A shear-based breakdown model for the hydraulic fracturing of hot dry rock

• Hydraulic fracturing experiments were conducted on granite under high-temperature and true triaxial stress conditions. • A transition in the microfracturing mechanism from toughness-dominated to viscosity-dominated at elevated temperatures was observed. • Compared with other breakdown models, a shear-based breakdown model better fits experimental data under in situ geothermal conditions. Hydraulic fracturing is currently an indispensable technique for enhancing reservoir permeability in hot dry rock (HDR). However, the fracture mechanisms of rocks under in situ high-temperature (up to 300 °C) and true triaxial stress conditions are poorly understood. In this work, a series of hydraulic fracturing experiments were performed on granite samples under high temperatures and true triaxial stress states. We found that the breakdown pressure of granite decreased with increasing temperature. The fracture mechanism transitioned from toughness-dominated to viscosity-dominated when the temperature increased to 200 °C. The diffusion of pore pressure into thermally induced microfractures at elevated temperatures resulted in a reduction in the effective stress surrounding the borehole. Tensile-shear failure, rather than pure tensile failure, was observed under conditions of high three-dimensional stress and elevated temperatures, which was attributed to an increase in the effective major principal stress. Accordingly, two shear-based breakdown models were derived on the basis of the stress field around the pressurized borehole under in situ geothermal conditions, which better fit the experimental data under in situ geothermal conditions than other breakdown models do. The experimental and theoretical analyses confirmed that three-dimensional stress and high-temperature effects are critical for hydraulic fracturing initiation under in situ geothermal conditions.