Creating Cloud‑Fracture Network by Flow‑induced Microfracturing in Superhot Geothermal Environments

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This article is a summary of the paper already published in the “Rock Mechanics and Rock Engineering” (Goto et al., 2021). Superhot geothermal environments exceeding 400 ºC at depths of ~2-4 km are seen as a geothermal frontier. However, there are concerns that the network of permeable fracture is possibly absent in a continental granitic crust because thermally relevant factors which can be impair the formation of the permeable fracture network. Hydraulic fracturing is a hopeful technique to exploit the geothermal energy from such environments via establishing a superhot enhanced geothermal system. Our previous study already revealed that a dense network of permeable fractures, referred to as a cloud-fracture networks, possibly formed by superhot hydraulic fracturing. Although the formation cloud-fracture network is seemed to be caused by the infiltration of low-viscosity water into preexisting microfractures according to our previous study. However, the detailed formation process of cloud-fracture network and the plausible criterion for cloud-fracture network formation is yet to be clarified. The applicability of the Griffith failure criterion is supported by hydraulic fracturing experiments with acoustic emission measurements of granite at 400 ºC under true triaxial stress and at 450 ºC under conventional triaxial stress. The present study provides, for the first time, a theoretical basis required to establish the procedure for hydraulic fracturing in the superhot EGS.