(1. School of Petroleum Engineering，China University of Petroleum(East China)，Qingdao，Shandong 266580，China；
2. School of Safety Engineering，China University of Mining and Technology，Xuzhou，Jiangsu 221116，China；3. State Key Laboratory of Coal Mine Disaster Prevention and Control，China University of Mining and Technology，Xuzhou，Jiangsu 221116，China)
Abstract：Clarifying the hydraulic fracture propagation mechanism of hot-dry-rock (HDR) reservoirs under the influence of thermal stress and natural fractures is of positive significance for effectively guiding the hydraulic fracturing construction design and efficiently developing geothermal energy. In this paper，a high-temperature true triaxial hydraulic fracturing simulation experiment system developed independently is applied to perform high-temperature true triaxial hydraulic fracturing experiments on granite specimens，and the effects of temperature changes on the rock fracture are analysed. Then a 3D thermo-hydro-mechanical coupling fracture propagation simulation model is established based on the continuum–discontinuum element method，and its accuracy is verified by the 3D fracture propagation theoretical model and the high-temperature hydraulic fracturing experiments. And the influence of thermal stress and natural fractures on hydraulic fracture propagation patterns in HDR reservoir is investigated by numerical simulation. The results show that the induced thermal stress generated by injecting 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 increases，the induced thermal stress increases significantly，and the rock fracture pressure of the experiments and the numerical simulations both decreases significantly. Considering the effect of induced thermal stress，the hydraulic fracture can active the natural fractures even at large horizontal geostress differences and approximation 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%. With the reservoir thermal expansion coefficient increases，the effect of thermal stress becomes more and more obvious，and even secondary fractures are generated in the direction perpendicular to the main fracture surface，and the stimulation reservoir volume increases significantly.
