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Estimates for the total gas resource in place in geopressured Tertiary sandstone reservoirs along the Gulf Coast range from 3,000 to 100,000 Tcf (85 to 2,832 trillion cu m). This wide range in estimates was the incentive for initiating an extensive research effort in Texas and Louisiana to obtain more reliable data on geologic, engineering, environmental, legal, and social aspects of developing the geopressured resource. These studies include investigation of the available heat and hydraulic energy present in these aquifers in addition to the methane. All resource calculations are based on interpretations of total sandstone thickness, lateral extent of reservoirs as defined by depositional and structural boundaries, porosity and permeability, reservoir drive, salinity, te perature, pressure, and methane solubility. Diverse estimates arise from inadequate knowledge concerning these critical parameters.
To obtain answers to the many questions, an extensive research program has been established at The University of Texas at Austin by the Bureau of Economic Geology and the Center for Energy Studies. The bureau has been conducting broad regional geologic studies (for resource assessment and geothermal fairway delineation)
and detailed, site-specific studies (to identify potential test-well sites).
The Tertiary strata of the Texas Gulf Coast comprise a number of terrigenous depositional wedges which dip and thicken into the gulf basin. Some of the wedges, Wilcox, Vicksburg, and Frio, thicken abruptly in a downdip direction as a result of contemporaneous movement along growth faults which developed near the ancient shorelines. Thick sections of sand and mud accumulated on the down side of these growth faults; expulsion of water from these downfaulted sediments was impeded by the faults and, with increased burial and overburden, high fluid pressures characteristic of this geopressured zone developed. The lack of water circulation in these geopressured reservoirs resulted in the increase in the temperature gradient from approximately 1°F/100 ft (1.9°C/100 m) in the hydros atic zone to 2°F/100 ft (3.7°C/100 m) in the geopressured zone.
The regional studies, followed by detailed local investigations, were pursued to delineate prospective areas for production of geopressured energy. A prospective area must meet the following minimum requirements: (1) reservoir volume of 3 cu mi (12.5 cu km), (2) minimum permeability of 20 md, and (3) fluid temperatures of 300°F (149°C). Several geothermal fairways were identified in the Frio Formation and Wilcox Group as a result of these studies. Only the Brazoria fairway, however, meets all of the specifications for a geothermal prospect in the Frio. The DeWitt fairway best meets the requirements in the Wilcox.
In the Brazoria fairway, located in Brazoria and Galveston Counties, Texas, several hundred feet of deltaic sandstones have fluid temperatures greater than 300°F (149°C). Permeabilities within these reservoirs are greater than 20 md; this high permeability is related to secondary leached porosity, which developed in the moderate to deep subsurface. The geothermal test well (Department of Energy and General Crude Oil 2 Pleasant Bayou) is located within the Austin Bayou prospect, Brazoria fairway. The reservoir consists of 250 to 300 ft (75 to 90 m) of sandstone with core permeabilities between 40 and 60 md and fluid temperatures from 300 to 350°F (149 to 177°C). The sandstone-shale section within the Austin Bayou area is represented by seven progradational sequences Each sequence is characterized by low-porosity prodelta and distal delta-front shale and sandstone at the base grading to porous distributary-mouth bar and delta-plain sandstone and shale at the top. The older and deeper depositional sequences represent only the distal part of the lobate delta, and the later events represent the entire deltaic complex. The 2 Pleasant Bayou well has been completed to total depth of 16,500 ft (5,029 m) and testing will begin during the last half of 1979.
A proposed well in the DeWitt fairway, DeWitt County, Texas, will test the deep sandstone reservoirs in the lower part of the Wilcox Group. More than 300 ft (90 m) of deltaic sandstone is present within a fault block 3 mi (5 km) wide and 15 mi (24 km) long.
The Pleasant Bayou geopressured geothermal test well is expected to provide the first reliable data concerning many aspects of producing energy from the water in the geopressured zone. However, this single well will not answer all of the questions nor prove or disprove the feasibility; several additional wells are planned for both Texas and Louisiana.
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