P roduction Logging for Reservoir Te s t i n g

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. Pete Hegeman Jacques Pe l i s s i e r- C o m b e s c u r e Sugar Land, Texas, USA Although production logs are most commonly run to diagnose downhole problems when surface flow - rate anomalies occur, these tools can also be used during dow n- hole transient tests to determine reservo i r properties. In essence, measuring the flow rate downhole, just above the producing zone, makes for better interpretation because wellbore storage problems are nearly eliminated. Analysis of the tra n s i e n t s can yield reservoir parameters such as perm e a b i l i t y, skin and pressure at one moment in the life of the reservo i r. 1 For help in preparation of this article, thanks to Gilbert Conort, 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. The techniques for analyzing transient tests rely only on pressure measurements and assume a constant flow rate during the test period. The constant flow - rate situation, in p ractice, prevails only during shut-in conditions. Thus, buildup tests have become the most commonly practiced well testing method. Buildup tests, how e ve r, are sometimes undesirable because the operator does not want the production lost or because the well may not flow again if shut in. In such c i rcumstances, draw d own tests are preferable. In practice, it is difficult to ach i e ve a constant flow rate out of the well, so these tests have been traditionally ruled out. There are seve ral advantages to testing a well, either at the surface or downhole, wh i l e f l owing. In producing wells, less production is 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 difficult to return to production once shut in. In layered reservoirs, testing under draw d ow n reduces the possibility of crossflow between producing layers, whereas during a buildup test, crossflow can easily occur and complicate interpretation. 2 So, testing a well under f l owing conditions can be beneficial. A c c u rately testing a well with only surface f l ow - rate measurements is difficult in pra c- tice. Surface production and testing equipment cannot hold a flow rate constant or measure flow rate accurately in a short time f rame. This equipment is better suited to measuring flow rates during long periods d ays, not minutes or seconds for commercial sales volumes or daily production data. Most surface facilities lack the accura cy to measure quick changes in flow rate necessary for transient interpretation. Testing a well with downhole pressure and f l ow sensors eliminates some of these complications because flow rate is measured just a b ove the producing interval. Production logging tools measure flow rate more accurately than surface facilities do, especially for any instantaneous or small rate ch a n g e s. F l ow rates and pressure changes are closely associated any change in one produces a corresponding change in the other. The challenge in well-test analysis is to distinguish between pressure changes caused by reservoir ch a racteristics and those caused 1. Piers GE, Perkins J and Escott D: A New Flow m e t e r for Production Logging and Well Te s t i n g, paper SPE 16819, presented at the 62nd SPE Annual Te ch n i c a l Conference 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 g Design and A n a l y s i s, Oilfield Rev i ew 4, no. 2 (April 1992): 28-45. 16

by varying flow rates. The pure reservoir signal can be determined by acquiring simultaneous flow and pressure measurements, wh i ch can easily be obtained in most wells using production logging tools. The PLT Production Logging Tool string, positioned at the top of the producing interval, records d ownhole flow rate and pressure data throughout the test. Well Te s t i n g The three components of the classic well testing problem are flow rate, pressure and the formation. During a well test, the reservoir is subjected to a known and controllable flow rate. Reservoir response is measured as pressure versus time. The goal is then to ch a racterize reservoir properties. Complications arise because flow rate is typically measured at the wellhead, but interpretation models are based on flow rate at r e s e r voir conditions. Under some ideal conditions, such as single-phase flow and constant wellbore storage, the surface flow ra t e can be related to downhole rate, allowing a good interpretation of the reservoir ch a ra c t e r- istics. If more than one phase, oil and wa t e r for example, flows in the reservoir or in the wellbore gas evolving out of solution then the interpretation becomes more difficult. Obtaining interpretable data under nonideal conditions often requires test dura t i o n s ranging from days to weeks so that conditions in the wellbore can stabilize. For a typical pressure buildup test, the test wo u l d h ave to be run until all afterflow and phase redistribution effects cease. Until then, r e s e r voir response is masked by wellbore effects (top right). M e chanisms that cause wellbore stora g e are compressibility of the fluids in the wellbore and any changes in the liquid level in the wellbore. After a well is shut in, flow from the reservoir does not stop immediately; ra t h e r, it continues at a diminishing rate until the well pressure stabilizes. We l l- bore storage also varies with time due to segregation of fluids. Two important advances have significantly i m p r oved control of well testing: dow n h o l e shut-in va l ves and downhole flow measurements. These techniques have eliminated most of the draw b a cks inherent in surface shut-in testing, such as large wellbore storage, long afterflow period and variations in wellbore 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 early in 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 wellbore storage 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-in test, radial flow is evident after 1 hr. Summer 1997 17

Flow profile from a multilayered re s e r v o i r. The PLT tool measures bottomhole pre s s u re and obtains a flow profile over the entire producing interval. D ro p - o ff memory logging tool. A batteryp o w e red memory production logging tool can be run on slickline and hung in a tubing nipple for an extended downhole test. There are many ways to shut in a well d ownhole, from drillstem-test tools to wireline- and slick l i n e - c o nve yed tools. Th e a dvantage is that no downhole measurement of flow rate is required; how e ve r, there are seve ral disadvantages. This method is p ractical only for a shut-in test, so production is lost and returning the well to production may be difficult. Moreove r, wirelineand slick l i n e - c o nve yed va l ves are complicated to operate and may leak or fail. Reasons for Downhole Measure m e n t s Another approach to well testing is to measure flow rate downhole with a stationary production logging tool at or near the top of the reservo i r. The advantage of this method is that the well does not have to be shut in for the transient test. Another advantage is that 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 t test and one during the test to inve s t i g a t e c r o s s f l ow effects. Although simultaneous measurement of d ownhole flow rates and pressures has been possible for some time with production logging tools, the use of such measurements for t ransient analysis in well testing is relative l y n e w. A continuously measured flow rate can be processed with measured pressures to p r ovide a response function that mimics what would have been measured as pressure if downhole flow rate had been constant. In many cases, particularly in thick or layered formations, only a small percentage of a perforated interval may be producing, often because of blocked perforations, the presence of low-permeability layers or poor pressure draw d own on a particular laye r. A c o nventional surface well test may indicate the presence of major skin damage, but from the conventional data alone, it wo u l d be impossible to determine the reason for the damage. Downhole flow measurements a l l ow reservoir engineers to measure flow profiles in stabilized wells and calculate skin effects due to flow convergence. Th u s, they can infer the true contribution that formation damage makes to the ove rall skin effect. This information can help design more effective stimulation treatments. D ownhole flow - rate measurements are usually obtained with spinner flow m e t e r s run either on slickline for dow n h o l e recording or on electric line for real-time surface readout (a b ove left). Continuous spinners are used to test high-rate wells, to perform flow measurements inside tubing, if needed, and to test wells in wh i ch restrictions may prevent operation of a fullbore spinner. Continuous spinners are inline and allow use of a combination of tools, including a fullbore spinner, below in the tool string. Fullbore spinners are routinely used and considered as the reference, and they are used in deviated wells. L a y e red Reservoir Te s t i n g Most of the wo r l d s oil fields have layers of permeable rock separated by impermeable shales or siltstones, and these layers usually h ave different reservoir properties. If all the l ayers are tested simultaneously and only d ownhole pressure is measured, it is impossible to obtain individual layer properties (a b ove). Thus, special testing techniques are needed for layered reservo i r s. 18

IPR curves of a multilayered re s e r v o i r. A selective inflow perf o rmance test was run to d 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 curve intersects the vertical axis. Two economical methods of using production logging tools to perform multilaye r e d r e s e r voir tests are selective inflow performance tests and layered reservoir testing. S e l e c t ive inflow performance tests are performed under stabilized conditions and are suitable for medium- to high-permeability l ayers that do not exhibit crossflow within the reservo i r. Layered reservoir testing is conducted under transient conditions. Pressure and flow measurements are used to determine the optimum production rate for all producing layers. The selective inflow performance test can p r ovide an estimate of the inflow performance relationship curve for each laye r. 3 A s the well is put through a stepped production s chedule with various surface flow rates, the production logging tool measures the bottomhole pressure and flow profile at the end of each step. From these production profiles, an inflow performance response (IPR) curve can be constructed for each layer using the data from all the flow profiles (l e f t). Although a selective inflow performance test p r ovides formation pressure and IPR for e a ch laye r, it does not give unique values for the 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 pressure is measured is virtually useless. All the layers have wellbore pressure as the common inner boundary, so the pressure alone does not convey enough information to determine the properties of the individual laye r s. (b e l ow). At best, analysis of the pressure will yield ave rage properties of the entire system. F l ow rate, not pressure, indicates the properties of the layers. Good zones make large contributions to the total flow, whereas the poor layers have only small contributions. The method used to test a multilaye r e d r e s e r voir is to measure the contribution of e a ch layer during the transient test. 3. S chnorr DR: More A n swers from Production Logging Than Just Flow Profiles, Transactions of the SPWLA 37th Annual Logging Symposium, New Orleans, Louisiana, USA, June 16-19, 1996, paper KK. L a y e red reservoirs. This pre s s u re profile shows diff e rential depletion of up to 800 psi [5515 kpa] between layers (A, B, C and D). Cro s s f l o w will develop in this reservoir when the well is shut in. Summer 1997 19

The layered reservoir test requires careful planning and rigorous wellsite logging procedures because of the numerous events that can occur during a test. Interpreting laye r e d r e s e r voirs is complex because it invo l ve s both the identification of the reservoir model and the estimation of unknown para m e t e r s s u ch as permeability, skin factor, reservo i r geometry and pressure for each laye r. Combining a selective inflow performance test with a layered reservoir test yields the best results for multilayered reservoirs, especially if there is multiphase flow inside the casing. Simplified layered reservoir test sequence. Layered reservoir tests are multiple-rate tests in which stationary measurements of downhole rate and pre s s u re are conducted above each layer, and flow profiles are acquired across all layers just before the surface flow rate is changed. In this two-layer test, the flowmeter was placed in two locations, above the topmost layer (Q t ) and between the layers (Q 2 ). In addition to measuring flow profiles, layered reservoir tests acquire downhole pressures and flow rates versus time during each f l ow period (a b ove). The PLT tool takes these measurements as it is stationed between layers and above the uppermost laye r. Ta k i n g measurements at these stations, in effect, s e p a rates the layers. Bottomhole pressure is recorded continuously, but the rate per laye r is measured only at a discrete time interva l. 4 O u t l o o k Well testing remains of fundamental importance in the development of oil and gas r e s e r ves, and production log flow measurements provide a valuable tool to eva l u a t e well and reservoir performance. The trend is for the continual refinement of data acquisition and interpretation techniques, with a push for downhole measurement wh e n e ve r possible. Recent tool advances improve measurements in deviated, multiphase-flow and low flow - rate wells wh i ch have often posed problems for traditional spinners. Horizontal wellbores and associated completion designs present seve ral challenges to profile interpretation for conventional production logging sensors and tech n i q u e s. Testing and interpretation are better understood in vertical wells than in horizontal w e l l s. 5 Wellbore storage effects, phase segregation and complex geometry in horizontal d rainholes complicate analysis of dow n h o l e f l ow - rate measurements. A dvances in numerical modeling techniques are ove r- coming some of the limitations by allow i n g better model matching and earlier determination of the flow regime. As a result, production logs can be used to choose intervals that should be tested select ive l y, and new selective test procedures will help analyze limited sections in horizontal wells. In the future, these selective tests and numerical modeling will help reservoir engineers better identify formation property va r i- ations along the dra i n h o l e. K R 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 Sharma SK: Horizontal Well Testing in India, Oilfield Rev i ew 2, no. 3 (July 1990): 64-67. 20