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IMPROVING FRACTURE INITIATION AND POTENTIAL IMPACT 

ON FRACTURE COVERAGE BY IMPLEMENTING OPTIMAL WELL 

PLANNING AND DRILLING METHODS FOR TYPICAL STRESS 

CONDITIONS IN THE COOPER BASIN, CENTRAL AUSTRALIA  Dr. Raymond L. Johnson, Jr Unconventional Reservoir Solutions 168 Ninth Ave, St Lucia QLD rayj@unconreservoirs.com.au  Dr. Hani Farouq Abul Khair  Australian School of Petroleum, University of Adelaide Santos Petroleum Engineering Building, University of Adelaide, Adelaide SA hani.abulkhair@adelaide.edu.au  Dr. Rob Jeffrey CSIRO Energy, CSIRO Bayview Avenue, Clayton, VIC Rob.Jeffrey@csiro.au  Dr. Jeremy Meyer Ikon Science Level 2, 60 Hindmarsh Square, Adelaide SA jmeyer@ikonscience.com  Carly Stark Petro‐king Australia Pty Ltd Level 2, 333 King William Street, Adelaide SA carly.stark@petro‐king.com.au  James Tauchnitz Petro‐king Australia Pty Ltd Level 2, 333 King William Street, Adelaide SA james.tauchnitz@petro‐king.com.au 

KEYWORDS 

CooperBasin,geomechanics,geomechanicalmodelling,hydraulicfracturing, fractureinitiation, fractureoptimization,insitustress,drilling,wellplanning

ABSTRACT 

DrillingconditionsinvolvinghighmeananddeviatorystressesandnaturalfracturesintheCooperBasinposedifficulties in drilling and introducewellbore rugosities, leaving a damagedwellbore subject to astresscageeffect. Fracture initiationshavebeenproblematic inverticalCooperBasinwells,exhibitinghigh initiation and treating pressure frac treatments, and high stress conditions pose greater risks innon‐vertical completions. Whilst far‐field fracture complexity should simplify, the near wellborecomplexity results in reduced fracture conductivity. We believe that current drilling practices andwellboreazimuthsmaybecontributingtosub‐optimalhydraulicfractureinitiationsandcomplexities.Current analyticalmodellingmethodologies can derive initiation pressures for circular wellbores, butrequiremore complex numericalmodels to include flaws and ellipticity to represent natural fractures

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and wellbore rugosities. This study compares initiation pressures and presents graphical resultscomparing circular and elliptical wellbore cases with flaws.Wewill outline the criteria used in thesemodelsandremarkonareasforfurtherresearchandmodeldevelopment.Finally,weproposeimproveddrillingtechniquestoachievemorestable,smootherwellbores,potentiallyreducingsomerugosityanddrillinginducedfractures.Then,usingdatafromrecentresearchandothercaseswith complex stress environments, it is proposed that initiation pressuresmight be reduced byincliningwellsforhydraulicfracturingtreatmentsinafavourablealignmenttothemaximumhorizontalstressdirection(H‐Max)andimplementingcompletiontechniquesthataidbetterfractureinitiation. 

EXTENDED ABSTRACT 

Introduction 

The stress and rock strength environment in the CooperBasin typically results in significantwellborebreakout.Forexample,itisatypicalpracticetorundualdensitytoolsoffsetby90degreeswithonetoolgetting stuck in the breakout while the other tool measures undamaged formation. These conditionsoften result ina rugosenon‐circularwellbores impacting thequalityof the cement jobandadamagedwellboresubjecttoastresscageeffect,therebyaffectingfractureinitiationconditions.Asaresult,fractureinitiationshavebeenproblematicinverticalwellsandposeanevengreaterriskfordeviated to horizontal completions. Whilst the fracture may become less complex in the far‐field, amajorityofCooperBasincompletionsexhibithighinitiation,hightreatingpressures,andnearwellbore(NWB)pressurelosses.Solutionstotheseproblemshavebeenproposedbymanyauthors.TheknockoneffectsoftheirapplicationsareoftennotsolvingtheissueofcomplexgeometriesandpotentiallylowerNWB fracture widths, reducing fracture effectiveness in connecting to the main fracture and to anystimulatedreservoirvolumes.Typically,analyticalmethods topredict fracture initiationassumeacircular,undamagedwellbore. Wewillprovide someanalytical andnumericalmodellingused toexplore the likelygeometries thatmightresult from damage to the stress cage from drilling induced tensile (DITF) and drilling induced petalfractures(DIPF),naturalfractureleakoff,andwellborebreakout.Next,wewilldiscussdrillingstrategiesthat create these effects and propose improved techniques to achievemore stable, smootherwellboreconditions. These would provide a less damaged stress cage from which trial completion strategiesmightbeappliedtoreducefracturecomplexity. Toidentifyareasforcompletionimprovement,wewillpresent data from recent research and results from other complex stress environments, which whencombinedwith other successfully implemented CooperBasin completion,might improve futuremulti‐stagehydraulicfracturingtreatments.Finally, whilst any journey starts with a single step, it is unlikely that any onemodelling, drilling, orcompletion strategywill individually unlock the problematic areas of the Cooper Basin. It is likely torequire the application of improvements in all areas in tandem to generate this solution. In the finalsection,wewillexploreareasforfutureresearchintheareasofgeomechanicalmodelling,drilling,andcompletionsthatmustbepursuedtoachievethisgoal. 

Analytical and Numerical Modelling Results 

As fracture initiation is a perceived problem, finite elementmodels and analyticmethods can provideinsightintofractureinitiationpressuresandinitialtrajectorieswherenaturalfracturesorwellboreflawsmaybeinfluencingtheinitiation.Tounderstandtheseeffects,weillustrateseparatewellborecaseswithandwithoutnaturalfracturesorflawsatvariousanglestothemaximumprinciplestress(i.e.,maximumhorizontalstress,H‐Max)basedonastrike‐slipstressstatecaserepresentingtypicalCooperBasinstressconditions. The data set used for the modelling (Table 1) is based on typical strike‐slip conditionsindicativeof theCooperBasin, andrepresentsvaluesexpectednear2500mconsistentwith publishedstressmodelsoftheCooperBasin(Reynoldsetal.,2006).

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Table 1. Input data for geomechanical and hydraulic fracture modelling 

Data ValueMaximum horizontal stress (H‐Max, MPa)  120 

Minimum horizontal stress (h‐min, MPa)  50 

Vertical Stress(vert, MPa)  65 

Young’s Modulus (E, GPa)  40 

Poisson’s Ratio ( dim)  0.25 

Maximum principal stress at damage initiation (MPa)  8 

Wellbore pressure applied to initiate (MPa)  35 

Wellbore diameter (inch)  8.5 

Wellbore depth  (m)  2500 

Estimated Reservoir Pressure  (MPa)  25 

AnalyticalmodelscanbeusedtoderiveDITFandfractureinitiationpressuresbasedonsolutionsderivedfromtheKirschequations(BrudyandZoback,1999)andoftenpredictdrillinginducedtensilefailureatlow pressure conditions, consistent with Cooper Basin image logs. However, observed fractureinitiationsaremuchhigherthananticipatedusingthesesameanalyticalmodels,requiringaneedtofindalternative,numericalmodelstoexplainthesebehaviours.To numerically simulate the fracture propagation from open‐hole circular and ellipticalwellbores, wehave presented results using an Extended Finite Element Method (XFEM) to model initiation andpropagation of fractures. This is a numerical technique introduced to model cracks independent ofmeshes(BelytschkoandBlack,1999); itallowssimulationofdiscretecrackinitiationsandpropagationalong arbitrary, solution‐dependent paths without re‐meshing requirements. Themethodology can besummarizedbytheequation(1):

∑ ∑ ……………………………………………(1)Where:isuisthedisplacementvector,NIareshapefunctions,uIarenodaldisplacementvectors,H()arejumpfunctions,aIarenodalenricheddegreeof freedomvectors,Fareasymptoticcrack‐tip functions,and are nodal enriched degree of freedom vectors. The initiation of a fracture occurs when themaximumprinciplestressreachesacriticalvalue,and=1,usingequation(2):

………………………………………………………………………………………………………(2)Where: isthenormalstressactingontheinitiatedfractureplaneand isthemaximumprincipalstress at fracture initiation. To represent borehole breakout an ellipticalwellbore shape is used and anaturalfractureorflawisintroducedfrom15to60°fromH‐Maxin15°increments.Usingthesemodels,we compare initiation pressures to a smoother circular or elliptical cases and visualize early fracturepropagationpaths.Figures2and3representfractureinitiationsfromsmoothcircularandellipticalwellbores. Infigure2,theinitiatedfractureisverticalandoriented30ᵒfromtheH‐Maxorientation,asexpectedforastrike‐slipstressregime.However,thefracturehasnotpropagatedadequatelytoescapethehoop‐stresseffectsandreorient itself in the H‐Max direction. In figure 3, the fracture initiated and propagated in the H‐Maxdirection.Thisindicatestheeffectofthewellboreshapeonredistributingthehoopstressandeffectsoninitiationpointsofhydraulicfractures.

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Figure2.Numericalmodellingresultsinacircularwellbore

Figure3.NumericalmodellingresultsinaanellipticalwellboreFigures 4 – 7 illustrate fracture initiations where natural fractures were introduced to the ellipticalwellbore in 15° increments from the H‐Max direction. In the 15 & 30ᵒ models, fracture propagationshows strong branching indicating high effects of heterogeneity and a general tortuosity towards thedirection of H‐Max. However, as the angle increased to 45‐60°, H‐Max prevented the propagation offracturesunderthesamepumpingpressureandstressshadowingpreventednewfractureinitiationsandlimitedsubstantialpropagationbyany fracture.Whilst thismodellingdoesshowsomedifferencesandsimilaritiestopriorwork,isatanearlystageandfurtherinvestigationsofmeshingandflawintroductionwillbeinvestigated.

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Figure 4. Numerical modelling results from an elliptical wellbore with natural fracture direction 15°fromH‐Maxdirection.

Figure 5. Numerical modelling results from an elliptical wellbore with natural fracture direction 30°fromH‐Maxdirection.

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Figure 6. Numerical modelling results from an elliptical wellbore with natural fracture direction 45°fromH‐Maxdirection.

Figure 7. Numerical modelling results from an elliptical wellbore with natural fracture direction 60°fromH‐Maxdirection.

Drilling Effects and Strategies 

Drillinginducedpetalfractures(DIPF)(e.g.,petal,centrelineandpetal‐centreline)canoccuraheadofthebitduringdrillingasaresultofstressimbalancescausedbyremovalofinsitustressandinteractionwithbit loading forces. Studiesbasedon theoretical reconstructionsof stressesat a corebit concluded thatfailureoccurredon theperipheryof thecoreandslightlyaheadof thebit (GangaRaoetal.,1979).Thestrikeofthesefracturesiscontrolledprimarilybyinsitustresses,butstudiessuggestthattheyarealsoinfluenced locallybyshearstresses inherent frombit rotation(e.g., rotarydrilling) (LorenzandFinley,1988).Thesestudiesconcludedthatregularspacedpetalfracturesoriginateduringabruptincreasesinverticalbitstresscausedbyregularly increasedweightonbitandcanextendat leastthreewellborediametersfromthesideofthewellbore;whicharethenpotentialsitesforfractureinitiationthatisnotalignedwith

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the far‐field stress direction. Lorenz and Finley noted the absence of such fractures from smootherdrilling techniques whilst other research into DIPF determined that various other drilling factorsincludingbittypealsoinfluencetheextentofwellboredamage(FuenkajornandDaemen,1992).Althoughpublicdataislimited,petalfractureswereobservedtobepresentinthesmallsampleofCooperBasincore imagesavailable(Figure8).AnecdotalevidenceinconjunctionwithphotographicindicationofexistenceintheCooperBasinleadsustobelievethatbasedontheparameters,drillingtechniquesmaybecontributingtotheextentofDIPF.Thissuggeststhatapplyingalowerandmoreconsistentweightonbit(e.g., turbinedrillingtechniques)andreducedcirculatingpressuresmayresult inareductionintheoccurrenceofDIPFandDITF,aidingfractureinitiationandnearwellborepropagation.Figure8.CorephotographsfromtheCooperBasinindicatingpotentialpetalfractures

Other Modelling, Completion Effects and Strategies 

Other authors have investigated fracture propagation using two‐dimensional numerical models toanalyse initiation and growth of hydraulic fractures from awellbore, aligning thewell eitherwith themaximumorintermediateprincipalstress(JeffreyandZhang,2010).Theirmodellingresultsshowthathydraulicfracturewidthisnarrowedatthewellboreformisalignedfracturesthatreorientastheygrowaway from the well; ultimately, the far‐field pressures of these fractures are equal to or below thoseexpected for an aligned planar fracture. Further, they noted that while wellbore pressure did notsignificantlyincreasewithsmoothly,curvingfractures;however,whenamodelledfractureissegmentedand includes offsets, then the pressure increases significantly with the reduction of width. Together,thesefactorscancontributetohighNWBpressuresandpotentialfailuretomaintainaconductiveNWBproppedfractures.Figure 9 from Jeffrey and Zhang (2010), summarises thismodellingwork and shows fracture growthpathsasafunctionofanon‐dimensionalparameter,Fthatcontrolscurvingofthepath(seeequation3).Therock isassumedtobehomogeneous,elasticand isotropic.The fracturere‐orientationoccursmoreslowly as the value ofF becomes smaller. Therefore, this result predicts that higher rate and higherviscositywillreducecurving(complexity)nearthewellbore.

Χ . √ …………………………………………………………………………………………………………(3)

whereH‐Maxandh‐minarethemaximumandminimumprincipalstresses,respectively,inthex‐yplane,R isthewellboreradius,E’=E/(1‐2) istheplanestrainYoung’smodulus,’=12where is thefluidviscosity,andQistheinjectionrateintobothfracturewingsperunitheightofthefracture.UsingtheparameterslistedinTable1,andassuminginjectionofwaterat2m3/minute(12.58bbl/min)per m of perforated zone length,F takes on a value of 1.67, implying rapid reorientation, primarilycausedby the largedeviatorystressconditions.Thissuggests thatensuring the initiationoccurs in theplaneofeventualhydraulicfracturegrowthwillbethemosteffectivecontrolsincetheinjectionrateandviscosity have an effect that depends on their product raised to the¼ power. However, use of higherviscosityfluidislikelytohaveothereffectsnotcapturedbyForinthis2Dmodelling.

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Figure9:Fracturepathscalculatedasafunctionofthenon‐dimensionalparameterF(afterJeffreyandZhang,2010).Forthecasesshown,h‐min=60MPaandH‐Max=80MPa.Jeffrey and Zhang’s research supports the use of viscous fluids and high injection rates to initiatehydraulic fractures and reduce near wellbore fracture tortuosity; this method along with orientedperforatingwasused inaCooperBasin trial to successfully reduceNWBeffects (Johnsonetal.,2002).Further,inanareaofshallowerdepthbutneverthelessacomplexstressenvironment,diagnosticresultshadproventhatacomplexfracturewouldforminverticalwells(Johnsonetal.,2010)basedonnaturalfractureinteractionwiththestressregime.Inthissamearea,orientingthewellintheH‐Maxdirectionineither27.5or60°inclinationresultedinlesscomplexpropagationascomparedtoaverticalwellbasedonmultiplehydraulicfracturingdiagnosticsusedinbothcases(Megordenetal.,2013).Combiningthesetechnologies,drillingawellintheazimuthofH‐Maxandcompletingthewellbyaligningthe perforations or slots with the H‐Max direction may result in lowered NWB pressures and moreeffectiveverticalfracturepropagationintheCooperBasinstressregimes.

Conclusions and Recommendations 

Reorientationof hydraulic fracturesNWB is known fromexperiments to result in segmentationof thefractureasthereorientationoccurs.Thepaththatisproducedishighlycomplexandisassociatedwithhigh entry losses during injection. Current numerical models are unable to reproduce details of thefracturesegmentationprocessand this isanareaof research thatmay lead tobetterunderstandingofinitiationand improvedunderstandingof thedevelopmentof associated fracture complexity along thepathofthefracturereorientation.Benefits may be realised by creating longitudinal or oblique fractures placed in inclined wells in theazimuthofH‐Maxincomplexstressregimes.Initially,placinglongitudinalorobliquefracturesmayseemacapitulationstrategywithlessperceivedbenefitsthanapplyingNorthAmericantransversefracturingtechniques. However, lowertreatingpressures,moreeffectivefracturelengthandincreasedwidthwilllikely developed vertically with longitudinal or oblique fractures as compared to case histories oftransversefracturingintheCooperBasinstressregimeswhereNWBcomplexitiesaswellasvertical,andhorizontalfracturecomponentswereallnoted,andlikelyleadtosub‐optimalfracturing.Based on this work, several areas of drilling and completion practices could be optimized to reducefractureinitiationpressuresandpotentiallyhydraulic fracturecomplexity. Drillingwithreducedpumppressures and using smoother, higher speed (turbine) drilling practices may reduce DITF and DIPF.Although borehole breakout will be difficult to eliminate based on the magnitudes of the horizontalstresses, particular care in assuring adequate wellbore cleanout, casing centralization, and reducedcement slurry shrinkage should be considered whether the wellbore is vertical or inclined. Finally, if

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drilling in thevertical,h‐minoranalternativedirection to those favourablyorientedwithH‐Max is stilldesirable, thenefforts toperforateor slot thewell in theH‐Max directionare recommended to initiatefracturesinthecorrectplane,avoidingasmuchNWBtortuosityeffectsaspossible.

REFERENCES 

Belytschko, T. and Black, T. (1999). Elastic crack growth in finite elements with minimal remeshing.

InternationalJournalforNumericalMethodsinEngineering,45(5),601‐620.Brudy,M.andZoback,M.D.(1999).Drilling‐inducedtensilewall‐fractures:implicationsfordetermination

of in‐situ stress orientation andmagnitude. International Journal ofRockMechanicsandMiningSciences,36(2),191‐215.

Fuenkajorn, K. and Daemen, J. J. K. (1992). Drilling‐Induced Fractures in Borehole Walls. Journal ofPetroleumTechnology,44(02),7.

GangaRao,H.V. S., Advani, S.H., Chang, P., Lee, S. C., andDean, C. S. (1979). In‐SituStressDeterminationBasedOnFractureResponsesAssociatedWithCoringOperations,ARMA‐79‐0683.Paperpresentedatthe20thU.S.SymposiumonRockMechanics(USRMS),Austin,Texas.

Jeffrey,R.G. andZhang,X. (2010).MechanicsofHydraulicFractureGrowthFromaBorehole,SPE137393.Paper presented at the Canadian Unconventional Resources and International PetroleumConference,Calgary,Canada.

Johnson,R.L.,Jr.,Aw,K.P.,Ball,D.,andWillis,M.(2002).Completion,PerforatingandHydraulicFracturingDesignChangesYieldSuccessinanAreaofProblematicFracPlacement‐theCooperBasin,Australia,SPE 77906. Paper presented at the SPE Asia Pacific Oil and Gas Conference and Exhibition,Melbourne,Australia.

Johnson,R.L.,Jr.,Scott,M.,Jeffrey,R.G.,Chen,Z.Y.,Bennett,L.,Vandenborn,C.,andTcherkashnev,S.(2010,19–22September).EvaluatingHydraulicFractureEffectiveness inaCoalSeamGasReservoirfromSurfaceTiltmeterandMicroseismicMonitoring, SPE133063. Paper presented at the SPE AnnualTechnicalConferenceandExhibition,Florence,Italy.

Lorenz,J.C.andFinley,S.J.(1988).SignificanceofDrillingandCoringInducedFracturesinMesaverdeCore,NorthwesternColorado:SandiaNationalLaboratories.

Megorden,M.P.,Jiang,H.,andBentley,P.J.D.(2013).ImprovingHydraulicFractureGeometrybyDirectionalDrilling in Coal Seam Gas Formation, SPE 167053. Paper presented at the SPE UnconventionalResourcesConferenceandExhibition‐AsiaPacific,Brisbane,Australia.

Reynolds, S. D., Mildren, S. D., Hillis, R. R., andMeyer, J. J. (2006). Constraining stressmagnitudes usingpetroleumexplorationdataintheCooper–EromangaBasins,Australia.Tectonophysics,415,17.

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Biographies

DrRaymondL.(Ray)Johnson,Jr.iscurrentlyPrincipalatUnconventionalReservoirSolutionsandservesasAdjunctAssociateProfessorattheUniversityofAdelaideandFellowattheUniversityofQueensland.HehasaPhDinMiningEngineering,MScinPetroleumEngineering,G.D.inInformationTechnology,andB.A,inChemistry.RayhasbeenactiveintheSocietyofPetroleumEngineers(SPE),pastchairoftheSPEQueenslandSection,2013co‐ChairoftheSPEUnconventionalReservoirConferenceandExhibitionAsiaPacific,andwillbe2015co‐Chairof theSPEUnconventionalReservoirConferenceandExhibitionAsiaPacific.

DrHaniAbulKhairreceivedhisBScinEarthandenvironmentalsciences,hisMScsedimentologyandhisPhD in petroleum geosciences at the University of Jordan, Jordan. In 2008, he worked as petroleumgeoscientist with Target Exploration UK, before taking a post‐doctoral position at the University ofJordan.In2010,hejoinedtheAustralianSchoolofPetroleumasaResearchAssociatefocusedonashalegasprojectfundedbyPrimaryIndustriesandResourcesSA(PIRSA).In2011,hestartedworkingwiththeSouth Australian Centre for Geothermal Energy Resources on a project involved geomechanics andfracturesindeepshalehorizons.

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Dr Rob Jeffrey holds a PhD in Geological Engineering from the University of Arizona. Prior to joiningCSIRO in 1989, he was employed by Dowell Schlumberger, working on hydraulic fracturing andspecifically fracturingofcoalbedmethanewells.Hecontinuedwith thisresearch interestatCSIROandhasrunarangeofprojectsinvestigatinghydraulicfracturemechanicsincoalandinnaturallyfracturedrocks. Rob was instrumental in introducing hydraulic fracturing to the mining industry for caveinducement and preconditioning of rock masses and this technology is now being used at mines inAustraliaandChile.RobiscurrentlyLeaderofthehydraulicfracturingTeamatCSIROinMelbourneandisamemberofSPEandARMA.

DrJeremyMeyercompletedaBScmajoringinAppliedMathematicsandPhysicsbeforecompletingaPhDingeomechanicsaspartofthe“StressGroup”atAdelaideUniversity.After initialworkat theAustraliaSchoolofPetroleum,JeremyformedJRSPetroleumResearchPty.Ltd.,whichisnowparkofIkonSciencewhereheisSrVPGeomechanics. Hehasspentthelast15yearsworkingasageomechanicsandimagelogspecialistthroughouttheAustralasianregioninavarietyofsectorsincludingconventionaloil&gas,coalseamgas,geothermalenergyandCO2sequestration.JeremyisamemberofSPE,PESA&AAPG

MsCarlyStark is theWellStimulationLeader forPetro‐KingAustralia, an independentChina‐basedoilandgas technologycompany.She is responsible for leadingwell stimulationcapabilityprojects for theproductionenhancementteamwithintheAustralasianregion.ShegraduatedwithhonoursinPetroleumandChemicalEngineeringfromtheUniversityofAdelaideandhaspreviouslyheldrolesatSchlumbergerandNationalOilwellVarcowheresheworkedintheWellServicesandDownholedivisions,respectively.HerprimaryfocuswascenteredonhydraulicfracturingincoalbedmethaneapplicationsanddrillinginCentralAustralia.CarlyisamemberofSPE.

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MrJamesTauchnitzistheCountryManagerforPetro‐KingAustralia,anindependentChina‐basedoilandgas technology company. He is responsible for Petro‐King operations from well planning, drilling,stimulations, completions, and operations in Australasia. James graduated from the University ofAdelaide with honours in Petroleum and Chemical Engineering and has previously held a number oftechnical and management positions for Arrow Energy (Shell and CNPC JV), APLNG (Origin,ConocoPhillips, Sinopec JV) and BHP Billiton. Most recently he was Well Factory Manager for ArrowEnergywhereheheldoverallProjectManagementandstakeholderengagementresponsibilities for theDaandineExpansionProject.JamesisamemberofSPEandAMICDA.