CHALLENGES IN THE ASSESSMENT OF ENHANCED GEOTHERMAL SYSTEMS RESOURCES

Resource assessment is an important component in the characterization and development of geothermal energy resources. Among the critical issues for assessing potential Enhanced Geothermal Systems (EGS) resources are (1) determining those conditions under which it is possible to replicate the high average permeability (10^-15 to 10^-13 m²) characteristic of natural hydrothermal reservoirs and (2) evaluating the heterogeneity of fracture permeability within EGS reservoirs and its influence on the geothermal recovery factor, R_g, which is defined as the ratio of produced thermal energy to the thermal energy contained in the stimulated volume comprising the reservoir. A variety of mechanical, chemical and thermal approaches to reservoir stimulation have been proposed and tested over more than three decades of research on EGS technology, with the primary focus at present on enhancing fracture permeability by elevating fluid pressure sufficiently to induce shear failure along pre-existing natural fractures. Models for the development of fracture permeability from the shear slip 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. Calibration of model parameters with results from EGS field experiments and demonstration projects suggests that sufficient permeability may be difficult to attain through shear stimulation at depths greater than approximately 6 km, particularly in regions characterized by high normal stress on pre-existing faults and fractures. In addition, the expected heterogeneity of fracture permeability within EGS reservoirs may limit R_g to values on the order of 0.05 to 0.1, which is at the lower end of the observed range for producing geothermal reservoirs. However, at present the application of these models is compromised by the limited number of cases in which model predictions can be compared to laboratory or in situ data. The key challenge for improved EGS resource assessments is acquiring and interpreting comprehensive laboratory and field data that can provide quantitative constraints on the recovery of heat from EGS reservoirs in diverse settings.