The pressure buildup behavior of multiple-layer systems that communicate only at wellbores may be different from that of single-layer systems. However, different properties of layered reservoirs cause such different pressure transient responses that there is no generally valid criterion pressure transient responses that there is no generally valid criterion for recognizing multiple-layer systems from transient tests.

Introduction

Pressure transient testing has been used in reservoir Pressure transient testing has been used in reservoir diagnosis for many years, yet, until quite recently, little attention was given to the transient behavior of layered systems with no crossflow (also called commingled systems). These are reservoir systems with two or more layers but with communication between the layers only through wellbores. The transient behavior of such systems can be much different from the behavior of single-layer system - or it can be very similar. Although there is information available about single-well, two-layer, closed circular systems in the literature, the information does not provide a valid general description of the transient pressure behavior of layered systems. The pressure behavior of systems of different geometry and layer properties varies from behavior similar to that for two-layer circular systems to behavior indistinguishable from that of single-layer systems. To illustrate this variation, we shall present simulated pressure buildup behavior for several layered-reservoir situations. The mathematically simulated results are for only a small fraction of the systems actually studied; they are presented as illustrative of the range of responses presented as illustrative of the range of responses seen.Information similar to some of the material presented here is available in Refs. 1, 4, and 7. Lefkovits presented here is available in Refs. 1, 4, and 7. Lefkovits et al. present an analytical solution for the two-layer problem, indicate several distinct differences between problem, indicate several distinct differences between one- and two-layer pressure buildup behavior, show that the time to pseudosteady state is much longer for two-layer systems than for single-layer systems, and suggest techniques for estimating permeability and average reservoir pressure. Cobb et al. present similar data and investigate various analysis techniques for pressure buildup tests in two-layer reservoirs with equal layer thickness and phi mu ct. Raghavan et al. later expanded this work to include layers of unequal thickness and provided a technique for estimating the layer permeability ratio from buildup test results. Refs. 1, permeability ratio from buildup test results. Refs. 1, 4, and 7 all consider a single well in the center of a closed, circular, two-layer system with constant and equal layer porosity; their main purpose is to illustrate behavior and propose interpretation techniques for those limited situations.Although we suggest some analysis techniques, it is not the goal of this paper to propose widely usable interpretative techniques for pressure buildup tests in noncommunicating layered reservoirs. Rather, we wish to show a wide variety of simulated buildup test results to illustrate that there is no general description for pressure buildup behavior in layered reservoirs. For completeness, we present some information for a closed square system that is similar to that given in Refs. 1, 4, and 7. However, results in this paper include a much wider range of conditions than reported previously. The number of layers, the porosities, the permeabilities, and the thicknesses are varied.

JPT

P. 1178

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