Transient pressure analysis of gas wells producing at constant pressure

Abstract

A comprehensive investigation of the validity of applying the constant-pressure liquid solution to transient rate-decline analysis of gas wells is presented. Pseudo-pressure, non-Darcy flow effects, and formation damage are incorporated in the liquid solution theory to simulate actual real gas flow around the wellbore. The investigation shows that for constant-pressure gas production, the conventional semilog plot of the inverse of the dimensionless rate versus the dimensionless time used for liquid solution must be modified to account for high-velocity flow effects. Especially when reservoir permeability is higher than 1 md and the well test is affected by non-Darcy flow and formation damage. In addition, a systematic method for determining formation permeability, mechanical skin factor, and non-Darcy flow coefficient from a single constant-pressure production test also is presented. The working equations are written to allow graphical analysis of the variable rate with time that is analogous to analysis of the constant-rate production test. The procedure is simple and straightforward. It does not require type-curve matching or correlations. The applicability of the proposed method is illustrated using several simulated examples. The input formation permeability varies from 0.1 to 5 md. The ratio of the bottomhole pressure to the initial reservoir pressure ranges from 0.1 to 0.8.