Modified Power Law Exponential Decline for Tight Gas

Abstract

Traditional decline methods do not work for tight gas wells. A hyperbolic decline is often used for forecasting the production rate and estimating the expected ultimate recovery (EUR) of a gas well. This method is only valid when the well is in the Boundary-Dominated Flow (BDF) regime, and hyperbolic b-values are in the range of 0 to 1. However, most of the production data from tight gas reservoirs is in the transient flow regime and not in the BDF regime. This results in values of b bigger than 1, and a forecast which can significantly overpredict the reserves. The recently developed “power law exponential decline” method seems to be promising in predicting the production rate over both transient and BDF regimes. In this paper, we examine the applicability of the power law exponential decline for different cases. We evaluate its behavior during both the transient and boundary-dominated flow regimes, and specifically for linear and radial transient flow. In the BDF regime, we also study the effect of reservoir size on the decline parameters. We compare the power law exponential decline with the decline from analytical reservoir models, and we modify it accordingly. Furthermore, the sensitivity of the power law equation to any of its parameters is studied. The validity of the power law exponential decline in forecasting production in tight gas wells is tested using synthetic data.