Effect of Aquifer Size on the Performance of Partial Waterdrive Gas Reservoirs

Abstract

Summary Predicting the advancement of a gas/water contact (GWC) in a waterdrive gas reservoir plays an important role in evaluating, forecasting, and analyzing the reservoir performance. This study was conducted to predict the behavior and the rise of the GWC, assuming that it remains horizontal, and to determine its effect on ultimate gas recovery. Several factors control the rise of the GWC. Some of the most important factors are the size of the aquifer, gas production rate, initial reservoir pressure, and formation permeability. These factors account for the abandonment of a number of gas reservoirs at extraordinarily high pressure. Several methods have been developed for predicting the volume of water influx into a reservoir; the van Everdingen-Hurst method is used in this study. The performance calculated in this study was based on the material-balance equation for gas reservoirs. The gas reservoir pressure was adjusted to the original GWC for the water-influx equation, and the trapped gas in the water-invaded zone was accounted for in the water-invaded region. A constant reservoir permeability of 300 md was used in all calculations. The results showed that when ra/rg≤2, the effect of the aquifer on gas reservoir performance can be neglected. Also, the rate at which the GWC advances is controlled by the aquifer size when ra/rg >2. Finally, regardless of the size of the reservoir, when ra/rg>2, the pressure in the unsteady-state water-influx equation has to be corrected to the original GWC. Failure to do so may result in an error of more than 100% in the cumulative water influx, which in turn could lead to the wrong conclusions regarding the performance of the gas reservoir.