A Summary of the Geothermal and Methane Production Potential of U.S. Gulf Coast Geopressured Zones From Test Well Data (includes associated papers 16455, 16460, 16480, and 16521 )

Abstract

Summary Extensive testing in 12 wells in Texas and Louisiana has produceda significant body of production and test data from the U.S. gulf coastgeopressured zones. Results indicate that these deep sediments containsome reservoir-quality sandstones capable of extended production ofmoderately hot brine saturated with dissolved natural gas. The best of thetest wells to date, the U.S. DOE Pleasant Bayou Well 2 in Brazoria County, TX, flowed at a sustained rate of about 18,000 B/D [2860 m3/d] for about 7months. The reservoir may cover an area as large as 58 sq miles [150 km2]. None of the other 11 wells equaled the performance of the PleasantBayou test. From an economic standpoint, dissolved methane shows littlepromise of competitive performance in the foreseeable future. As a promise of competitive performance in the foreseeable future. As a geothermal energy source, the temperatures are disappointingly low. Theresource overall appears to be large but diffuse. Introduction Hottman's 1966 patent apparently represents the firstformal recognition of the deep gulf coast sediments, known as geopressured zones, as an energy resource. Thepatent claims that under certain conditions, overpressured patent claims that under certain conditions, overpressured formations (i.e., geopressured formations) are capable ofproducing hot water and dissolved methane gas under producing hot water and dissolved methane gas under natural flow. The patent was assigned to Hottman'semployer, Shell Oil Co., and no specific implementationsteps were ever reported. In the years following, the resource received theattention of the U.S. Geological Survey (USGS). The USGSwork was documented in a series of publications preparedby Jones, and his associates during 1968–74. After the OPEC oil embargo of 1973, geopressure, along with other alternative energy sources, began toreceive relatively large research support from the U.S.DOE and its predecessors. That work culminated in aseries of well flow tests, the results of which have removedmuch of the speculation surrounding the resource. Description of Geopressured Sediments In the gulf coast basin, as in many similar geologicalstructures throughout the world, buried sediments aresaturated with water, generally under hydrostatic pressure. Under certain conditions, however, the fluid may betrapped within the pore spaces, either by faulting ordepositional conditions, and attain pressures approaching lithostatic. Such formations are said to be geopressured. Along the Texas and Louisiana gulf coast, geopressuredconditions are general at depths below about 10,000 ft[3050 m]. Geological formations in these horizonsconsist of sandstone and shale; the latter accounts for about90% the total volume. Where reservoir-quality sandstonesexist in this sequence, production wells are potentially capable of flowing water at high rates. For reasons associated with the depositional history, formation temperatures are also higher than normal andconstitute some geothermal potential, Perhaps mostimportant is the dissolved methane with which the containedwater is apparently saturated. Questions about theusefulness of the resource involve primarily productioneconomics, including the producibility and production lifeof the potential reservoirs, the temperature of the producedwater, and the quantity of producible methane containedin solution. While geopressured conditions are known toexist over an area covering at least 100,000 sq miles[259 000 km2], the magnitude of the technically andeconomically producible fraction of the resource is open toconsiderable doubt. Estimates of production costs havenot been encouraging. Requirements of a GeopressuredProduction Well Production WellOn the basis of early study of the resource, pioneerresearchers at Louisiana State U. (LSU) considered theproduction of geopressured aquifers. In a classic paper production of geopressured aquifers. In a classic paper on the subject, Parmigiano, using established principlesof petroleum reservoir engineering, developed methodsfor predicting the performance of single wells or of wellsin clusters under geopressured conditions. JPT P. 1365