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 pressure, and formation permeability. These factors account for the abandonment of a number of gas reservoirs at extraordinarily high pressureSeveral 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 t pressure in the unsteady-state water-influx equation has to be corrected t 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. Introduction Several methods for predicting the depletion performance of waterdrive gas reservoirs have been published in the literature. Bruns et al. studied the effect of water influx on the p/z-vs.-cumulative-gas-production curves. From their study, they concluded p/z-vs.-cumulative-gas-production curves. From their study, they concluded that it is dangerous to extrapolate the p/z charts on a straight line without considering the possibility of water influx. Agarwal et al. used a material-balance model to study the effect of water influx on natural gas recovery. On the basis of their calculations, they concluded that gas recovery depends on production rate, residual gas saturation, aquifer strength, aquifer permeability, and the volumetric sweep efficiency of the encroaching water zone. Dumore predicted the future behavior of a bottomwater-drive gas reservoir. In his study, he neglected the total compressibility and assumed that all the gas in the reservoir and the free gas in the water-invaded zone was at the same average reservoir pressure and that the rising of the GWC remains horizontal all the time. On the basis of his study, he concluded that the straight-line relationship between p/z and Gp obtained from reservoir pressure data in the early life of the reservoir does not necessarily indicate a gas-depletion-type production mechanism but is consistent with a moderate water influx. The calculated p/z curve (in his example of a gas reservoir) deviated very little from the straight line until more than 50% of the initial gas in place had been produced. When the pressure dropped to 90 atm [9120 kPa], the calculated G, was about pressure dropped to 90 atm [9120 kPa], the calculated G, was about 85 % of the initial gas in place, and the GWC had risen to about 70% of the closure of the reservoir. In 1968, Knapp et al developed a two-phase, two-dimensional model to predict gas recovery from aquifer storage fields. The model was used to study the effects of heterogeneity, aquifer strength, and gas production rates. From the results of their study, they concluded that gas recovery is a function of gas production rate, aquifer strength, and heterogeneity. Their conclusions agree with those of Agarwal et al. regarding the gas production rate and aquifer strength. Shagroni studied the effect of formation compressibility and edge water on gasfield performance. On the basis of the results of his study, he concluded that it is incorrect to extrapolate the early part of the p/z vs. Gp. curves as a straight line to p/z=0.0 to part of the p/z vs. Gp. curves as a straight line to p/z=0.0 to estimate the initial gas is place without considering the possibility of water influx and the effect of formation compressibility, and that the sensitivity of the performance curve (p/z vs. Gp) to formation compressibility increases as the initial reservoir pressure increases. An experimental study of residual gas saturation under waterdrive was performed by Geffen et al. in 1952. The results of their experimental study indicated that residual gas saturation under waterdrive varies from 15 to 50% pore space, depending on the type of sand. Pepperdine used a mathematical model to study the performance of the Devonian gas fields in northeastern British Columbia. performance of the Devonian gas fields in northeastern British Columbia. From the results of the mathematical model and the analysis of the actual field data, he concluded that to achieve maximum gas recovery, the depletion process should be increased as much as possible by production practices, and that the important factor in the low efficiency of gas recovery was water influx rather than the coning phenomenon in the portion of the Clarke Lake field that was phenomenon in the portion of the Clarke Lake field that was modeled. Lutes et al studied the performance of a strong waterdrive gas reservoir (Katy V-C Reservoir in the U.S. gulf coast) with a modified material-balance equation that accounts for higher pressure in the water-invaded zone. They reached conclusions similar to that of Pepperdine regarding the effect of rapid blowdown of waterdrive gas reservoirs. They also monitored the advancement of the GWC of the Katy gas reservoir as it was put on accelerated blowdown, and an almost vertical front was observed. Givens used a simulation model to determine the effects of well density, production rates, water influx, water coning, and rock and fluid properties on the depletion performance of dry gas reservoirs with bottomwater drive. On the basis of his results, he concluded that the best performance reservoirs with bottomwater drive will not necessarily be obtained by high producing rates and that the presence of bottomwater drive in gas reservoirs lowers the ultimate presence of bottomwater drive in gas reservoirs lowers the ultimate recovery and increases the producing life of the gas reservoirs. The objective of this study is to predict the depletion performance of partial waterdrive gas reservoirs; particularly to study the effect of aquifer size, gas production rate, and initial reservoir pressure on the rate at which the GWC advances and on gas recovery. SPERE p. 380