Thermal and mechanical properties of fracture-filled granite with different structural forms under high temperature and high pressure

The reservoir of deep hot dry rock (HDR) geothermal energy is not intact rock mass, which includes a large number of fractures induced by geological tectonic movement. The tectonic fractures are subsequently filled by hydrothermal fluid. This kind of reservoir is so-called fracture-filled reservoir. To guide deep hot dry rock geothermal mining, this paper used a 600 ℃ high-temperature and high-pressure rock mass triaxial testing machine independently developed by the Taiyuan University of Technology to study the thermal and mechanical characteristics under high temperature (500 ℃) and high pressure (20 MPa confining pressure) of four types of granite from the Luya Mountain, Shanxi, China, including parent rock (type I granite), hydrothermal fluid backfill (type II granite), cementation interface between the backfill and the parent rock laterally positioned through the specimen (type III granite), and cementation interface between the backfill and the parent rock longitudinally positioned through the specimen (type IV granite). The research shows that the thermal expansion coefficient of four types of granite can be divided into three stages with the increase of temperature: the slow fluctuation stage at low temperature, the rapid increase stage at low and medium temperature, and the rapid decrease stage at medium and high temperature. The elastic modulus of four types of granite increases slowly and then decreases rapidly with the increase of temperature. The dissolution pore structure and the presence of minerals with low bonding strength lead to the lowest elastic modulus of hydrothermal fluid backfill. In addition, the critical temperature of the closure of preexisting fractures in the fracture-filled granite is about 200 ℃. The critical temperature for the backfill near the cementation interface to restore the weak-plane structure characteristics is about 250 ℃. Finally, the failure modes of four types of granite under high temperature and high pressure are obtained. The coarse grain boundary of the parent rock, the dissolution pores of the hydrothermal fluid backfill and the cementation interface in the fracture-filled granite are very likely to form a large-scale efficient water channel during the reservoir construction by hydraulic fracturing, which provides a new idea for the theory and technology of reservoir construction for HDR geothermal exploitation. © 2024 China Coal Society. All rights reserved.