ABSTRACT

Drilling of the first well designed to test geopressured geothermal resource for a sustained period of time was initiated in July 1978. Regional and site-specific geologic and engineering studies supportive of this site were conducted by the University of Texas Bureau of Economic Geology and Department of Petroleum Engineering/Center for Energy Studies with funds from the U.S. Department of Energy. The area sought during these studies needed reservoir volume of 3 cu mi (12.5 cu km), minimum permeability of 20 md, and fluid temperatures of 300°F (149°C). The Brazoria fairway, Brazoria and Galveston Counties, Texas, best met these specifications and the Austin Bayou Geothermal Prospect was developed in this fairway. Funds for drilling the Nos. 1 and 2 Pleasant Bayou geothermal test wells in the Austin Bayou Prospect were provided by DOE; the operator of the wells is General Crude Oil Co.

The geopressured sandstone reservoirs are in the lower part of the Frio Formation between the depths of 14,000 and 17,000 ft (4,200 and 5,100 m). The sandstone units are at the top of at least seven progradational deltaic cycles; cumulative thickness of all permeable sandstone units is 250 to 300 ft (75 to 90 m). Each deltaic cycle is composed of a gradational vertical succession characterized by low-permeability prodelta and distal delta-front sandstone and shale at the base grading to permeable distributary-mouth-bar and delta-plain sandstone and shale at the top.

Fluid will be produced from the 2 Pleasant Bayou well, stripped of methane and heat, then injected into Miocene sandstones at 6,000 to 7,000 ft (1,800 to 2,100 m) depth in the nearby 1 Pleasant Bayou disposal well. The produced water is expected to have salinities ranging from 50,000 to 80,000 ppm, temperature from 300 to 350°F (149 to 177°C), pressure from 10,000 to 15,000 psi (68,950 to 103,425 kPa) and 40 cu ft (1.2 cu m) of methane per barrel. The 2-year test period will evaluate the feasibility of long-term production of large quantities of water (ultimately 40,000 bbl/day) from geopressured reservoirs.

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