The enhanced geothermal system (EGS) has a great potential to generate electric power from a rock mass with a relatively low initial permeability in both conventional and superhot/supercritical (> ca. 400°C) geothermal environments. Creating a permeable fracture network is critical for efficient exploitation of geothermal energy from the initially low-permeability rock mass. Our previous works of laboratory-scale hydraulic fracturing experiments on granite at supercritical temperatures of water have demonstrated formation of a dense network of permeable fracture distributed throughout the entire rock body, so called cloud-fracture network. It has been suggested that this fracture network occurs as a result of continuous infiltration of low-viscosity water into preexisting microfractures followed by creation and merger of the subsequent fractures. However, the occurrence of flow-induced microfracturing and its plausible criterion has not been clarified so far. We have therefore conducted a hydraulic fracturing experiment on granite at superhot geothermal conditions, together with acoustic emission measurement, to address these points. Moreover, we have conducted a fracturing experiment on granite at conventional geothermal conditions using CO2 instead of water because it has been suggested that the formation of the cloud-fracture network attributes primary to injection of the low-viscosity fluid (i.e., high-temperature water). Results in both fracturing experiments using water and CO2 have indicated the occurrence of flow-induced microfracturing and have clarified that the well-known Griffith failure criterion largely predicts the fluid pressure required to initiate this fracturing. The present findings will contribute to creating permeable fracture networks in granite for both conventional and superhot EGS.

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