抄録
Geothermal energy is one of the most environment-conscious resources among the natural resources. Recently, the development of a supercritical geothermal system at great depth has been proposed to enhance the geothermal heat extraction. In order to design the supercritical geothermal reservoir whose temperature and pressure conditions exceed the critical point of water, the formation behavior of the geothermal reservoir under the great depth conditions has to be examined. In this study, we develop a new numerical analysis code for analyzing the hydraulic fracturing behavior in deep-seated rock mass. This code consists of two parts: "flow analysis" which computes the pressure distribution in the induced crack, and "crack propagation analysis". The former is based on a finite difference method. The later is based on a finite element method with embedded crack element. In the "crack propagation analysis", the mixed-mode fracture behavior with process zone formation is modeled. A shear dilation is accounted for in the fracture model. The numerical results show that the crack growth behavior, i.e. the mode of crack propagation depends on the depth. Under a typical tectonic stress condition, the crack growth mode is dominated by the opening fracture mode above the depth of 4 - 5 km, whereas the influence of the shear fracture mode increases with increasing the depth. This result may suggest that the current target of supercritical geothermal reservoirs may be formed mainly under the opening fracture mode.
