fuel259(2020)15. parameters,Mech.Res.Commun.38(3)(2011)153–157.
[9] T.Ishida,Q.Chen,Y.Mizuta,Effectofinjectedwateronhydraulicfracturingdeduced [36] Q.Q.Ni,M.Iwamoto,Wavelettransformofacousticemissionsignalsinfailureof
fromacousticemissionmonitoring,PureAppl.Geophys.150(3–4)(1997)627–646. modelcomposites,Eng.Fract.Mech.69(6)(2002)717–728.
[10] S.Kramadibrata,M.A.Rai,B.Sulistianto,R.K.Wattimena,P.N.Hartami,K.Matsui,De- [37] R.Gutkin,C.J.Green,S.Vangrattanachai,S.T.Pinho,P.Robinson,P.T.Curtis,On
terminationofthreedimensionalinsitustressregimeusinghydraulicfracturing acousticemissionforfailureinvestigationinCFRP:patternrecognitionandpeakfre-
andacousticemissionmethods,Contrib.RockMech.NewCent.1and2(2004) quencyanalyses,Mech.Syst.Signal.Pr.25(4)(2011)1393–1407.
1147–1152. [38] P.J.Groot,P.A.M.Wijnen,R.B.F.Janssen,Real-timefrequencydeterminationof
[11] Q.G.Li,B.Q.Lin,C.Zhai,Theeffectofpulsefrequencyonthefractureextensiondur- acousticemissionfordifferentfracturemechanismsincarbon/epoxycomposites,
inghydraulicfracturing,J.Nat.GasSci.Eng.21(2014)296–303. Compos.Sci.Technol.55(4)(1995)405–412.
[12] Y.P.Liang,Y.H.Cheng,Q.L.Zou,W.D.Wang,Y.K.Ma,Q.G.Li,Responsecharacteristics [39] C.R.Ramirez-Jimenez,N.Papadakis,N.Reynolds,T.H.Gan,P.Purnell,M.Pharaoh,
ofcoalsubjectedtohydraulicfracturing:anevaluationbasedonreal-timemonitor- Identificationoffailuremodesinglass/polypropylenecompositesbymeansofthe
ingofboreholestrainandacousticemission,J.Nat.GasSci.Eng.38(2017)402–411. primaryfrequencycontentoftheacousticemissionevent,Compos.Sci.Technol.
[13] P.G.Ranjith,W.A.M.Wanniarachchi,M.S.A.Perera,T.D.Rathnaweera,Investigation 64(12)(2004)1819–1827.
oftheeffectoffoamflowrateonfoam-basedhydraulicfracturingofshalereservoir [40] X.-L.Yao,Y.-B.Zhang,X.-X.Liu,P.Liang,L.Sun,Optimizationmethodforkeychar-
rockswithnaturalfractures:anexperimentalstudy,J.Pet.Sci.Eng.169(2018) acteristicsignalofacousticemissioninrockfracture,RockSoilMech.39(1)(2018)
518–531. 375–384.
[14] J.Kaiser,AStudyofAcousticPhenomenainTensileTest,PhDThesisTechnicalUni- [41] M.Cai,P.K.Kaiser,H.Morioka,M.Minami,T.Maejima,Y.Tasaka,etal.,FLAC/PFC
versity,1950. couplednumericalsimulationofAEinlarge-scaleundergroundexcavations,Int.J.
[15] S.Stanchits,J.Burghardt,A.Surdi,Hydraulicfracturingofheterogeneousrockmon- RockMech.Min.44(4)(2007)550–564.
itoredbyacousticemission,RockMech.Rock.Eng.48(6)(2015)2513–2527. [42] T.T.Jiang,J.H.Zhang,G.Huang,S.X.Song,H.Wu,Effectsofbeddingonhydraulic
[16] J.N.Albright,C.F.Pearson,Acousticemissionsasatoolforhydraulicfracturelocation fracturingincoalbedmethanereservoirs,Curr.Sci.India113(6)(2017)1153–1159.
-experienceattheFentonHillhotdryrocksite,Soc.Petrol.Eng.J.22(4)(1982) [43] D.Hong,LawofHydraulicFracturingCrackInitiationintheMulti-LayeredCoal
523–530. Seams,MasterChongqingUniversity,2014.
[17] M.Seto,T.Kiyama,T.Narita,M.Kouno,K.Shiota,H.Nabeya,etal.,Acousticemission [44] Q.T.Hu,S.T.Zhang,G.C.Wen,L.C.Dai,B.Wang,Coal-likematerialforcoalandgas
inhydraulicfracturingofcoalmeasurerock,J.MMIJ105(9)(1989)661–666. outburstsimulationtests,Int.J.RockMech.Min.74(2015)151–156.
[18] V.Arumugam,R.N.Shankar,B.T.N.Sridhar,A.J.Stanley,Ultimatestrengthprediction [45] Y.H.Guo,R.J.Cao,L.H.Zhu,Researchonsimilarmaterialinphysicalspecimenpe-
ofcarbon/epoxytensilespecimensfromacousticemissiondata,J.Mater.Sci. trographyofrock,Adv.Mater.Res.616–618(2013)346–349.
Technol.26(8)(2010)725–729. [46] G.Yin,X.Li,J.Lu,Z.Song,Afailurecriterionforlayeredcompositerockundertrue
[19] J.W.Liu,X.Z.Wu,X.X.Liu,Y.Y.Yu,W.Hu,L.B.Yin,Time-frequencycharacteristicand triaxialstressconditions,Chin.J.RockMech.Eng.36(2)(2017)261–269.
signalrecognitionofacousticemissiongeneratedfromdifferentrockbrittlefailure, [47] B.Jordi,C.Álvaro,I.Xavier,P.Antoni,SaljeEKH,S.Wilfried,etal.,Statisticalsimilar-
Nonferr.MetalsSci.Eng.4(6)(2013)73–77. itybetweenthecompressionofaporousmaterialandearthquakes,Phys.Rev.Lett.
[20] X.Jingna,N.Guanhua,X.Hongchao,L.Shang,S.Qian,D.Kai,Theeffectofaddingsur- 110(8)(2013)(088702).
factanttothetreatingacidonthechemicalpropertiesofanacid-treatedcoal,Pow- [48] J.M.Chambers,W.S.Cleveland,B.Kleiner,P.A.Tukey,GraphicalMethodsforData
derTechnol.356(2019)263–272. Analysis,Wadsworth,1983.
[21] S.G.Cao,Y.B.Liu,L.I.Yong,L.Q.Zhang,Experimentalstudyonacousticemission [49] W.T.Thomson,TheoryofVibrationwithApplications,Prentice-Hall,1981.
characteristicsofcoalrockatdifferentconfiningpressure,J.Chong.Univ.32(11) [50] R.K.Miller,P.Mcintire,AcousticEmissionTesting,2008.
(2009)1321–1327. [51] Y.Aimene,C.Hammerquist,A.Ouenes,Anisotropicdamagemechanicsforasym-
[22] X.Wang,E.Wang,X.Liu,X.Li,H.Wang,D.Li,Macro-crackpropagationprocessand metrichydraulicfractureheightpropagationinalayeredunconventionalgasreser-
correspondingAEbehaviorsoffracturedsandstoneunderdifferentloadingrates, voir,J.Nat.GasSci.Eng.67(2019)1–13.
Chin.J.RockMech.Eng.37(6)(2018)1446–1458. [52] L.Qin,S.Li,C.Zhai,H.Lin,P.Zhao,M.Yan,etal.,Jointanalysisofporesinlow,inter-
[23] X.Li,L.Zhonghui,W.Enyuan,L.Yunpei,L.Baolin,C.Peng,etal.,Patternrecognition mediate,andhighrankcoalsusingmercuryintrusion,nitrogenadsorption,andnu-
ofminemicroseismic(MS)andblastingeventsbasedonwavefractalfeatures,Frac- clearmagneticresonance,PowderTechnol.362(2020)615–627.
tals26(3)(2018)(1850029-1–18).