Abstract

Active water influx into a gas reservoir reduces ultimate recovery from that which would be expected under volumetric conditions due to reduced sweep efficiency and residual gas which is trapped at high pressure. Previous investigations on Gulf Coast gas fields presented in the literatures have demonstrated reservoir management techniques which can increase the expected ultimate recovery over 10% of the original gas in place. This paper presents new work in the form of a detailed case history which extends this emerging technology into aquifer gas storage reservoirs. By their nature, aquifer gas storage fields are water-drive reservoirs. Thus, prudent reservoir management is required to reduce the detrimental effects of trapped gas and poor volumetric sweep.

A detailed reservoir characterization and numerical simulation study are presented for a midcontinent aquifer gas storage field. It is demonstrated that rate optimization during both injection and withdrawal cycles can significantly improve the performance of the storage reservoir. Performance improvements are realized in the form of a larger working volume of gas, a reduced cushion volume of gas, and a decrease in field water production. This research has significant implications for the business facet of the natural gas industry. By utilizing these reservoir management techniques, gas storage operators will be able to minimize their base gas requirements, improve their economics, and determine whether the best use for a particular storage field is base loading or for meeting peak day requirements.

Introduction

Since the 1960's, the detrimental effects of water encroaching into gas reservoirs have been well documented. Significant water influx reduces gas recovery by trapping residual gas at high pressure. Water influx can also promote vertical coning and areal cusping of water which traps mobile gas through reduced volumetric displacement efficiency. Gas recovery in this type of reservoir is largely a function of reservoir management.

Over the past few years much attention, predominantly through GRI funded research, has been devoted to field operating techniques which optimize ultimate hydrocarbon recovery in the presence of an invading water front. As part of that research, this study was conducted in an effort to extend existing technology and to develop new technology for aquifer gas storage fields. Specifically, the evaluation addresses the impact of various reservoir management strategies on peak day field rates, cycled gas volumes, and water handling requirements.