Rock fracture toughness testing under coupled stress-temperature conditions remains unaddressed by the ISRM-suggested methods, limiting understanding of water-injection-driven rock fractures in deep horizons. To bridge this gap, hydraulic fracturing experiments on Jining granite employed hollow double-wing crack specimens under coupled stress-temperature conditions mimicking burial depths (2200–4500 m) were performed. Key findings reveal: (1) Confining pressure densifies the granite microstructure, promoting transgranular failure and enhancing fracture toughness; (2) Heated water, rather than elevated temperature alone, drastically reduces fracture toughness via thermochemical reactions—grain boundary weakening and mineral alteration—with these effects intensifying with increasing depth; (3) Competition between geostress strengthening and water-induced degradation creates a counterintuitive depth-dependence, i.e., fracture toughness peaks near 3000 m; (4) Despite the increasing degradation from water–rock interactions at greater depths, geostress-induced strengthening remains dominant across studied depths, resulting in fracture toughness under conditions mimicking deep horizons still exceeding that under ambient conditions simulating the Earth's surface. These findings advance the understanding of coupled stress–temperature–fluid effects on fracture toughness and provide practical guidance for hydraulic-fracturing design in deep geothermal reservoirs. • The fracture toughness measured by ISRM methods does not represent that of rocks under in-situ conditions. • Heated water weakens granite strength via chemical reactions, while geostress enhances it. • Fracture toughness is governed by a competition between thermal weakening and stress strengthening.
Li et al. (Sun,) studied this question.