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

• Published in: Rock mechanics and rock engineering Vol. 54; no. 6; pp. 2959 - 2974
• Main Authors: Goto, Ryota, Watanabe, Noriaki, Sakaguchi, Kiyotoshi, Miura, Takahiro, Chen, Youqing, Ishibashi, Takuya, Pramudyo, Eko, Parisio, Francesco, Yoshioka, Keita, Nakamura, Kengo, Komai, Takeshi, Tsuchiya, Noriyoshi
• Format: Journal Article
• Language: English
• Published: Vienna Springer Vienna 01.06.2021; Springer Nature B.V
• Subjects: Acoustic emission; Acoustic emission testing; Axial stress; Civil Engineering; Criteria; Earth and Environmental Science; Earth Sciences; Emission measurements; Enhanced geothermal systems; Fractures; Geophysics/Geodesy; Geothermal power; Geothermal resources; Granite; Hydraulic fracturing; Microfracture; Network formation; Original Paper; Viscosity; Water temperature; Superhot geothermal energy; Hydraulic fracturing; Enhanced geothermal system; Supercritical geothermal energy; Granite
• ISSN: 0723-2632; 1434-453X
• DOI: 10.1007/s00603-021-02416-z

Superhot geothermal environments with temperatures of approximately 400–500 °C at depths of approximately 2–4 km are attracting attention as new kind of geothermal resource. In order to effectively exploit the superhot geothermal resource through the creation of enhanced geothermal systems (superhot EGSs), hydraulic fracturing is a promising technique. Laboratory-scale hydraulic fracturing experiments of granite have recently demonstrated the formation of a dense network of permeable fractures throughout the entire rock body, referred to as a cloud-fracture network, at or near the supercritical temperature for water. Although the process has been presumed to involve continuous infiltration of low-viscosity water into preexisting microfractures followed by creation and merger of the subsequent fractures, a 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 °C under true triaxial stress and at 450 °C 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.