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PetroleumScience2004Vol.1No.4ATheoreticalStudyofRockDrillingwithaHighPressureWaterJet*NiHongjianandWangRuihe(CollegeofPetroleumEngineering,UniversityofPetroleum,DongYing,ShandongProvince257061,China)ReceivedMarch22,2004Abstract:Basedonanalysesofexperimentalresultsofwaterjetdrilling,thefluidmotionlawinrockporesandthetendencyofenergydistribution,therock-breakingprocessunderhighpressurewaterjetdrillinghasbeenstudiedsystematically.Theresearchindicatesthatthemaininteractionbetweentherockandwaterjetisinterfacecoupling,thattheimpactingloadandthestaticpressureofthewaterjetacttogethertomaketherockbreak,andthatthestresswaveisthemainfactor.Waterjetdrillingcanbedividedintotwostages:Attheinitialstage,thestresswaveplaysthemainroleandmostoftherockbreakingtakesplace;atthelaterstage,theexistingrockdefects,forinstance,micro-holesandmicro-cracks,arepropagatedandmergedtomakemacroscopicdamage,andthenthediameterofthejet-drilledholeisexpanded.Keywords:Waterjet,rock,fluid-wallinteraction,drilling*Thisproject(50234030)wasgrantedfinancialsupportfromNationalNatureScienceFoundationofChina.1.IntroductionItisanimportantapplicationofthejettechnologytobreakrockdirectlywithahigh-pressurewaterjet,whichprofoundlyinfluencesthedevelopmentoffieldslikepetroleumengineering,miningengineeringandconstructionengineering(Shen,1998).Intheoreticalresearchofthewaterjetdrilling,anessentialproblemistoexplorethemechanismandprocessofthewaterjetdrilling.Varioustheoriesabouttherockbreakingduringthewaterjetdrilling,forinstance,thestretch-waterwedgedrillingtheoryandthecompactedcore-splitdrillingtheory(Xu,1984),havebeenpresenteduptonow.However,mostofthesetheoriesremainatthehypothesislevel.Theywereconfinedonlytodescribingtherockbreakingphenomenonanddidnottouchtheessenceoftherockbreaking,soitishardforthemtoguidetheapplicationofthewaterjetdrillingandtheexplorationofthephysicalmechanismofrockbreaking.Toimprovethestatusoftheoreticalresearchandpracticalapplication,basedontheanalysisofthecouplingbetweenrockandwaterjet,andthejetpropertiesduringthewaterjetdrilling,theauthorsattempttoestablishabasictheoryandaresearchmethodtoanalyzethewholeprocessofrockbreaking.2.AnalysisofthecouplingbetweenwaterjetandrockTheessenceofthewaterjetdrillingistheinteractionbetweenwaterjetandrock,andtheprocessofrockbreaking.Thefluidvelocitydecreasesveryquicklyafterthewaterjetcontactstherockatahighspeed,andthepartialjetenergytransmitsintherockwithstresswaves.So,thereisapressuredifferencebetweentheporefluidintherockandthefluidontherocksurface,underwhichthefluidseepstotherock.Therearetwokindsofcouplingbetweentherockandthefluidduringthewaterjetdrilling.Thefirstisthecouplingbetweentherockskeletonandtheporefluid,inwhichthesolidandthefluidregionspartiallyorcompletelyoverlaptogether,anditishardtodistinguishthemfromeachother.Thesecondisthecouplingontheinterfacebetweenthesolidandthefluid.Rockdamageorbreakagecloselyrelatestothesetwokindsofcoupling,andmostoftheexistingtheoriesaboutrockbreakingwerealsoestablishedonthecouplingbetweenwaterandrock.But,whichofthesetwokindsofcouplingisprimaryisindisputebyfar.Mostoftheexistingtheoriesholdtheopinionthatthefirstkindofcouplingplaysthemainroleduringthewaterjetdrilling.Rockisatypicallyporousmedium,whichhasplentyofdefects,suchasmicro-holesandmicro-cracks.Somecurrentresearchalsoshowsthatthesedefectsobviouslyaffecttherockbreaking.Thus,consideringtheporefluidflowintherock,somepeoplebelievethattherock-breakingmechanismunderahighpressurewaterjetistheinteractionbetweentheporefluidandtheinitialdefects,makingthesedefectspropagategradually,andtheexpansionresultsintherockdamageandbreakage.Basedonthisopinion,somerockbreakingtheoriesandtheoreticalmodelswerepresented.Thesearebasicallyhalf-qualitativeorhalf-quantitative,suchasthestretch-waterwedgeATheoreticalStudyofRockDrillingwithaHighPressureWaterJetVol.1No.473drillingtheoryandthecuttingrockmodel(Crow,1973;Rehbinder,1976,1978).Butthepracticalapplicationshowsthatthiskindoftheoryagreespoorlywiththereality.Wemakeanassumptionasfollows:Thefluidisisotropic;thevolumeoftherockislarge；theporosityandtheseepagerateareconstant;andboththerockandthefluidareinelastic.Therefore,thefluidflowingprocessintherockduringthewaterjetdrillingcanbeapproximatelyshowninFig.1.Fig.1FluidflowingmodelinrockunderawaterjetWhereRwistheradiusofthejet-drilledhole,Reistheradiusoftherock,wpisthestaticpressureofajet,andepisthesupplypressureontherockedge.Thesteadyseepageflowingintherockcanbedescribedbyequationsasfollows(Zhang,etal.,1998):(1)(2)φπ332rQt=(3)whereKistheseepagerateofrock,µistheviscosityoffluid,vistheseepageflowvelocity,ristheseepageflowradius,Qistheflux,φistheporosityofrock,andtisthetime.Boundaryconditions:r=Rw，p=pw，r=Re，p=pe。Theviscosityofwatertakes1mP·s,andweassumeRw=0.15cm，Re=50cm，pe=0.Kandφtakesthevalueintermsofthereferences(Xiong,etal.,1999).Thecalculatedresults(showninFig.2)showthatwiththeincreaseintime,theflowingdistanceoftheporefluidincreasespromptlyatfirst,andtherateofincreasedecreasesgradually.Atanytime,thehighertherockporosity,thelongeristhedistance