pc bt yc +mr dsm-slSPE27975Practical Well TestInterpretationLouis Mattar, FeketeAssocs. Inc.SPEMember-ht 1~, Wc@tyofPetroleum Errginaere,Inc.Thkpaper wee preparadfor ~ntetion at the Universityof TuksCentennialPetroleumEngineeringSymposiumheld in Tulsa, OK, U.S.A., 2S-31Auguef1S94.This paper wee eefectedfor p&smation by an SPE ProgramCamStea folkwing reviewof informatknoontalnadin an ebetrsofeubmlffedby the author(e).Contenteof the paper,se W-M, hsve not been reviewedby lhe SoolefyOf Pefrokum Enginsateend are subjectto comotion by the author(a).The material,se preesntad,doesnot neweserlly refkotanypositionofthe SoMatyof PetrofeumEngineere,ireoffkera, or membem.Peperepreatmfedet SPE meetingsare wbjecf to pubfksfionreviewby EdlforlalOommineesof the societyOf~fdellm En@naara.P~ tOc@y je~toenabatrectof notmorethenWOwords.Illuetretionemaynotbecopied.Thesbatrecfaftoufdwnfeinconapbwouseoknowfadgmentof where and by whomthe papal apresented.Write Ltbrarisn,SPE, P.O. Sox S3SSSS,Richerdeon,TX 750SS.3SSS,U.S.A. Telex, 1S3245SPEUT.AbstractWa!! Test !nterpreta?!on!nvdves much more thanPressure Transient Analysis. It includes scrutinkingthe field notes, thesurface operations, theequipmentand the wellbore configuration.It means incorporatinggeological informationand production information;adjusting the measureddatatoreservoir conditionsand accountingfor multiphaseeffectsboth inthewellbore and inthe reservoic recognizingthat wellboreeffectsduring a buildupare differentfromthose duringadrawdown;in short, integrating thepractical issueswiththe theoretical analysis.FieJd exampleswill be presentedto illustrate theseeffects. :gncirinM.-e- - -h. . GA.- **Q ~n !eadtO thewrongpressuretransient analysis.IntroductionReservoirengineering integrates many looksinto theresewoir; for example core analysis, log analysis,pressure analysis andproduction analysis. Each oneof thesegives its own viewinto thereswvoir. Someonly investigatea fewinches (core, logs) while othersreflect a much larger (100s of feet) radius ofinvestigation(pressure, production). Thelookscanbe supportiveof each other and fit in with other looksinto the reservoir, such as geology or seismicinterpretations. However, quiteoften, theselookscanbe contradictoryof eachother, inwhichcasesomeoftheev~ence must berejected, ignoredorexplainedaway. It isthe responsibilityof the amlyst tointegrateall these perspectivesof the resetvoir in orderto arriveat the best poesibiei t i t & pi e* & t k 3r kThe same analogy applies within each discipline. Forexampleinthefieldof pressureanalysis, PressureTransient Analysis (P.T.A.)mustbeviewedasbeingonlypartof thepicture, not thewholepicture. WellTestInterpretation (VV.T.I.)must encompass thefieldnotes describing how the test was conducted,previoustests, initiil and final static gradient, thewellbore configuration, multi-phaseaspects in thewellbore, wellbore dynamics(references 1,2and3),material balance and production information, inadditionto P.T.A.P.T.A. has become a very sophisticatedtool, andbecause of@advancednature, it tends to be placedon apedestal, to theneglect of other more useful yetsimpter Information. Forexample, aP.T.A. ona flowand builduptest may indicatea dual porosityreservoir.Whilethismay be the case (andthere are manyw el l bor e dynamics thatmaycontra-indicate that),wemayhaveoverbookedthe fact that staticgradientsbeforeandafterthetest indicatedepletion(anon-economicreservoir -G mAII I nnr Q~& k $~ueI hanc e Lh~ u~, ~-.becomes a red herring).Thisarticleisnot intendedto belittlethe valueofP.T.A., rather it attempts toemphasize thepoint thatthis powerful tool must notbeusedinisolation, andthat itsvalue is greatly enhanced when it is usedwithin1752 PRACTI CAL WELL TEST I NTERPRETATI ON SPE 27975the ~r~der persp@ive of We!!Test Interpretation(W.T.I.). In therest of thepaper, I will discuss severalcases (unrelatedtoeach other) each illustratingsomepractical aspect of wefttesting.1. MeasureInitialPressureFigure1represents synthetic dataofa buildup test(150 hr. flow31hr. shut-in) as publishedin the late Dr.Rame~s paper (reference 4). The dtiiculty, asexpressed in that publication, was thattherewasnocorrect Homer straight line, and at best thepermeability that couldbederivedfromthisHornerplot was 37.6 mD as compared to the true permeabilityof 48 mD.Inthis particularexample, the initial pressurewasknown but was not displayed on the Horner plot. If weplacetheinitial pressureonthex-axis= 1(infhiteshut-intime) as in Figure2, it becomes evidentthat thepressureisreturning tothe initial pressure. If thesemifogstraightline [sanchored on the initialpressure,then thevalue ofpermeability calculated is 44.8mD.This is significantlybetter than the previouscalculationandis acceptablyclosetothetrueanswer. [Ifyouknowwhere you have come from, it is easier to knowwhere you are going to.]gauge or a straingauge. Mattar et al (reference 1) giveanexample ~- strain gtwgeand aquart! gauge fromdifferent manufacturers which track each otherflawlessly. Figure 5shows datafromastrain gaugeand a (supposedly much more accurate) quartzgauge; the one is buildingup whilethe other isdedining; thedifference betweenthemissome200kPa, andindependent measurements show thestraingauge to be the correct one. ~he quality of service ismore importantthan the type of the instrument.]4. Is TemDemturelmDortant or Not?Electronic pressure gauges are known to betemperature sensitiie, quartzmuchmore thanstraingauges. Thus it is importantto have a good handle onthe temperature when measuring the pressure,otherwisesignificantinterpretationerrorscan occur.Figures 6and7show twodifferent tests, eachusingtwostrdngauges by thesamemanufacturers. Bothtestsshowatemperatureanomaly. InFigure 6thepressuresfromboth recorderstrack excellentlyinspite of the temperaturediscrepancy. In Figure 7 thereis a temperature anomaly of the same magnitude, yetthe pressuresdo not track, divergingat early time andconverging at latetime. [Sometimes itmatters andsometimesnot.]2. DesianYour Test5. Freauencvof SamolingInthis case history,there were 6 oil wells ina pod, onein the centre and five in acircle around it. In order toestablish htervvell properties and confirmcommunication, apuise testwascorxkmtsd with thecentral well as the active well. Out of the fweobservationwells, two (2) respondedas expected fromthe design (Figure 3), two (2) showed a very confusedresponse (Figure 4), and one (1) showed no responseatall totheactive well, butseemed torespond toawell producingfromanother pod! ~hings dontabvaysgo the way you planthem.]3. Select Your PressureGauaeSemite CommmyA lot of emphasis is placedon selecting thepressuregauge for a particular test. Often it is more importantto select theright service company than to select therightgauge. Eventhebest of gaugesneedtobeproperlycalibratedand maintained. The questionoften arises astowhetheroneshoulduseaquartzUnlike analoggauges, digital electronic gaugesdonot provide a continuouspressuretrace, but they giveindividualdata pointsat each sample time. Inorder tooptimize thenumber of data points, asophisticatedsamplingprogramcanbe designed, whereby alargenumber of pressures aremeasured when there arerate changes, from one rateto another, or fromflowtoshut-in(or vice-versa), and fewer data points are takenelsewhere. It has often beenobservedthat a changeinthe frequency of samplingcan result in a change intk recorded pressuretrend as illustrated in Figure 8.This is obviously not a reservoireffect but the artifactof the electrorks of the pressure recorder. Aiso themore complex the samplingprogramis, the more ft ispronetooperator error. Moreover, havinga complexPm-programmed sampling schemeremoves alotofthe flexibilityneeded during some tests. For example,if thetest had beendesigned to flowfor 12 hoursandthenbeshut-in, buthaffway through theflowperiod,the well started loading up or hydratingand it becamenecessary to modifythe test, the complexpre-set176SPE27975 L. MATTAR 3sampling frequency program may be totallyinappropriatefor themodified test, and much neededvaluabledatamaybemissed asaresult. [Keepitsimple.]6. WellboreTubularConfiauretionFigure 9 shows data from two downhoie recorders. itisobviousthat the moreyouflow, the higherthepressuregets! Thisstmnge observationisreadilyexplained if it i s r ec ogni zed t hat t he r ec or der i ssi t uat ed some90 mbelow theslidingsfeeve, throughwhichgasis flowing from theannuius totheWb!ng.The space between the sfldingsleeve and therecorder rundepth is slowlyfillingup withwaterthroughout thetest. If thepressure at theend of thetest is corrected for this hydrostatic column of water,then the final reservoir pressure is some 300I@a lessthanthe initialreservoir pressure, indicatingseveredepletionduringthe test (inspiteof the apparentincreasingpressure). [Conduct a static gradientbefore and after a test, and account for tubularconfigurations.]7. Rermat&ble AnomaliesFigure 10 is a plot showing two buildup testsconducted on the same OIL well 6 monthsapart. Theybothshowa significantnon-resewoir anomaly (60kPa) some 7 hours intothe test. At the end of the testastatic gmdient run in thewell showed GAS andnotOIL tobepresent in thewellbore. Thecauseof theanomalyis obviously awellbore dynamic associatedwithphasesegregation andliquidefftux. [Wellboredynamics can be very obvious, biif they maybesubtieand inconspicuous.]8. Drawdown(Injetilon) versusBuildugfFalioW]P,T.A. is basedon thedmwdown(Injection) equationand Buildup (Falloff) analysisutilizesthis solutionalong withthe principJeof superposition. In theory, theanalysis of drawdown (injection) datashould yield thesame answersas buildup (fafloff)data. In pmctice, thishappens only mrely. Very often, the analysisfrom theflow period gives different resuftsfrom the analysis ofthe shut-in period. Figures 11 and 12 are examples ofsignificantly different analysesof the injectionandfalloff dataonthesametest. Suchinconsistenciesmust beresolved byinformation external tothetest;forexample, whatis thetruereservoir pressure? didtheinjection createahydraulic fmcturewhichhealsduringthefalloff? nhe P.T.A. modelsmaybetooideaii%d to representa real-lifesituation.]9. MvetariousDate!nS*Q 9f ail effofts, we were unable to findanexplanationfor the periodicityobserved on the bi i i i dupt est of a dry gasweii show-n inFigwe %3. Bothelectronic pressure recorders showed the sameoscillations. (We may never knowall the answers.]10. Non-UniaueSolutions:Many well tests can be interpretedusingseveraldifferent reservoir models. Selecting theappropriatemodel often cannot be done without recoursetoexternal information eg. geological deposition.@environment. Figures14 and 15 show2 ditYerentmodefssimulating the sametest data withequallyacceptable results.In THEORY, every reservoiris unique, and its behavioris different from other reservoir. However, inPRACTICE, the differencesbetweenmanyreservoirmodelsare sosmall that they are markedbythescatter of practical data measurements. In the event ofthe multiplicityof models, the late H.J. Rame~sphilosophymust be heeded, namely [start withasimple model andproceedtoa progressively morecomplex model, only as you need to.]Conclusions(1) Ifyoutorturethe data long enough, it willconfessto anything.(2) PressureTmnsient Analysis (P.T.A.) should beutifized witininthebroader%arnework of WellTest Interpretation(W.T.L).(3) W.T.!. >> P.T.A.4 PRACTI CAL WELL TEST I NTERPRETATI ONFM~i Wi CW31.2.3.4.MATTAR, L CriticalEvaluationandProcessingof Data Priorto PressureTransientAnaiysis,Presented at the 67th Annual TechrkalConference and Exhibitionof the Society ofPetroteumEngineers, Washington, B.C., October4-7, 1992.MAITAR, LandSANTO, M.S,: How WellboreDynamicsAffect PressureTransient Analysis,The Journal of Canadian PetroleumTechnology,Vol 31, No. 2, February, 1992.MAITAR, L and ZAO~ K.: ~b pd~vPressure Derivative (PPD) - ANewDiagnosticTool inWell TestInterpretation,TheJournal ofCanadianPetroleum Technology, VOI 31, No. 4,April, 1992.RAMEY, H.J.: Advances in Practical Well TestAnalysis,J.P.T., June 1992.SPE27975178HORNERPRESSURE BUILDUP PLOT32002800-)2400 :.-IJl? 2000-AG1A2AIA1600 AAIA1200AA1,,!!I ,i !,800Iii,04 103102,Ql 1.0(t+At)/At Figure 1HORNERPRESSURE BUILDUPPLOT3200Anchor onPi16001200IIAAAAAAAAAAPiJ800F I ,,l .1,,,!I i )!!,~4 103102,Ql 1.0(t+At)/At Figure 2179Observation Weii Response (weiis 1and 2)pmour . at*~ ,,0~4 w 10100-~,,a10050=,,,,,10000(2.0 120Figure 3Observation Weii Response (weiis 3 and 4)10200~.. . . . . ..- ------------- ,10150.010100 L10050,~; ::,,29Time .(hrjFigure 4180Comparison of Quartz and Strain Gauge14800kQuar t z-1----- --______ ------------ ------ ------- -..-,./-kIiL14000LiIinconsistent Temperature but Consistent PressureFigure S18000~771i*. . . . .& .--- -- ,=?..* c ~... ------- 70150001 ~ \--------#. .No,,.*-----~/* =t i r -e ----/*/pr * * ..--_ -r g ~-, - Gr -e* o--=-+.-i n P69,--~ 12000 , ~ -/-~/,//~~/~.,J ..;n; #(--- I 0689000 .-, $Ld-w ,/0/ /9/ #w ,,,,,,,,,,(41676000 /h4* * S t,~ t, 18/,Il =. .0 . . . . . . . . . . . . .115~ 125130135,4066~(-)~o. . ~~ 100 105 110..- ----- /.-. \. #z I f t m,~r ,r /Figure 6181Inconsistent Temperature - Inconsistent pressure4501 1656690r6660i/ ------- ------------------6570 ;141Pressure Difference >=---------------------------- -1//o ~ 1]6540516051.5 52 52.5 53 53.5 5454.5 55 55.5 5.5Time, (hr)14450Effect of Sampling Frequency on114448144461444414442I,..~---L.2 1.. ..... ....-:..... ...../.4 1SiiilingFrM=:3 minute........................./ftJ1.................. ...:,/-/l IG(~ 6ep). &Jsl z# JapJ03a&j-COMPOS I 1-E MODEL,04 _1(33:x.0u-30.Ial3a10:~/~Vpaaterskl -&100.00= 5.000\c: =100.00$r l =L104. 49k2 =47. 18: JI t , t 1 , m , , , L,01 L~~d~--,01 1021(,0-2,()-1 1.0A tFigure 15