Evaluation Of An Energy Resource - Geopressured Geothermal Zone, Texas Gulf Coast
Authors: Bebout Don G. , Loucks Robert G. , Gregory A.R. 
Published: December 2007

Regional and detailed geological studies of the Frio, Vicksburg, and Wilcox Formations along the Texas Gulf Coast have been conducted in order to delineate areas favorable for the production of geopressured geothermal energy. These studies have been completed for the Frio Formation, and several geothermal fairways haze been identified that contain thick sandstone zones with temperatures higher than 300 deg. F. The Brazoria Fairway was found to meet best the specifications believed necessary for a successful test of the resource, and the Austin Bayou Prospect has been outlined in this fairway.

A geothermal test well is scheduled to be drilled in the Austin Bayou Prospect, and several geopressured sandstones will be tested between depths of 13,500 and 16,500 ft.

Introduction

Luring the last several years, estimates energy resource in place in the geopressured zone along the Gulf Coast have ranged widely from 100,000 to 3,000 quads (for comparison, the United States now consumes 20 quads of natural gas per year); the energy occurs in the form of hot water (geothermal) and methane dissolved in the water. These wide-ranging reserve estimates emphasize the lack of knowledge that now exists in the public domain concerning the geological and engineering aspects of the geopressured zone and resulted in the initiation of an extensive study of the resource in Texas conducted by The U. of Texas at Austin and funded by the U.S. Dept. of Energy, Div. of Geothermal Energy. The Bureau of Economic Geology has conducted research on the geology and environmental studies, and the Dept. of Petroleum Engineering and the Center for Energy Studies have guided engineering research and studies of legal, institutional, and sociological aspects. These studies were pursued in order to delineate areas favorable for production of geopressured geothermal energy. A prospective area must meet the following minimum requirements: reservoir volume of 3 cu miles, minimum permeability of 20 md, and fluid temperatures of 300 deg. F.

The geological phase of the geopressured geothermal project is divided into two main parts:the study of the regional distribution of reservoir sandstones in geopressured Frio, Vicksburg, and Wilcox Formations and identification of potentially prospective areas (fairways) that contain potentially prospective areas (fairways) that contain thick sandstones with fluid temperature higher than 300 deg. F andthe detailed study of these fairways in order to locate favorable areas in which to drill a test well.

Both parts have been completed for the Frio Formation, and the results are summarized in thus paper. A report including the details of this research has been published by the Bureau of Economic Geology.

GEOTHERMAL FAIRWAYS

Several geothermal fairways were identified from the Frio study (Fig. 1). The Hidalgo Fairway is located in Hidalgo, Cameron, and Willacy Counties and contains many thick, laterally extensive deltaic sandstone bodies with fluid temperatures greater than 300 deg. F, but extremely low permeabilities. The Armstrong Fairway, located in Kenedy County, contains a number of thick sandstones that extend over an area of 50 sq miles and have probable core permeabilities of 20 md, but fluid temperatures of less than 300 deg. F. The Corpus Christi Fairway, located primarily in Nueces County, contains sandstones with temperatures greater than 300 deg. F, but the sandstones are thin, are of limited lateral extent, and have low permeability. The Matagorda Fairways contain sandstones with high fluid temperatures, but the sandstone beds are thin and extremely limited in areal extent. In the Brazoria Fairway, the section deeper than 13,500 ft contains several hundred feet of sandstone with fluid temperatures greater than 300 deg. F (Fig. 2) and permeabilities between 40 and 60 md. The results of permeabilities between 40 and 60 md. The results of the fairway studies emphasize that the major limiting factor in all of the above fairways is finding adequate permeability in reservoirs with fluid temperatures of 300 deg. F. Only the Brazoria Fairway has recorded permeabilities higher than 20 md and, thus, meets all of the specifications for a geothermal prospect. prospect.