Natural Gas Production from Geothermal Geopressured Aquifers

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Abstract The energy crisis has resulted in an increased interest in the use of alternative energy sources for electric power generation. Geothermal energy is commonly mentioned as a supplement to existing and projected major sources of fuel. A unique form of geothermal energy exists in geopressured aquifers underlying the United States Gulf Coast. Water from such geopressured aquifers often contains natural gas. This paper deals with the production of natural gas from such reservoirs. production of natural gas from such reservoirs. We have classified geothermal geopressured reservoirs into three types based upon the natural gas content. In Type 1, the reservoir water is undersaturated with natural gas. However, it could still contain more than 40 standard cubic feet of natural gas per reservoir barrel of water. In Type 2, the reservoir water is fully saturated with natural gas and the reservoir may contain a small gas cap. Type 3 is a geopressured gas reservoir. The water is nearly immobile in the reservoir but the adjacent and underlying shales contain water with gas in solution, that may move into the reservoir. Producers have been most interested in Type 3. This paper presents computer studies of the production of multiphase fluids (natural gas, gas in solution and water) from geothermal geopressured reservoirs. The two phase, two dimensional simulator was developed for simulating reservoir production behavior of both nonisothermal and production behavior of both nonisothermal and isothermal deformable reservoirs, and can be used for areal or cross-sectional studies. Computer runs were made to generate a variety of data. For Type 1 and Type 2 reservoirs, reservoir pressure variations with natural gas and water production were generated. It was found that substantial amounts of natural gas can be produced over a long period of time. Where the water is hot enough the accompanying water may be used to generate electrical energy or for space heating or desalinated for agricultural purposes. The model was used to make areal studies of a bounded hypothetical geopressured gas reservoir, with no shale water influx using different compaction coefficients. It was found that BHP/Z versus cumulative gas production curve changed significantly with production curve changed significantly with increase in compaction coefficients. The model was also used to make cross-sectional studies to assess the effect of shale water influx from adjacent and underlying shales. Finally the model was used for simulating the reported production history of Anderson 'L', a geopressured gas reservoir in south Texas. Good agreement was obtained between the observed BHP versus cumulative production as well as BHP/Z versus cumulative production.