Geopressured geothermal reservoirs are characterized by high temperatures and high pressures with correspondingly large quantities of dissolved methane. In many cases, the reservoirs are comprised of multilayer systems of thin sandstones and thicker shales. Below the low permeability shale layers, large quantities of gas may have accumulated over time. Two methods utilizing reservoir simulation techniques have been used to estimate recoverability factors of geothermal brine and methane based on well log data from a specific reservoir in Texas. The first assumes a simplified reservoir that has three layers: upper shale, sandstone, and lower shale with the sandstone layer thickness equal to the net sandstone in the reservoir interval. The second method uses a detailed reservoir model that accounts for multiple sandstone and shale layering. This method includes 12 layers of sandstone or shale with the layer depths determined from a well log. These two methods are used to answer the question on the sensitivity of the results to the level of detail that is included in the reservoir model. Based on a comparison of the recovery of fluid and methane from the detailed and simplified model, the influence of incorporating reservoir heterogeneity was determined. It was found that incorporating multiple thin layers of lower permeability sandstone can noticeably impact the results of the reservoir simulation. The heterogeneous model resulted in greater flow rates of both geothermal brine and total methane. Both models demonstrate that the geopressured geothermal reservoir is capable of producing hot geothermal fluid at flow rates over a long duration that are sufficient for electricity production from binary power plants. The results indicate that simplified models of geopressured geothermal reservoirs that approximate actual reservoir details can be applied to give a reasonable, albeit conservative, estimate of the recoverable resource over broad areas using generalized data sets.
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