大深部岩体における水圧破砕き裂進展挙動の数値シミュレーション法に関する研究

抄録

Geothermal energy is one of the most environment-conscious resources among the natural resources. Recently, the development of a supercritical geothermal system 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 condition 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 FDM. The later is based on FEM 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 result shows that the crack growth behavior, i.e. the mode of crack propagation changes from mode I to mode II as the depth increases. Under a typical tectonic stress condition, the crack growth mode is dominated by the mode I component above 4-5 km depth, whereas the influence of mode II component increases with increasing the depth. This result may suggest that the current target of supercritical geothermal reservoirs may be formed mainly under the mode I fracture.