Abstract

As the search for additional natural gas continues, recovery of known reserves must be improved if the goal of avoiding shortages is to be met. One area requiring further industry attention for enhanced gas recovery is the reservoir associated with an active aquifer. In reservoirs of this type, pressure maintenance and entrapment of gas by encroaching water greatly reduce recovery.

This paper discusses the mechanism by which the aquifer reduces gas recovery and the magnitude of gas possibly left in place. Three methods of improving recovery are analyzed including (1) planned water production, (2) accelerated gas production and (3) production, (2) accelerated gas production and (3) gas replacement/displacement by other gas of lower economic value. The study shows criteria for technical and economic feasibility of the methods. Examples are used to illustrate the evaluation procedures. procedures

Introduction

Enhanced gas recovery has traditionally been used to describe methods of unconventional gas recovery from tight gas sands, Devonian shales, coalbed methane and methane from geopressured aquifers. These sources of gas have been estimated by experts to contain between 200 and 3,300 TCF of potentially recoverable gas in the United States. Both the industry and Government have addressed and actively pursued the development of these reserves. However, pursued the development of these reserves. However, numerous difficult issues, particularly the technology, risk and economics, remain barriers to progress in this direction. We are probably still far from commercially developing these reserves.

Meanwhile, conventional gas reserves continue to decline rapidly as demand soars. In the cases where reservoir pressure depletes naturally with gas production, recovery efficiency tends to be high, production, recovery efficiency tends to be high, often approaching 90 percent. Where there is water encroachment, recovery may be as low as 10 percent of the original gas in place. The remaining reserves would be unrecoverable unless external aid is provided to reduce the water influx and induce pressure provided to reduce the water influx and induce pressure depletion.

In the past, the natural gas price did not encourage wide application of external aid to produce these known reserves. With the increasing price and value of gas, recovery of such reserves should be given more attention than it currently receives.

Since huge investments may be required to enhance the recovery of these reserves, detailed studies requiring sophisticated evaluation approaches and computer applications are often required. The field engineer who is closest to the data and information on the reservoirs probably has the responsibility of initiating such studies. Unfortunately, his time is limited and routine operations may prevent him from acquiring and applying the necessary tools.

The aims of this work, then, are (1) to provide some information to the field engineer regarding the magnitude of gas reserves he may be leaving in his water drive reservoir, (2) to review the possible methods he can apply to improve his gas recovery, (3) to show procedures for initial reservoir evaluation which may be used as a basis for detailed studies, and (4) to discuss the economic analysis method necessary to justify the project.

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