SPE Members Abstract A detailed engineering and geologic evaluation of an offshore Gulf Coast gas reservoir with water influx is presented. The study was undertaken to analyze various production management strategies in order to optimize the ultimate recovery of the reservoir given the detrimental effects of the water influx Without implementing any reservoir management techniques, the recovery factor of the reservoir is estimated at 66%, much lower than would be expected under volumetric depletion performance. It is demonstrated that producing high volumes of water from downdip wells and adding an additional well high on the structure can significantly increase the ultimate gas recovery from the reservoir. This is achieved by lowering the reservoir pressure which liberates trapped residual gas and by recovering mobile attic gas. However, accelerated gas production does not appear to be beneficial in this particular case due to a reduced volumetric sweep efficiency associated with the accelerated rate case. Economic analyses show that recompletion of an additional well at a higher structural position is the optimum strategy for this particular reservoir. Due to the limited extent of the aquifer, this single well will effectively lower reservoir pressure, liberate gas trapped at residual saturation and recover mobile gas remaining at the top of the structure. Introduction Many natural gas reservoirs located throughout the world experience water influx and water production. The types of reservoirs in which this occurs and the effect this influx has on ultimate recovery is variable in terms of depositional environment and reservoir characteristics. Significant water influx tends to maintain reservoir pressure and reduce ultimate recovery by trapping residual gas at high pressure. Active water influx can also promote water coning and areal cusping, which increases individual well water production and traps gas through reduced volumetric displacement efficiency. Ultimate recovery in this type of reservoir is largely a function of reservoir management. There are several techniques an operator can use in an attempt to increase the ultimate recovery from a waterdrive gas reservoir. Accelerated gas production, selective recompletion of existing wells or the drilling of new wells, and continued production of certain wells at very high water cuts may all result in increased recovery. In many cases, it is necessary to consider the economics of the project not on a well by well basis but on an overall field basis. Any action taken which increases the ultimate gas recovery of a reservoir obviously has the potential for increasing the net present value of that reservoir as well. P. 9^

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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.