The Technical and Economic Feasibility of Enhanced Gas Recovery in the Eugene Island Field by Use of the Coproduction Technique
Authors: Arcaro D.P. , Bassiouni Z.A. 
Published: December 2007

Summary.
Coproduction, the simultaneous production of gas and water, is used to control water influx into water-drive gas reservoirs. The coproduction technique can increase gas recovery by as much as 20%. Reservoir selection criteria and study methodology are presented and illustrated with a U.S. gulf coast reservoir.
Basic material-balance analysis, tank-model simulation, and a preliminary economic analysis demonstrated the technical and economic preliminary economic analysis demonstrated the technical and economic feasibility of the process for a case study of the Louisiana gulf coast Eugene Island Block 305 10,300-ft [3140-m] -sand gas reservoir. This study also shows that the coproduction technique could result in a substantial increase in recovery efficiencies in many other water-drive gas reservoirs under specific economic conditions.

Introduction
Conventional production in a water-drive gas reservoir terminates when the producing wells load up with water, leaving high-pressure bypassed gas in the watered-out areas and in the gas cap updip of the watered-out wells. Generally, recovery from water-drive gas reservoirs is much lower than that from depletion-type gas reservoirs.
Water-drive gas reservoirs generally have much lower recoveries than depletion-drive reservoirs. In a water-drive gas reservoir, the reservoir pressure is maintained by the encroaching water. The stronger the water drive, the higher the reservoir pressure remains. Because residual gas saturation is independent of pressure, larger amounts of gas (residual gas) are trapped at the higher pressure than if a lower stabilization pressure could be pressure than if a lower stabilization pressure could be reached.
An existing technique that would benefit recovery in these strong water-drive gas reservoirs is the accelerated blowdown method. In essence, the concept is to out-run the water influx by producing gas at accelerated rates. Reservoir pressure is reduced before the aquifer can respond fully. Usefulness of the process, however, is limited in many cases. Deliverability controls because of sales contracts or production facilities may disallow high production rates. High-permeability reservoirs show production rates. High-permeability reservoirs show dampened efficiencies as a result of high water mobility. Reservoirs with considerable permeability variations show a reduced effectiveness of the method because of the uneven advancement of water. Water-coning and wellsanding problems may cause many operational problems. Often the scale of economic investment required problems. Often the scale of economic investment required to implement this process defers application.
In the proposed coproduction process, as the downdip wells begin to water out, they are converted to high-rate water producers, while the updip gas wells maintain gas production. The coproduction process enhances recovery production. The coproduction process enhances recovery in three ways:production of water lowers reservoir pressure, and more gas is produced because of expansion; pressure, and more gas is produced because of expansion;water production slows the advance of the water front; andpreviously immobile gas in the swept zone might become mobile again as the pressure is lowered.
The process is applicable in all moderate-to-active water-drive gas process is applicable in all moderate-to-active water-drive gas reservoirs. Reservoirs not yet watered out, however, present the greatest economic potential. present the greatest economic potential.

Coproduction Technique
The coproduction process is defined as the simultaneous production of gas and water. Initial attempts of enhanced production of gas and water. Initial attempts of enhanced gas recovery by coproduction focused on the depressurization of a totally watered-out reservoir by withdrawing large volumes of water. This process is technically feasible and economically applicable in some cases. In the case of unfavorable gas relative-permeability characteristics, however, extremely large volumes of water must be removed to mobilize the gas. Also, the cost involved to rework a shut-in field and to handle large amounts of two-phase gas and water production at high water/gas ratios might be prohibitive.
Our study directs the application of the process to waterdrive gas reservoirs not yet totally watered out. The process involves the conversion of downdip wells to water producers as they water out, while gas production updip producers as they water out, while gas production updip is maintained. The production of downdip water enhances recovery by (1) slowing the advance of the water front, thus delaying the watering out of wells, (2) reducing reservoir pressure so more gas can expand and be produced, and (3) reducing pressure in the swept zone so that previously immobile gas can expand and perhaps be produced. previously immobile gas can expand and perhaps be produced. The updip gas wells can, if warranted, be produced at a high rate, thus incorporating the benefits of the accelerated blowdown method.
JPT
P. 585