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 withlarge pressure changes accompanied by a severe wellbore storage effect, such asin tight reservoirs, use is made of pseudopressure and pseudotime transforms. The purpose of the pseudovariables is to linearize "effectively" thegoverningdiffusivity equation so that proper test interpretation can bemade. In this study, normalized pseudovariables or transforms are introduced. Useof these new pseudovariables produces results identical to those obtained withthe conventional pseudovariables. However, the proposed pseudovariables offersome distinct practical advantages. For example, units of pressure and time areretained, thus giving a physical "feel" for the pseudovariabledefinitions. Other features include use of liquid equations for solving the gasflow, 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 firstmethod involves logarithmic convolution of pressure with flow rate, bothmeasured downhole by use of a radial model. A trial-and-error approach is usedto obtain the rate-dependent skin on a convolution plot. The second methodrequires a transient flow-after-flow test of short duration: a standard graphictechnique allows the estimation of the desired reservoir parameters. Once the reservoir parameters are estimated with either method, the absoluteopen-flow potential (AOFP) of a well can be established by assuming a reservoirradius because the AOFP is relatively insensitive to this assumption.