Geothermal Potential of Deep Sedimentary Basins in the United States

Abstract

Geothermal energy development has a promising future as part of a broad energy supply mix to meet growing demand in the United States and globally. Currently developed hydrothermal systems are a significant energy source, but these systems have limited geographic extent. Research is underway, including ongoing pilot projects, to evaluate the potential for EGS, or Engineered Geothermal Systems, to drill into hot crystalline rock, and create fractured reservoirs suitable for water injection and production cycles. However, a challenge to the economics of these systems is the drilling and fracturing cost. Co-production of geothermal energy associated with oil operations has been demonstrated successfully at Teapot Dome, where produced water is of sufficient quantity and adequate temperature to generate electricity with binary/hybrid systems. However, this approach has yet to be embraced by the oil industry. A potential new path toward expanded geothermal energy production is to use known porous and permeable reservoir rocks in appropriate sedimentary basins, where those packages of rocks have sufficient temperature, thickness, porosity, and permeability, existing at depths that are not so great that drilling costs make the potential system uneconomic. This presentation describes a DOE-funded project to identify, screen, and model these potential systems, incorporating geology, engineering, and economic modeling disciplines. From a geologic perspective, 17 basins in the western U.S. have been examined. Stratigraphic columns were compiled, including unit depths and thicknesses, along with thermal profiles. Target reservoir sections at appropriate depths and temperatures have been evaluated with respect to porosity and permeability, primarily from available core data, supplemented with wire-line log analysis. For screening purposes, thresholds of 125 °C temperatures were applied to meet economic targets. Results indicate that many of those basins should be excluded, for example, the Bighorn Basin of Wyoming has favorable porous and permeable reservoir rocks and good temperatures for geothermal energy production, but these occur at nearly 6 km, too deep for economic drilling costs. The temperature at the 4 km threshold is only 100-110 °C, in the marginal range for binary geothermal power systems. Based on this work, basins meeting the criteria are the Williston, Denver, Great Basin, Fort Worth, Sacramento, Gulf Coast, and Imperial Valley.