Experiments and analysis of hydraulic fracturing in hot dry rock geothermal reservoirs using an improved large-size high-temperature true triaxial apparatus

Hydraulic fracturing has become the main technology for the efficient development of geothermal energy in hot dry rock (HDR), however, few studies on the propagation behavior and mechanism of HDR hydraulic fractures under high-temperature conditions have investigated. In this paper, a large-size high-temperature true triaxial hydraulic fracturing physical modeling apparatus is designed, and hydraulic fracturing experiments with it are performed to investigate the fracture initiation and propagation behavior in natural granite samples collected from Gonghe Basin, the first HDR site in China. The experimental results show that the designed high-temperature apparatus provides a constant-temperature condition during the whole hydraulic fracturing process and the maximum temperature can reach 600 °C, showing its ability to simulate realistic temperatures and pressures in both ultra-deep and HDR formations. Although the tensile strength of the rock samples remains almost unchanged at a temperature of 200 °C, the cooling effects of the fracturing fluid in high-temperature rock can induce the formation of microfractures and significantly reduce the rock strength, thus lowering the breakdown pressure and increasing the complexity of the hydraulic fracture morphology. Compared with traditional oil and gas reservoirs, the hydraulic fractures in HDR are rougher and the specific surface area of a single fracture is larger, which can be helpful for heat extraction. This study provides a basis for understanding hydraulic fracture geometries and field construction design in HDRs.