--> Evaluating the potential for Enhanced Geothermal Systems development in the United States

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Evaluating the potential for Enhanced Geothermal Systems development in the United States

Abstract

The successful implementation of Enhanced Geothermal Systems (EGS) technology has the potential to dramatically expand both the magnitude and spatial extent of geothermal energy production in the United States. Although several outstanding scientific and technical issues must be addressed before EGS becomes a viable option in future energy planning, field and laboratory experiments, modeling, and pilot projects both in the United States and overseas are providing a framework for the U.S. Geological Survey (USGS) to implement an improved methodology for updating the EGS resource assessment of the United States. Prominent among the issues for the EGS resource assessment are (1) determining those conditions under which it is possible to replicate the high average permeability (approximately 10-14 to 10-12 m2) characteristic of natural hydrothermal reservoirs, (2) evaluating the likely heterogeneity of fracture permeability within EGS reservoirs and its influence on the geothermal recovery factor, which is the ratio of produced thermal energy to the thermal energy contained in the stimulated volume comprising the reservoir, and (3) improving estimates of temperature in the upper crust to better quantify the thermal energy available at those depths viable for EGS reservoir creation. Models for the development of fracture permeability induced by hydraulic stimulation indicate that production from EGS reservoirs will be sensitive to the influence of effective stress and rock properties on the processes of shear fracture formation and closure. Comparisons of model parameters with results from EGS field experiments and demonstration projects suggest that sufficient permeability may be difficult to attain through shear stimulation at depths greater than approximately 5 km, particularly in regions characterized by high normal stress on pre-existing faults and fractures. As a result, away from areas of hydrothermal and volcanic activity with anomalously high temperatures at shallow depths, the most promising targets for EGS development in crystalline bedrock are in areas characterized by both high conductive heat flow and extensional to strike-slip stress regimes. Although there are gaps in the spatial coverage of heat-flow measurements in much of the United States and uncertainty in the estimation of thermal properties at depth, analysis of the existing thermal data indicates that even with the possible constraints outlined above, large areas of the western United States will be suitable for EGS development. The key remaining challenge for improving our understanding of the EGS resource is acquiring and interpreting comprehensive laboratory and field data to provide quantitative constraints on the creation and operation of EGS reservoirs that can be translated into commercial engineering practice. The Department of Energy (DOE) Frontiers of Research in Geothermal Energy (FORGE) initiative for a dedicated EGS test site is an important step forward in addressing this challenge.