Oilfield Review16

Pete HegemanJacques Pe l i s s i e r- C o m b e s c u r eSugar Land, Texas, USA

Although production logs are most com-monly run to diagnose downhole problemswhen surface flow - rate anomalies occur,these tools can also be used during dow n-hole transient tests to determine reservo i rproperties. In essence, measuring the flowrate downhole, just above the producingzone, makes for better interpretationbecause wellbore storage problems arenearly eliminated. Analysis of the tra n s i e n t scan yield reservoir parameters such as per-m e a b i l i t y, skin and pressure at one momentin the life of the reservo i r.1

The techniques for analyzing transient testsrely only on pressure measurements andassume a constant flow rate during the testperiod. The constant flow - rate situation, inp ractice, prevails only during shut-in condi-tions. Thus, buildup tests have become themost commonly practiced well testingmethod. Buildup tests, how e ve r, are some-times undesirable because the operator doesnot want the production lost or because thewell may not flow again if shut in. In suchc i rcumstances, draw d own tests are prefer-able. In practice, it is difficult to ach i e ve aconstant flow rate out of the well, so thesetests have been traditionally ruled out.

There are seve ral advantages to testing awell, either at the surface or downhole, wh i l ef l owing. In producing wells, less productionis lost because the well is not shut in. Ke e p-ing the well on production is especially va l u-able for poor producers that may be difficultto return to production once shut in. In lay-ered reservoirs, testing under draw d ow nreduces the possibility of crossflow betweenproducing layers, whereas during a builduptest, crossflow can easily occur and compli-cate interpretation.2 So, testing a well underf l owing conditions can be beneficial.

A c c u rately testing a well with only surfacef l ow - rate measurements is difficult in pra c-tice. Surface production and testing equip-ment cannot hold a flow rate constant ormeasure flow rate accurately in a short timef rame. This equipment is better suited tomeasuring flow rates during long periods—d ays, not minutes or seconds—for commer-cial sales volumes or daily production data.Most surface facilities lack the accura cy tomeasure quick changes in flow rate neces-sary for transient interpretation.

Testing a well with downhole pressure andf l ow sensors eliminates some of these com-plications because flow rate is measured justa b ove the producing interval. Productionlogging tools measure flow rate more accu-rately than surface facilities do, especiallyfor any instantaneous or small rate ch a n g e s .

F l ow rates and pressure changes areclosely associated—any change in one pro-duces a corresponding change in the other.The challenge in well-test analysis is to dis-tinguish between pressure changes causedby reservoir ch a racteristics and those caused

P roduction Logging for Reservoir Te s t i n g

Using production logging tools to test wells provides a more accurate analysis of re s e rv o i r

parameters, such as permeability and skin damage. Measuring flow rate and pre s s u re

immediately above a producing zone not only reduces wellbore storage effects but also

makes it practical to run transient tests without shutting in a well and halting pro d u c t i o n .

1. Piers GE, Perkins J and Escott D: “A New Flow m e t e rfor Production Logging and Well Te s t i n g ,” paper SPE16819, presented at the 62nd SPE Annual Te ch n i c a lConference and Exhibition, Dallas, Texas, USA,September 27-30, 1987.

2. D e r u y ck B, Ehlig-Economides C and Joseph J: “Te s t i n gDesign and A n a l y s i s ,” Oilfield Rev i ew 4, no. 2 (April1992): 28-45.

For help in preparation of this article, thanks to GilbertConort, Schlumberger Wireline & Testing, Montrouge,France; DeWayne Sch n o r r, Schlumberger Wireline &Testing, A n ch o rage, Alaska, USA; Keith Burgess,S chlumberger Wireline & Testing, Sugar Land, Te x a s ,USA; and Gérard Catala, Schlumberger Wireline & Te s t-ing, Clamart, Fra n c e .P LT (Production Logging Tool) is a mark of Sch l u m b e r g e r.

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by varying flow rates. The pure reservoir sig-nal can be determined by acquiring simulta-neous flow and pressure measurements,wh i ch can easily be obtained in most wellsusing production logging tools. The PLT Pro-duction Logging Tool string, positioned atthe top of the producing interval, recordsd ownhole flow rate and pressure datathroughout the test.

Well Te s t i n gThe three components of the classic welltesting problem are flow rate, pressure andthe formation. During a well test, the reser-voir is subjected to a known and control-lable flow rate. Reservoir response is mea-sured as pressure versus time. The goal isthen to ch a racterize reservoir properties.

Complications arise because flow rate istypically measured at the wellhead, but inter-pretation models are based on flow rate atr e s e r voir conditions. Under some ideal con-ditions, such as single-phase flow and con-stant wellbore storage, the surface flow ra t ecan be related to downhole rate, allowing agood interpretation of the reservoir ch a ra c t e r-istics. If more than one phase, oil and wa t e rfor example, flows in the reservoir or in thewellbore—gas evolving out of solution—thenthe interpretation becomes more difficult.

Obtaining interpretable data under non-ideal conditions often requires test dura t i o n sranging from days to weeks so that condi-tions in the wellbore can stabilize. For a typ-ical pressure buildup test, the test wo u l dh ave to be run until all afterflow and phaseredistribution effects cease. Until then,r e s e r voir response is masked by wellboreeffects (top right) .

M e chanisms that cause wellbore stora g eare compressibility of the fluids in the well-bore and any changes in the liquid level inthe wellbore. After a well is shut in, flowfrom the reservoir does not stop immedi-ately; ra t h e r, it continues at a diminishingrate until the well pressure stabilizes. We l l-bore storage also varies with time due tosegregation of fluids.

Two important advances have significantlyi m p r oved control of well testing: dow n h o l eshut-in va l ves and downhole flow measure-ments. These techniques have eliminatedmost of the draw b a cks inherent in surfaceshut-in testing, such as large wellbore stor-age, long afterflow period and variations inwellbore storage (r i g h t) .

■We l l b o re storage effects. We l l b o re storage and skin effects distort the data collected earlyin a transient test. Interf e rence from other wells or boundaries affects later parts of the test.In the purple zone, radial flow occurs, allowing determination of formation perm e a b i l i t y .

■Downhole shut-in. The main advantages of downhole shut-in are minimization of wellborestorage effects and the reduced duration of the afterflow period. In the surface shut-in test,w e l l b o re storage masks the radial flow plateau for more than 100 hr. In the downhole shut-intest, radial flow is evident after 1 hr.

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Oilfield Review18

There are many ways to shut in a welld ownhole, from drillstem-test tools to wire-line- and slick l i n e - c o nve yed tools. Th ea dvantage is that no downhole measure-ment of flow rate is required; how e ve r, thereare seve ral disadvantages. This method isp ractical only for a shut-in test, so produc-tion is lost and returning the well to produc-tion may be difficult. Moreove r, wireline-and slick l i n e - c o nve yed va l ves are compli-cated to operate and may leak or fail.

Reasons for Downhole Measure m e n t sAnother approach to well testing is to mea-sure flow rate downhole with a stationaryproduction logging tool at or near the top ofthe reservo i r. The advantage of this methodis that the well does not have to be shut infor the transient test. Another advantage isthat the stationary production log can be

combined with a traditional flow survey ve r-sus depth conducted prior to the tra n s i e n ttest and one during the test to inve s t i g a t ec r o s s f l ow effects.

Although simultaneous measurement ofd ownhole flow rates and pressures has beenpossible for some time with production log-ging tools, the use of such measurements fort ransient analysis in well testing is relative l yn e w. A continuously measured flow rate canbe processed with measured pressures top r ovide a response function that mimicswhat would have been measured as pressureif downhole flow rate had been constant.

In many cases, particularly in thick or lay-ered formations, only a small percentage ofa perforated interval may be producing,often because of blocked perforations, thepresence of low-permeability layers or poorpressure draw d own on a particular laye r. Ac o nventional surface well test may indicatethe presence of major skin damage, butfrom the conventional data alone, it wo u l dbe impossible to determine the reason forthe damage. Downhole flow measurementsa l l ow reservoir engineers to measure flowprofiles in stabilized wells and calculateskin effects due to flow convergence. Th u s ,they can infer the true contribution that for-mation damage makes to the ove rall skineffect. This information can help designmore effective stimulation treatments.

D ownhole flow - rate measurements areusually obtained with spinner flow m e t e r srun either on slickline for dow n h o l erecording or on electric line for real-timesurface readout (a b ove left). Continuousspinners are used to test high-rate wells, toperform flow measurements inside tubing,if needed, and to test wells in wh i chrestrictions may prevent operation of a full -bore spinner. Continuous spinners are in-line and allow use of a combination oftools, including a fullbore spinner, belowin the tool string. Fullbore spinners are rou-tinely used and considered as the refer-ence, and they are used in deviated wells.

L a y e red Reservoir Te s t i n gMost of the wo r l d ’s oil fields have layers ofpermeable rock separated by impermeableshales or siltstones, and these layers usuallyh ave different reservoir properties. If all thel ayers are tested simultaneously and onlyd ownhole pressure is measured, it is impos-sible to obtain individual layer properties(a b ove). Thus, special testing techniques areneeded for layered reservo i r s .

■D ro p - o ff memory logging tool. A battery-p o w e red memory production logging toolcan be run on slickline and hung in a tub-ing nipple for an extended downhole test.

■Flow profile from a multilayered re s e r v o i r. The PLT tool measures bottomhole pre s s u reand obtains a flow profile over the entire producing interval.

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Two economical methods of using produc-tion logging tools to perform multilaye r e dr e s e r voir tests are selective inflow perfor-mance tests and layered reservoir testing.S e l e c t ive inflow performance tests are per-formed under stabilized conditions and aresuitable for medium- to high-permeabilityl ayers that do not exhibit crossflow withinthe reservo i r. Layered reservoir testing isconducted under transient conditions. Pres-sure and flow measurements are used todetermine the optimum production rate forall producing layers.

The selective inflow performance test canp r ovide an estimate of the inflow perfor-mance relationship curve for each laye r.3 A sthe well is put through a stepped productions chedule with various surface flow rates, theproduction logging tool measures the bot-tomhole pressure and flow profile at the endof each step. From these production profiles,an inflow performance response (IPR) curvecan be constructed for each layer using thedata from all the flow profiles ( l e f t) .Although a selective inflow performance testp r ovides formation pressure and IPR fore a ch laye r, it does not give unique values forthe permeability and skin factor of indiv i d-ual formation laye r s .

If a reservoir has multiple layers, a tra n-sient test in wh i ch only downhole pressureis measured is virtually useless. All the lay-ers have wellbore pressure as the commoninner boundary, so the pressure alone doesnot convey enough information to deter-mine the properties of the individual laye r s .(b e l ow). At best, analysis of the pressure willyield ave rage properties of the entire system.F l ow rate, not pressure, indicates the prop-erties of the layers. Good zones make largecontributions to the total flow, whereas thepoor layers have only small contributions.The method used to test a multilaye r e dr e s e r voir is to measure the contribution ofe a ch layer during the transient test.

■IPR curves of a multilayered re s e r v o i r. A selective inflow perf o rmance test was run tod e t e rmine the IPR curve for each of the four producing layers in a well. The static pre s-s u re of each layer can be estimated from the point at which the individual IPR curveintersects the vertical axis.

■L a y e red reservoirs. This pre s s u re profile shows diff e rential depletionof up to 800 psi [5515 kPa] between layers (A, B, C and D). Cro s s f l o wwill develop in this reservoir when the well is shut in.

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3. S chnorr DR: “More A n swers from Production LoggingThan Just Flow Profiles,” Transactions of the SPWLA37th Annual Logging Symposium, New Orleans,Louisiana, USA, June 16-19, 1996, paper KK.

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In addition to measuring flow profiles, lay-ered reservoir tests acquire downhole pres-sures and flow rates versus time during eachf l ow period (a b ove). The PLT tool takes thesemeasurements as it is stationed between lay-

ers and above the uppermost laye r. Ta k i n gmeasurements at these stations, in effect,s e p a rates the layers. Bottomhole pressure isrecorded continuously, but the rate per laye ris measured only at a discrete time interva l .4

The layered reservoir test requires carefulplanning and rigorous wellsite logging proce-dures because of the numerous events thatcan occur during a test. Interpreting laye r e dr e s e r voirs is complex because it invo l ve sboth the identification of the reservoir modeland the estimation of unknown para m e t e r ss u ch as permeability, skin factor, reservo i rgeometry and pressure for each laye r.

Combining a selective inflow performancetest with a layered reservoir test yields the bestresults for multilayered reservoirs, especially ifthere is multiphase flow inside the casing.

O u t l o o kWell testing remains of fundamental impor-tance in the development of oil and gasr e s e r ves, and production log flow measure-ments provide a valuable tool to eva l u a t ewell and reservoir performance. The trend isfor the continual refinement of data acquisi-tion and interpretation techniques, with apush for downhole measurement wh e n e ve rpossible. Recent tool advances improvemeasurements in deviated, multiphase-flowand low flow - rate wells wh i ch have oftenposed problems for traditional spinners.

Horizontal wellbores and associated com-pletion designs present seve ral challenges toprofile interpretation for conventional pro-duction logging sensors and tech n i q u e s .Testing and interpretation are better under-stood in vertical wells than in horizontalw e l l s .5 Wellbore storage effects, phase segre-gation and complex geometry in horizontald rainholes complicate analysis of dow n h o l ef l ow - rate measurements. A dvances innumerical modeling techniques are ove r-coming some of the limitations by allow i n gbetter model matching and earlier determi-nation of the flow regime.

As a result, production logs can be used tochoose intervals that should be tested selec-t ive l y, and new selective test procedures willhelp analyze limited sections in horizontalwells. In the future, these selective tests andnumerical modeling will help reservoir engi-neers better identify formation property va r i-ations along the dra i n h o l e . — K R

■Simplified layered reservoir test sequence. Layered reservoir tests are multiple-rate testsin which stationary measurements of downhole rate and pre s s u re are conducted aboveeach layer, and flow profiles are acquired across all layers just before the surface flowrate is changed. In this two-layer test, the flowmeter was placed in two locations, abovethe topmost layer (Qt) and between the layers (Q2) .

4. “ L ayered Reservoir Te s t i n g ,” Middle East Well Eva l u a-tion Rev i ew no. 9 (1990): 22-47.

5. Clark G, Shah P, Deruyck B, Gupta DK and SharmaSK: “Horizontal Well Testing in India,” Oilfield Rev i ew2, no. 3 (July 1990): 64-67.

Oilfield Review