Abstract: Large-scale hydraulic fracturing is the most effective way to construct the artificial reservoir in the development of hot dry rock geothermal energy. The key issue is to reveal the hydraulic fracture mechanism of rock under high temperature and pressure. The high-temperature-vapour-driven failure experiments were carried out on granite under uniaxial stress, and the preliminary results on the failure mechanism were obtained under thermo-mechanical coupling. Results indicate that high temperature can greatly cause the failure of granite by weakening the strength and reducing the breakdown pressure. Compared with the breakdown pressure of hydraulic fracturing at room temperature, they were decreased by at least 58% at the high injection rates of 430℃ and 350℃ vapours, while they were reduced by 75% at the low injection rates of 400℃ and 450℃ vapours in the vapour driven failure experiments. The process of high-temperature-vapour-driven failure can be divided into two stages, namely, thermal fracture damage and macrocrack propagation. In the thermal cracking stage, the thermal stress resulted in thermal cracks randomly around the borehole. With the increase of vapour injection time, the thermal cracking range gradually extended, and the microcracking density enhanced, which facilitated the fracture propagation. In the second stage, the fractures were primarily generated on both sides of the borehole and then propagated along the final trace of macrocracks until the failure of the specimen. Compared with hydraulic fracturing at room temperature, the failure induced by vapour at a low injection rate was a slow ductile tensile failure. The fracture extended asymmetrically along the borehole, and the width was smaller than that of hydraulic fracturing at room temperature.

Key words: hydraulic fracturing, granite, hot dry rock, acoustic emission