ABSTRACT

Tertiary strata of the Texas Gulf Coast include several terrigenous depositional wedges, some of which thicken abruptly at their downdip ends as a result of contemporaneous movement of growth faults and underlying salt. The Frio Formation is one of these wedges.

Broad regional studies, followed by detailed local investigations, were pursued in order to delineate prospective areas for production of geopressured geothermal energy. A prospective area must meet the following minimum requirements: reservoir volume of 3 cu mi (12.5 cu km), minimum permeability of 20 md, and fluid temperatures of 300°F (149°C). Several geothermal fairways were identified as a result of the Frio study. Only the Brazoria fairway, however, meets all of the specifications for a geothermal prospect.

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 site is located within the Austin Bayou prospect, Brazoria fairway. The reservoir will consist 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 depositional sequences. Each sequence is composed of a gradational vertical succession 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 depositional sequences represent the distal half of a lobate delta, and the later events represent the entire deltaic complex.

More than 10 billion bbl of water are in place in these sandstone reservoirs of the Austin Bayou prospect; there should be approximately 400 Bcf of methane in solution in this water. Only 10% of the water and methane (1 billion bbl of water and 40 Bcf of methane) will be produced without reinjection of the waste water into the producing formation. Reservoir-simulation studies indicate that more than 50% of the methane can be produced with reinjection.

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