Paper 1. Practical Well Test Interpretation

12
pcbtyc+mrdsm-sl SPE 27975 Practical Well Test Interpretation Louis Mattar, Fekete Assocs. Inc. SPE Member -ht 1~, Wc@tyofPetroleum Errginaere,Inc. Thk paper wee preparadfor ~ntetion at the Universityof Tuks CentennialPetroleumEngineeringSymposiumheld in Tulsa, OK, U.S.A., 2S-31 Auguef1S94. 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,does not neweserlly refkot anypositionofthe SoMatyof PetrofeumEngineere,ireoffkera, ormembem.Peperepreatmfedet SPE meetingsare wbjecf to pubfksfionreviewby EdlforlalOommineesofthe society Of~fdellm [email protected]~ tOc@y je ~ toenabatrectofnotmorethenWOwords.Illuetretionemaynotbecopied.Thesbatrecfaftoufdwnfeinconapbwouseoknowfadgment of where and by whomthe papa lapresented.Write Ltbrarisn,SPE, P.O. Sox S3SSSS,Richerdeon,TX 750SS.3SSS,U.S.A. Telex, 1S3245SPEUT. Abstract Wa!! Test !nterpreta?!on !nvdves much more than Pressure Transient Analysis. It includes scrutinking the field notes, the surface operations, the equipment and the wellbore configuration. It means incorporating geological information and production information; adjusting the measured data to reservoir conditions and accounting for multiphase effects both in the wellbore and in the reservoic recognizingthat wellbore effects during a buildup are differentfrom those during a drawdown; in short, integrating the practical issues with the theoretical analysis. FieJd examples will be presented to illustrate these effects. :gncirinM.-e- - -h .. A .- **Q ~n !ead tO the wrong pressuretransient analysis. Introduction Reservoirengineering integrates many “looks”into the resewoir; for example core analysis, log analysis, pressure analysis and production analysis. Each one of these gives its own view into the reswvoir. Some only investigate a few inches (core, logs) while others reflect a much larger (100’s of feet) radius of investigation (pressure, production). The “looks”can be supportive of each other and fit in with other “looks” into the reservoir, such as geology or seismic interpretations. However, quite often, these “looks” can be contradictory of each other, in which case some of the ev”~ence must be rejected, ignored or explained away. It isthe responsibilityof the amlyst to integrateall these perspectivesof the resetvoir in order to arrive at the best poesibie itit&pie*&tk3rk The same analogy applies within each discipline. For example in the field of pressure analysis, Pressure Transient Analysis (P.T.A.) must be viewed as being only part of the picture, not the whole picture. Well Test Interpretation (VV.T.I.)must encompass the field notes describing how the test was conducted, previous tests, initiil and final static gradient, the wellbore configuration, multi-phase aspects in the wellbore, wellbore dynamics (references 1, 2 and 3), material balance and production information, in addition to P.T.A. P.T.A. has become a very sophisticated tool, and because of@ “advanced”nature, it tends to be placed on a pedestal, to the neglect of other more useful yet simpter Information. For example, a P.T.A. on a flow and builduptest may indicate a dual porosityreservoir. While this may be the case (and there are many wellbore dynamics that may contra-indicate that), we may have overbooked the fact that static gradients before and after the test indicate depletion (a non- economic reservoir - m AII I nnrQ~& k$~ue Ihance Lh~ u~, ~-. becomes a red herring). This article is not intended to belittle the value of P.T.A., rather it attempts to emphasize the point that this powerful tool must not be used in isolation, and that its value is greatly enhanced when it is used within 175

description

Analisis de presion transiente, en un pozo petrolero. escrito por SPE. bajado de la plataforma onepetro.

Transcript of Paper 1. Practical Well Test Interpretation

Page 1: Paper 1. Practical Well Test Interpretation

pcbtyc+mrdsm-sl

SPE 27975

Practical Well Test InterpretationLouis Mattar, Fekete Assocs. Inc.

SPE Member

-ht 1~, Wc@tyofPetroleumErrginaere,Inc.

Thk paper wee preparadfor ~ntetion at the Universityof Tuks CentennialPetroleumEngineeringSymposiumheld in Tulsa, OK, U.S.A., 2S-31 Auguef1S94.

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 notbeen reviewedby lhe SoolefyOfPefrokum Enginsateend are subjectto comotion by the author(a).The material,se preesntad,does not neweserlly refkotany positionofthe SoMatyof PetrofeumEngineere,ireoffkera, or membem.Peperepreatmfedet SPE meetingsare wbjecf to pubfksfionreviewby EdlforlalOommineesof the societyOf~fdellm [email protected]~ tOc@y je~ toenabatrectofnotmorethenWOwords.Illuetretionemaynotbecopied.Thesbatrecfaftoufdwnfein conapbwouseoknowfadgmentof where and by whomthe papa lapresented.Write Ltbrarisn,SPE, P.O. Sox S3SSSS,Richerdeon,TX 750SS.3SSS,U.S.A. Telex, 1S3245SPEUT.

Abstract

Wa!! Test !nterpreta?!on !nvdves much more thanPressure Transient Analysis. It includes scrutinkingthe field notes, the surface operations, the equipmentand the wellbore configuration. It means incorporatinggeological information and production information;adjusting the measured data to reservoir conditionsand accounting for multiphase effects both in thewellbore and in the reservoic recognizingthat wellboreeffectsduring a buildup are differentfrom those duringa drawdown; in short, integrating the practical issueswith the theoretical analysis.

FieJd examples will be presented to illustrate theseeffects. :gncirinM.-e- - -h. . ●A .- **Q ~n !ead tO the wrong

pressuretransientanalysis.

Introduction

Reservoirengineering integrates many “looks”into theresewoir; for example core analysis, log analysis,pressure analysis and production analysis. Each oneof these gives its own view into the reswvoir. Someonly investigate a few inches (core, logs) while othersreflect a much larger (100’s of feet) radius ofinvestigation (pressure, production). The “looks”canbe supportiveof each other and fit in with other “looks”into the reservoir, such as geology or seismicinterpretations. However, quite often, these “looks”can be contradictory of each other, in which case

some of the ev”~ence must be rejected, ignored orexplained away. It is the responsibilityof the amlyst tointegrateall these perspectivesof the resetvoir in orderto arriveat the best poesibie itit&pie*&tk3rk

The same analogy applies within each discipline. Forexample in the field of pressure analysis, PressureTransient Analysis (P.T.A.) must be viewed as beingonly part of the picture, not the whole picture. WellTest Interpretation (VV.T.I.)must encompass the fieldnotes describing how the test was conducted,previous tests, initiil and final static gradient, thewellbore configuration, multi-phase aspects in thewellbore, wellbore dynamics (references 1, 2 and 3),material balance and production information, inadditionto P.T.A.

P.T.A. has become a very sophisticated tool, andbecause of@ “advanced”nature, it tends to be placedon a pedestal, to the neglect of other more useful yetsimpter Information. For example, a P.T.A. on a flowand builduptest may indicatea dual porosityreservoir.While this may be the case (and there are manywellbore dynamics that may contra-indicate that), wemay have overbookedthe fact that static gradientsbefore and after the test indicate depletion (a non-economic reservoir - ● m AII I nnrQ~& k$~ueIhance Lh~ u~, ~-.becomes a red herring).

This article is not intended to belittle the value ofP.T.A., rather it attempts to emphasize the point thatthis powerful tool must not be used in isolation, andthat itsvalue is greatly enhanced when it is used within

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2 PRACTICAL WELL TEST INTERPRETATION SPE 27975

the ~r~der persp@ive of We!! Test Interpretation

(W.T.I.). In the rest of the paper, I will discuss severalcases (unrelated to each other) each illustratingsomepracticalaspect of wefttesting.

1. Measure Initial Pressure

Figure 1 represents synthetic data of a buildup test(150 hr. flow 31 hr. shut-in) as published in the late Dr.Rame~s paper (reference 4). The dtiiculty, asexpressed in that publication, was that there was nocorrect Homer straight line, and at best thepermeability that could be derived from this Hornerplot was 37.6 mD as compared to the true permeabilityof 48 mD.

In this particular example, the initial pressure wasknown but was not displayed on the Horner plot. If weplace the initial pressure on the x-axis = 1 (infhiteshut-intime) as in Figure 2, it becomes evidentthat thepressure is returning to the initial pressure. If thesemifogstraightline [sanchored on the initialpressure,then the value of permeability calculated is 44.8 mD.This is significantlybetter than the previouscalculationand is acceptably close to the true answer. [If youknow where you have come from, it is easier to knowwhere you are going to.]

gauge or a straingauge. Mattar et al (reference 1) givean example ~- strain gtwge and a quart! gauge fromdifferent manufacturers which track each otherflawlessly. Figure 5 shows data from a strain gaugeand a (supposedly much more accurate) quartzgauge; the one is building up while the other isdedining; the difference between them is some 200kPa, and independent measurements show the straingauge to be the correct one. ~he quality of service ismore importantthan the type of the instrument.]

4. Is TemDemture lmDortant or Not?

Electronic pressure gauges are known to betemperature sensitiie, quartz much more than straingauges. Thus it is importantto have a good handle onthe temperature when measuring the pressure,otherwisesignificantinterpretationerrorscan occur.

Figures 6 and 7 show two different tests, each usingtwo strdn gauges by the same manufacturers. Bothtests show a temperature anomaly. In Figure 6 thepressures from both recorders track excellently inspite of the temperature discrepancy. In Figure 7 thereis a temperature anomaly of the same magnitude, yetthe pressuresdo not track, diverging at early time andconverging at late time. [Sometimes it matters andsometimesnot.]

2. Desian Your Test5. Freauencv of Samoling

In this case history,there were 6 oil wells in a pod, onein the centre and five in a circle around it. In order toestablish htervvell properties and confirmcommunication, a puise test was corxkmtsd with thecentral well as the active well. Out of the fweobservationwells, two (2) responded as expected fromthe design (Figure 3), two (2) showed a very confusedresponse (Figure 4), and one (1) showed no responseat all to the active well, but seemed to respond to awell producing from another pod! ~hings don’tabvaysgo the way you plan them.]

3. Select Your PressureGauae Semite Commmy

A lot of emphasis is placed on selecting the pressuregauge for a particular test. Often it is more importantto select the right service company than to select theright gauge. Even the best of gauges need to beproperly calibrated and maintained. The questionoften arises as to whether one should use a quartz

Unlike analog gauges, digital electronic gauges donot provide a continuous pressuretrace, but they giveindividualdata pointsat each sample time. In order tooptimize the number of data points, a sophisticatedsampling program can be designed, whereby a largenumber of pressures are measured when there arerate changes, from one rate to another, or from flow toshut-in (or vice-versa), and fewer data points are takenelsewhere. It has often been observed that a changein the frequency of sampling can 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 sampling program is, the more ft isprone to operator error. Moreover, having a complexPm-programmed sampling scheme removes a lot ofthe flexibilityneeded during some tests. For example,if the test had been designed to flow for 12 hours andthen be shut-in, but haffway through the flow period,the well started loading up or hydrating and it becamenecessary to modifythe test, the complex pre-set

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SPE 27975 L. MATTAR 3

sampling frequency program may be totallyinappropriate for the modified test, and much neededvaluable data may be missed as a result. [Keep itsimple.]

6. Wellbore Tubular Confiauretion

Figure 9 shows data from two downhoie recorders. itis obvious that the more you flow, the higher thepressure gets! This stmnge observation is readilyexplained if it is recognized that the recorder is

situated some90 m below the sliding sfeeve, throughwhich gas is flowing from the annuius to the Wb!ng.The space between the sflding sleeve and therecorder run depth is slowly filling up with waterthroughout the test. If the pressure at the end of thetest is corrected for this hydrostatic column of water,then the final reservoirpressure is some 300 I@a lessthan the initial reservoir pressure, indicating severedepletion during the test (in spite of the apparentincreasing pressure). [Conduct a static gradientbefore and after a test, and account for tubularconfigurations.]

7. Rermat&ble Anomalies

Figure 10 is a plot showing two buildup testsconducted on the same OIL well 6 monthsapart. Theyboth show a significant non-resewoir anomaly (60kPa) some 7 hours into the test. At the end of the testa static gmdient run in the well showed GAS and notOIL to be present in the wellbore. The cause of theanomaly is obviously a wellbore dynamic associatedwith phase segregation and liquid efftux. [Wellboredynamics can be very obvious, biif they maybe subtieand inconspicuous.]

8. Drawdown (Injetilon) versusBuildug fFalioW]

P,T.A. is based on the dmwdown (Injection) equationand Buildup (Falloff) analysis utilizes this solutionalong with the principJeof superposition. In theory, theanalysis of drawdown (injection) data should yield thesame answers as 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 analyses of the injection andfalloff data on the same test. Such inconsistenciesmust be resolved by information external to the test;

for example, what is the true reservoir pressure? didthe injection create a hydraulic fmcture which healsduring the falloff? nhe P.T.A. models may be tooideaii%d to representa real-lifesituation.]

9. Mvetarious Date

!n S*Q 9f ail effofts, we were unable to find anexplanationfor the periodicityobserved on the biiiiduptestof a dry gas weii show-n in Figwe %3. Bothelectronic pressure recorders showed the sameoscillations. (We may never know all the answers.]

10. Non-Uniaue Solutions:

Many well tests can be interpreted using severaldifferent reservoir models. Selecting the appropriatemodel often cannot be done without recourse toexternal information eg. geological deposition.@environment. Figures 14 and 15 show 2 ditYerentmodefs simulating the same test data with equallyacceptable results.

In THEORY, every reservoiris unique, and its behavioris different from other reservoir. However, inPRACTICE, the differences between many reservoirmodels are so small that they are marked by thescatter of practicaldata measurements. In the event ofthe multiplicity of models, the late H.J. Rame~sphilosophy must be heeded, namely [start with asimple model and proceed to a progressively morecomplex model, only as you need to.]

Conclusions

(1) “If you torture the data long enough, it willconfessto anything”.

(2) Pressure Tmnsient Analysis (P.T.A.) should beutifized witininthe broader %arnework of WellTest Interpretation(W.T.L).

(3) W.T.!. >> P.T.A.

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4 PRACTICAL WELL TEST INTERPRETATION

FM~iWiCW3

1.

2.

3.

4.

MATTAR, L “CriticalEvaluation and Processingof Data Prior to Pressure Transient Anaiysis,”Presented at the 67th Annual TechrkalConference and Exhibition of the Society ofPetroteumEngineers,Washington, B.C., October4-7, 1992.

MAITAR, L and SANTO, M.S,: “How WellboreDynamics Affect Pressure Transient Analysis,”The Journal of Canadian Petroleum Technology,Vol 31, No. 2, February, 1992.

MAITAR, L and ZAO~ K.: ~b pd~vPressure Derivative (PPD) - A New DiagnosticTool in Well Test Interpretation,”The Journal ofCanadian Petroleum Technology, VOI 31, No. 4,April, 1992.

RAMEY, H.J.: “Advances in Practical Well TestAnalysis,”J.P.T., June 1992.

SPE 27975

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HORNERPRESSURE BUILDUP PLOT

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Observation Weii Response (weiis 1 and 2)

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Comparison of Quartz and Strain Gauge

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Inconsistent Temperature - Inconsistent pressure

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Apparent Increasing Reservoir Pressures Attributed to

Tubular Configuration

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Inconsistent Fall-off Model

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COMPOS I 1-E MODEL

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