Testing Exploration Wells by Objectives

Testing should never become a mindless exercise in which operations personnel blindly follow proceMany papers have been published extending the dures written by others. The people who are theoretical understanding of pressure transient implementing the test should be actively trying to analysis. Most of these papers have concentrated on achieve the test objectives. It is often difficult or analysis of buildup tests for reservoir permeability, Impossible to predict how to best achieve the test wellbore damage, and reservoir pressure. In many objectives. Often it will be necessary to alter test cases, the state of the art of analytical methods is procedures to conform to well performance, equipment, far superior to the qualityof the data obtained with problems, weather, or other unforeseen events. present day procedures and instrumentation. This paper reviews the state of the art of testing The test engineer’s role is central to successful exploration wells and presents a systematic approach testing. It is important for one person to be to better formation evaluation. This systematic responsible for test design, equipment, procedures, approach consists of establishing specific objectives data collection, data quality control, test analysis, for obtaining formation evaluation from well testing. and presentation of results. When several people Once the objectives are set, guidelines are given to perform these functions, often important data are lost design the test to meet all the objectives. The or forgotten and several of the test objectives may be responsibility of test design rests on a well trained overlooked. In addition, if the same individual runs test engineer who also has the responsibility of test the test and analyzes the data, he should be in the supervision, data quality control, data analysis, and best position to ensure all of the data are collected test documentation. Guidelines are presented for and to explain the test results. procedures and instrumentation and a specific field example is presented that illustrates how to plan and Successfully testing exploration wells requires conduct an exploration well test. constant attention to detail. A successful test requires good test design and careful data quality INTRODUCTION control. As a result, it would be very difficult indeed to run one exploration well test without first Much useful reservoir information can be gathered stating the test objectives. from a production test of an exploration well. Frequently however, much of the important data is lost TEST OBJECTIVES due to improper planning or poor choices of procedures and equipment. Proper test design and procedures While it is not possible to compose a list of should maximize the quality of information obtained objectives which will suffice for every test, six from a test. This paper explores the practical objectives seem to occur in most exploration wells. aspects of well test planning resulting from over 20 The conrnonexploration well test objectives are: years of our experience testing expioratiorr wells .....”1 A..,.r A WI luw,Ve. I 1. To determine the nature of the formation fluids. Before any test can be designed, the objectives 2. To measure the productivityof the well. of the test must be clearly and completely stated. 3. To measure the reservoir temperature and Throughout various phases of the test -design, pressure. implementation, data collection, and analysis, all 4. To obtain suitable samples for laboratory personnel involved should be aware of the test analysis. objectives and should strive to fulfill them. 5. To obtain reservoir description (permeability, reservoir heterogeneities). References and illustrations at end of paper. 6. To estimate completion efficiency. 2 TESTING EXPLORATION WELLS BY OBJECTIVES SPE 13184 Safety has not been included in this list because it is understood to be of paramount importance in every operation. The determination of formation fluids is important in any wildcat well. The first question to be answered is: does the formation contain oil, gas, or water and how much of each can be produced. Testing is often curtailed when water is produced. Closely connected to the first objective is the measurement of the formation productivity. The flow rates of oil, gas, and water are important to each test. These measurements of well productivity will be used primarily to evaiuate the reservoir. The third objective, measuring reservoir temperature and pressure, is probably the most difficult. The reservoir temperature is needed for fluid property determination, while the reservoir pressure will be used not only for pressure transient analysis, but also in later reservoir engineering studies. Attempted measurement of the initial pressure is frequently unsuccessful, but this information is often vital to test analysis. Samples of produced fluids are needed not only for laboratory analysis, but also for fluid property infnvqatinn . . . 0 “! . -“ ,“,, . k!hefl s~able flew has been achievedj separator samples are often adequate. However, bottom-hole samples may be needed to meet this objective. !$iGst Gf the ifif~vfititiijfi .-41-I.>.. . at allaule iil tile literature is aimed at the fifth objective. To obtain reservoir description, bottom-hole flowing and shut-in pressures are analyzed. These data can be interpreted to give permeability-thickness, to estimate flow efficiency, and to indicate heterogeneities in the formation. The quality of the conclusions drawn from these data is only as good as the data itself. The completion efficiency, which is very important in production wells is of lesser importance in exploration wells. In many cases, little effort is made to minimize damage or stimulate the well until after it is known that valuable hydrocarbon reserves exist. The degree of damage may however be important for subsequent planning of development wells. One important item, estimation of reserves, has been omitted from our list of objectives. It is important to know the size of reserves; however, it is often impractical or impossible to determine the reserves from short flow tests. Reserves which can be determined solely from a short exploration well test are small and typically have negative impact on the development of a prospect. Extended production tests to evaluate reserves are not normally part of exploration well testing. The list of objectives is neither comprehensive nor will all of these objectives necessarily apply. Since each well has its own objective, the objectives should be clearly stated before the test is designed and the equipment is selected. TEST DESIGN The test engineer should take the specified objectives and design a test to meet them. A dualflow-dual-shut-in procedure shown in Figure 1, consisting of a short flow period with a shut-in to fi~~~~~~ ~~~ ~~j~j~~ p~~~~~re ~fidz ~Qng f~~w nfarinti ,r-. --with a subsequent shut-in is typically used. Two other issues which must be addressed in the design are the selection of pressure gauges and the choice of shut-in techniques. The initial flow and shut-in are designed to establish cotmnunicationwith the formation and measure “+ial nwncc,,w.a the j~~e,u, ~l==-WI=. ?~~ initial f~~~ narinrl chnl]lrl ,,, , ., , w, be as short as possible. r-, ,“” -----,Long flow periods will require iong shut-in periods to reach pressure stabilization. For gas wells, it is often advisable to surface gas to avoid the complication of phase segregation in the wellbore. The initial shut-in period should be at least one hour and at least four times the duration of the flow period. Surface recording bottom-hole pressure gauges are invaluable in obtaining accurate measurements of the initial pressure. The major flow period should be long enough to evaluate a representative volume of the formation. For most formations of interest, a flow period of six to twelve hours is sufficient. This should encompass a period of at least six hours of stable operation to ensure a reasonable estimate of productivity and to obtain samples for analysis. The major shut-in period ~ho~]d be ~ ~/~ to Z times the duration of the flOW period. No provision has been made here for multirate testing. Multirate tests are designed to meet . . .-.h+ae++,,a.~~~~ Speclflc ““.JGG.,V=. z). ma e,,w+nn +hn +,m-h,,lnn~a ~= ,R,=a=u, ,,,s~,,=~u,“u ,=,,titi fac$or in gas wells or evaluating different tubing strings. As a result, they are rarely needed in exploration well testing= However> if the test objectives call for multirate testing, the test should be so designed. As part of the test design, the choice between surface and bottom-hole shut-in must be made. Surface shut-in has the advantages of simplicity and the opportunity to suspend surface-recording bottom-hole gauges in the well while testing. Bottom-hole shut-in has the advantages of lower surface pressure during shut-in and less wellbore storage. If high surface pressures during shut-in will pose a safety problem or if wellbore storage will distort valuable early time .-1. t’ ~~efi ua a, h-++-”, h,..l UULL”,II-IIU Iii! ~~ti~cjn ehn ,1A ha ,,ecirl altub,u “G “a=”. However, in most cases the advantages of surface shut-in, particularly surface recording gauges, will dominate. Typical bottom-hole equipment for surface and bottom-hole shut-ins are shown in Figures 2 and 3. Selection of pressure gauges is important. In general, select the best available gauges. Typically, this means using one of several high precision electronic gauges available. However, it is important to note that mechanical gauges are often sufficiently accurate to evaluate many wells. An acceptable evaluation is normally obtained when the sensitivity Df the gauges is less than i~ percent of the anticipated slope on the semilog plot of pressure versus time. In addition to checking the gauge sensitivity, SPE 13184 R. S. BARNUM& SAUL VELA it is important to run more than one gauge. It f~ desirable to change orifice plates and calibrate flow highly embarrassing to fail to meet several objectives meters against the test tanks while performing the because the only gauge malfunctioned. test to ensure accurate measurements. One of the potential variables to be considered in test design is the perforating technique. Uhere possible, underbalanced perforating gives better results. However, safety considerations, equipment limitations, or other factors may force overbalanced perforation with a casing gun. The final step in test design is the specification of equipment. Typically, the equipment specified will consist of flow lines, a choke manifold, a heater, a 3-phase separator, test tanks, a transfer pump, and burners. It is important to consider the risk of hydrogen sulfide and to have hydrogen sulfide rated equipment whenever possible. When testing from a floating drilling vessel, the surface equipment will be augmented by a subsea test tree and possibly by a subsea lubricator valve. Typical surface equipment is shown schematically in Figure 4. The test engineer should know the limitations of each piece of equipment in order to safely conduct the test. DATA COLLECTION AND THE TEST ENGINEER Now that the test has been designed to meet the specified objectives, the test engineer’s primary responsibility is to conduct a safe test, effectively gathering the data needed for analysis. In addition, he must monitor the quality of the data, check for pr~p~r ~qu~pm~nt fun~ti~~ing: and ensure quality samples are taken. The first step in the data collection process occurs before the test. The test engineer must be sure that all of the pressure gauges, flow meters, and thermometers are functioning and properly calibrated. Normally, thermometers are calibrated in ice water, pressure gauges are checked against a dead weight tester, differential pressure gauges are checked with a mercury manometer, and flow meters are calibrated with a tank both before and after a test. Once the test begins, the test engineer tries to bring the well production to the maximum safe stable rate as quickly as possible. The desire is to have as long a stable flow period as is possible within test design limitations. A production plot, a plot of flow rate and tubing pressure or bottom-hole pressure versus time as shown in Figure 5, can be an invaluable aid in assessing the stability of the test. After the test has started, careful documentation of data and events is as important as anything else. Sc!meguidelines for data collection are presented in Table 1. The X’s in Table 1 denote the appropriate times for data collection. It is particularly important early in the flow period to monitor not only the nature of the-produced fluids with samples, but alSO tO pE7tOdiC~ll~ C!leCk the concentration of hydrogen sulfide either with length of stain tubes or by titration. Data quality control does not end with calibration. Constant vigilance is required throughout the test to be sure that all of the equipment is functioning properly. It may even be One of the most time consuming jobs of the test engineer is sampling. While monitoring the test, collecting data, and ensuring data quality, the engineer must find time to collect quality samples. With the aid of the production plot, he must first decide if operation is sufficiently stable to permit the use of pressurized separator samples. After three to five hours in which separator rates have varied less than ten percent, pressurized separator samples should be obtained. If separator operations are not sufficiently stable, bottom-hole samples may be required. Before the flow period is complete, the test engineer should be confident that all of the data and samples needed to meet the objectives of the test have been obtained. Wherever possible, the test engineer should not be constrained by previous design. Rather, the successful achievement of the test objectives should govern how long the test lasts. Once the well is shut-in, the test engineer should begin monitoring bottom-hole pressures, if available, and attempting to analyze the pressure buildup data.