Abstract

An analytical model is presented which predicts the performance of gas reservoirs producing under water-drive conditions. The model incorporates a modified water influx technique which accounts for pressure gradients and relative permeability effects pressure gradients and relative permeability effects across the water invaded region of the reservoir. Under certain conditions in gas reservoirs, these effects can cause significant deviations in predicted performance from that behavior projected using conventional water influx theory. The conditions necessary for this to occur and the improvements realized by using the modified approach are discussed in detail. In addition, the results of the analytical model are compared to solutions generated using a radial, numerical simulator.

Previous work has shown that for water-drive gas reservoirs, ultimate recovery increases with decreasing permeability, trapped gas saturation and aquifer size, and increasing fluid withdrawal rates. However, these parameters are all interrelated. Gas recovery cannot be determined based on one factor without considering the influence of the others. Thus, this paper details the development of new parameters which incorporate the key factors that influence gas recovery. These parameters describe the shape of the p/z performance curves for the reservoir and allow the p/z performance curves for the reservoir and allow the engineer to estimate the ultimate gas recovery for a particular reservoir/aquifer configuration. particular reservoir/aquifer configuration

Introduction

The significance of an active water drive on the ultimate recovery and p/z behavior of a gas reservoir has been investigated by numerous authors over the past few decades. Water encroachment reduces gas recovery by trapping residual gas behind the advancing water front at pressures higher than abandonment pressure under volumetric behavior. In a classic paper, Geffen, et al. discussed laboratory experiments which showed that this residual gas saturation could be as high as 50 percent under certain conditions.

In 1965, Bruns, et al. and Agarwal, et al. demonstrated the effect water influx has on the p/z versus cumulative gas produced curve for a gas reservoir. The authors illustrated that those factors which work to lower reservoir pressure and/or retard water influx will lead to an increase in ultimate gas recovery. Thus, a reduced permeability, a smaller aquifer or a greater gas withdrawal rate will all lead to a higher recovery factor.

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