Abstract: Crack initiation and propagation in hydraulic fracturing process of hot dry rocks is affected by thermal stress induced by the low temperature of injected cryogenic fracturing fluid as well as its injected water pressure. Firstly, the interaction between injected water pressure and thermal stress during the injection of cryogenic fracturing fluid and its effect on crack initiation are studied by THM coupling analysis. Subsequently, a THM-D coupling simulation considering the meso-structure of rock is conducted to investigate the hydraulic fracturing process of hot dry rocks under different thermal conditions. The results show that the thermal stress which is generated by both temperature gradient in the rock itself and the non-uniform expansion of rock particles, appears as tensile stress around the wellbore. The high injection pressure will prohibit the initiation of multiple cracks induced by thermal stress and the existence of thermal stress around the wellbore will weaken the injection pressure. As the rise of rock temperature, more cracks emerge near the wellbore at the crack initiation phase and the propagation velocity of fracture network along the maximum geo-stress direction decreases while the reconstruction scale enlarges. Simultaneously, the generated multiple cracks also increase the crack extension pressure.

Key words: hot dry rocks, thermal stress, hydraulic fracturing, rock damage, coupled THMD model