Mechanism of hydraulic fracture propagation in hot dry rocks under the influence of thermal stress and natural fractures

Understanding the mechanism of hydraulic fracture propagation in hot-dry-rock(HDR) reservoirs，particularly under the influence of thermal stress and natural fractures，is crucial for effectively guiding hydraulic fracturing design and enhancing geothermal energy development. First，an independently developed high-temperature true triaxial hydraulic fracturing simulation experiment system is utilized to conduct high-temperature true triaxial hydraulic fracturing tests on granite samples，examining how variations in temperature influence rock fractures. And a 3D thermo-hydro-mechanical coupling model for fracture propagation simulation is created based on the continuum-discontinuum element method. The model<italic>'</italic>s accuracy is verified through comparison with a 3D theoretical model of fracture propagation and high-temperature hydraulic fracturing experiments. Subsequently，numerical simulations of hydraulic fracturing in HDR reservoirs are performed to study the effects of thermal stress and natural fractures on the hydraulic fractures propagation. The results indicate that the induced thermal stress caused by the injected cold water can reduce the reservoir fracture pressure and fracture extension pressure，increase the fracture width and decrease the fracture length. As the temperature of the rock sample rises，the induced thermal stress increases considerably，leading to a significant decrease in the rock fracture pressure observed in both experiments and the numerical simulations. When accounting for the effect of induced thermal stress，hydraulic fracturing can active the natural fractures even when there are significant horizontal geostress differences and large approach angles. Compared with the HDR reservoir without natural fractures，the stimulation reservoir volume of the reservoir with 200 randomly distributed natural fractures increases by 63.8%. As the thermal expansion coefficient of the reservoir rises，the influence of thermal stress becomes increasingly pronounced，leading to the formation of secondary fractures that develop perpendicular to the main fracture surface，and the stimulation reservoir volume increases significantly.