Pulse-Test Response of a Two-Zone Reservoir

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Woods, E.G., Member AIME, Esso Production Research Co., Houston, Tex. Abstract A mathematical investigation of pressure response of two-zone reservoirs indicates apparent transmissibility (kh/ ) obtained by pulse testing is always equal to or greater than the total transmissibility of the zones, and that apparent storage (phi ch) is always equal to or less than the total storage of the zones. These apparent zone properties approach total properties as vertical fluid communication between zones increases. The presence of non uniform wellbore damage in the zones alters the division of flow between zones, and consequently, alters their apparent transmissibility ratio. In the absence of wellbore damage. the flow-rate ratio is a good estimator of the transmissibility ratio of the zones. A procedure is proposed for advantageously using differences in reservoir properties determined by single-well tests and pulse tests to describe flow properties of two-zone reservoirs. A numerical properties of two-zone reservoirs. A numerical example is included. Introduction Pulse tests, interference tests, and single-well pressure buildup or drawdown tests have been used pressure buildup or drawdown tests have been used to estimate reservoir properties. These pressure transient tests are normally analyzed with mathematical models which assume that the reservoir is a homogeneous single layer. Various techniques for analyzing single-well test data to obtain information about the properties of layered reservoirs have been shown by others to have limited applicability. This mathematical study was undertaken to determine what errors could be caused by interpreting pulse tests (in a multizone reservoir) with a single-layer model. Pulse testing is based on the measurement and interpretation of a pressure response in one well to a transient pressure disturbance introduced by varying flow rate at an adjacent well. The measured pressure response is usually a few hundredths of a pressure response is usually a few hundredths of a pound per square inch. Pulse-test terminology is pound per square inch. Pulse-test terminology is shown in Fig. 1; Johnson et al. give a complete description of pulse testing. Measured at the wellhead or in the wellbore, pressure response is a function of reservoir pressure response is a function of reservoir transmissibility (T=kh/mu) and diffusivity (n = k/phi cmu) in the region between the two wells; from these two quantities reservoir storage ( = /n=phi ch) can be derived. The analysis presented here discusses additional reservoir information made available by pulse testing and shows that single-well test and pulse-test results can be combined to give more information about a two-zone reservoir than either type of test alone. Also, procedures are given for estimating the magnitude of error if test results of a two-one reservoir are interpreted with the assumption that it is a one-zone, vertically homogeneous, reservoir. Discussions of theoretical work, field data requirements, interpretation procedure, and a numerical example follow. Details of the mathematical model are given in the Appendix. THEORETICAL STUDY - TWO-ZONE MODEL Reservoir Model - Assumptions and Boundary Conditions A reservoir model consisting of two zones penetrated by two wells, each of which is completed in both zones was assumed (Fig. 2). SPEJ p. 245

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