Gas Well Test Analysis: Use of Normalized Pseudovariables

Abstract

Summary Pressure data from gas wells are normally analyzed by use of pseudopressure, which is a transformation of measured pressure. When tests are associated with large pressure changes accompanied by a severe wellbore storage effect, such as in tight reservoirs, use is made of pseudopressure and pseudotime transforms. The purpose of the pseudovariables is to linearize "effectively" the governing diffusivity equation so that proper test interpretation can be made. In this study, normalized pseudovariables or transforms are introduced. Use of these new pseudovariables produces results identical to those obtained with the conventional pseudovariables. However, the proposed pseudovariables offer some distinct practical advantages. For example, units of pressure and time are retained, thus giving a physical "feel" for the pseudovariable definitions. Other features include use of liquid equations for solving the gas flow problems—i.e., constants of the working equations remain unchanged. This study also proposes two methods for estimating the rate-dependent skin, together with permeability, mechanical skin, and well deliverability. The first method involves logarithmic convolution of pressure with flow rate, both measured downhole, by use of a radial model. A trial-and-error approach is used to obtain the rate-dependent skin on a convolution plot. The second method requires a transient flow-after-flow test of short duration; a standard graphic technique allows the estimation of the desired reservoir parameters. Once the reservoir parameters are estimated with either method, the absolute open-flow potential (AOFP) of a well can be established by assuming a reservoir radius because the AOFP is relatively insensitive to this assumption.