| Abstract |
Resource assessment and classification is a key element in the characterization and development of energy resources, including geothermal energy. Stakeholders at all levels of government, within the geothermal industry, and among the general public need to be able to understand basic characteristics of the geothermal resource, such as location, quality, feasibility of development, and potential impacts. 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/Engineered 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. Recent EGS resource assessments have incorporated one of two approaches. The first approach is based on an analogy with thermal energy recovery from naturally fractured geothermal reservoirs and sets Rg as a constant with a mean value of 0.05. The second approach assumes that a constant amount of thermal energy is recovered during the life of a project, regardless of the temperature of the reservoir. Models for the development of fracture permeability from hydraulic stimulation indicate that production from EGS reservoirs will be sensitive to the processes of shear fracture formation and closure at high levels of effective stress. The implications of these models are that reservoir performance (and consequently Rg) will also depend on in situ stress, depth, and rock properties. This dependence may limit the viability of EGS resource development at great depth in the crust and needs to be incorporated in EGS assessment methods. |