Variable Rate Pressure Drawdown Analysis for Gas Wells

Abstract

ABSTRACT Pressure transient tests of gas wells, in general, are peculiar in their nature. They possess additional degree of difficulty when compared with the oil well tests. The peculiarity of gas well tests over oil well tests is due to the non-linearity of the diffusivity equation and to the presence of non-Darcy flow or rate-dependent skin effect around the wellbore. Moreover, the constant flow rate requirement in conventional pressure drawdown tests adds another dimension of difficulty to the tests, because this condition is hard to fulfill in field applications. In recent years, several approaches have been proposed to solve these problems. For instance, normalized pseudopressure and pseudo-time have been used to linearize the diffusivity equation. Variable rate tests were proposed as an effective substitute for single point test. Trial-and-error procedure or at least two constant rate tests were used to determine the rate-dependent skin. Even though the trial and error procedure and the two rate tests provide good estimates of the non-Darcy skin, they are time consuming and hard to apply in practice. This paper presents a simple technique to analyze variable flow rate data of gas wells with turbulence and damage. The proposed method uses normalized pseudo-variables to avoid the non-linearity in the diffusivity equation and it is straight-forward in the sense that it does not require a trial-and-error procedure or two rate tests. The method involves the construction of two different plots. The formation permeability is obtained from the slope of the first plot, whereas the inertial turbulent flow coefficient and the mechanical skin factor are obtained from the slope and intercept of the second plot. The proposed method is simple and provides accurate and reliable results in a short testing period when compared with other methods. A field case and a simulated example are used to illustrate the applicability of the proposed method.