Reserve Estimate for Geopressured and Water Drive Gas Reservoirs

Abstract

The accurate estimate of initial gas in place and gas reserves at an early stage is of great importance for decision making concerning the future development and management of gas reservoirs. For gas reservoirs at normal pressures and without water influx from surrounding aquifers, a plot of p/z versus the cumulative gas production Gp yields a straight line that can be extrapolated to give the initial gas in place at zero pressure or the gas reserves at the value of p/z corresponding to the abandonment reservoir pressure. In geopressured gas reservoirs where the reservoir pressures are abnormally high, rock and water compressibilities play important role in reservoir behavior at the early stage and the plot of the p/z vs Gp is no longer a straight line but rather a curve that is convex downward. Also, for gas reservoirs subject to water influx, the plot is a curve that is convex in the upward direction and tends to flatten and may stabilize at a constant pressure if equilibrium between gas withdrawal and water influx rates is achieved. In such cases, the conventional method to estimate initial gas in place and gas reserve is not applicable since no straight line is established.In this work the material balance equation with rock compressibility effect and water influx is considered and differentiated to obtain an expression for the slope of the tangent to the p/z - Gp curve. The value of the slope at the initial pressure is used to obtain a relation between the actual and extrapolated values of the initial gas in place for geopressured reservoirs and to obtain Gi directly for water drive reservoirs. The application of the method to estimate gas in place from field data for cases of geopressured and water drive gas reservoirs is outlined.