Impulse Testing

Abstract

Summary. This paper proposes a new method for evaluating hydrocarbon wells. The formation is subjected to a rate impulse created by briefly flowing the well or injecting into the formation. The pressure response to an ideal rate impulse (i.e., to an instantaneous source of strength unity) is given by the derivative of the pressure response to a conventional step-rate change. This general solution is used in developing an interpretation method based on recently published type curves of the derivative of pressure. Impulse testing is particularly useful for wells that do not flow to the surface and for wells where extended flow may not be desirable (because of sand problems, for example). Applied in conjunction with underbalanced perforating, this method provides a low-cost evaluation of the reservoir and wellbore condition. Several field examples are presented to illustrate the analysis technique. Introduction Well testing is recognized as a unique means to evaluate the characteristics of oil and gas reservoirs under dynamic conditions. Testing procedures are well established for wells flowing to the surface for long durations (several hours to several months), and recent analysis developments, such as the pressure derivative method, have improved the interpretation techniques. In addition, the use of onsite computers for data acquisition and analysis ensures that the test is conducted efficiently toward achieving its objectives. When flow to the surface is not possible or desirable for a significant duration, however, test interpretation may become difficult or inconclusive. This paper proposes a method that is well adapted to test wells that do not flow to the surface or where extended flow may not be desirable. The formation is subjected to an impulse rate created by briefly flowing the well or injecting into the formation. The method requires measuring the quantity of fluid produced or injected and the corresponding pressure variations as produced or injected and the corresponding pressure variations as a function of time. The interpretation technique relies on recently published type curves of the pressure derivative. Because the initial flow of a drillstem test (DST) can be short enough to be considered an impulse, the method is illustrated with two example DST's that are analyzed by classic methods for the final shut-in and the impulse technique for the initial shut-in. Additional examples illustrate the use of this technique in conjunction with tubing-conveyed perforating (TCP) and with backsurge operations. Background Hydrocarbon-bearing formations are commonly tested by measuring the downhole pressure and flow rates as a function of time during a given sequence of opening and closing of the well. The physical characteristics of the fluids are also evaluated. These measurements are analyzed during the test using on-site computers to provide information for efficiently conducting the test toward achieving its objectives. Several methods are commonly used for interpretation. The best known is the Horner method; part of the data are in infinite-acting radial flow and display a semilog straight line that is used to determine the physical characteristics of the formation. Type curves representing the global pressure response of flowing or shut-in wells for a variety of reservoir configurations are also used; a log-log plot allows diagnosis of the reservoir behavior and identification of the different characteristic flow regimes, such as infinite-acting radial flow. This is generally used to justify the validity of a semilog (Horner) analysis. More recently, a method based on the analysis of the derivative of pressure with respect to the appropriate time function was proposed; it uses a log-log plot of the slope of the conventional superposition plot (generalized Horner) as a function of time. This method benefits from the advantages of both log-log and semilog analysis. It provides on a single plot an analysis of the global response with improved definition, because the derivative magnifies small phenomena of interest and gives an excellent indication of the reservoir behavior. DST 1-Final Shut-In. To illustrate the use of the derivative method, the final buildup of DST 1 is analyzed as shown in Fig. 1. The match is done with the type curve for a well with wellbore storage and skin in a homogeneous reservoir. Early-time data are affected by changing wellbore storage. At the beginning of the shut-in, the compressibility of free gas in the wellbore is predominant and results in a high wellbore storage coefficient. predominant and results in a high wellbore storage coefficient. As pressure builds up, the free gas dissolves in the oil and the predominant compressibility decreases, resulting in a lower storage predominant compressibility decreases, resulting in a lower storage coefficient. This phenomenon is more frequently encountered when the well is damaged and is shut in at the surface, which is the case for this example. The derivative shows clearly that infinite-acting radial flow was attained after about 45 minutes of shut-in. Table 1 lists the data; calculations and results of the analysis are given in Appendix A. DST 2-Final Shut-In. For wells not flowing to the surface, the analysis of transient pressure data is difficult. Fig. 2 presents the pressure recorded during DST 2 on a well that did not flow to the surface. The data are listed in Table 2. A closed-chamber system was used to evaluate the rates during the initial and final flow periods. Fig. 3 presents an analysis of the final shut-in using the derivative of pressure. Analysis of this test was more difficult because of the poor quality of the data recorded by mechanical gauges; no infinite-acting radial flow was identified and more than one solution could be obtained. Furthermore, a conclusive analysis of the initial shut-in is difficult because of the short duration of the initial flow. The next section presents a new technique to interpret the initial shut-in, thus making the overall analysis of the test more reliable. Impulse Testing Description. Impulse testing consists of a short injection or production period, followed by a falloff or a buildup period. It production period, followed by a falloff or a buildup period. It requires accurate measurements of the pressure variations with time and of the total quantity of the fluid injected or produced. SPEFE P. 534