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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.

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