A variety of mechanical, chemical and thermal approaches to reservoir stimulation have been proposed and tested over more than three decades of research on Enhanced Geothermal Systems (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. A critical issue in assessing the potential EGS resource is quantifying Rg, the geothermal recovery factor, which is defined as the ratio of produced thermal energy to the thermal energy contained in the fractured volume comprising the reservoir. One approach to EGS resource assessments incorporates the assumption that a constant amount of thermal energy is recovered during the life of a project, regardless of the temperature of the reservoir, thereby concluding that there is a decrease in Rg with increasing reservoir temperature and a reduced potential associated with deep, higher temperature resources. By contrast, production experience and simulations of thermal energy recovery from naturally fractured geothermal reservoirs indicate that Rg, which typically falls in the range from 0.05 to 0.2, is primarily a function of internal reservoir structure, not temperature. Because the thermal energy content of the crust increases linearly with increasing temperature, if the characteristics of Rg for naturally fractured reservoirs apply to EGS reservoirs, proportionally greater resource potential is associated with the deeper, hotter portions of the Earth’s crust, despite the costs and challenges associated with creating and exploiting reservoirs at greater depths and higher temperatures. However, other aspects of production from deep, hot EGS reservoirs need further evaluation, such as the relative effects on productivity of declining fluid viscosity with increasing temperature, fracture closure at higher levels of effective stress, and the increased rates of mineral precipitation and dissolution at higher temperatures. These aspects may limit the viability of deep EGS resources.

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