Study on the influence of temperature on fracture propagation in ultra-deep shale formation

•A three-dimensional numerical model considering the temperature was proposed.•The influence mechanism of temperature and stress on fracture propagation was analyzed.•The temperature limit affecting crack propagation was determined.•The effect of temperature on the interaction between the hydraulic fracture and the bedding was analyzed quantitatively. Creating a complex fracture network by hydraulic fracturing was an essential technic for the development of ultra-deep reservoirs. Due to the effect of thermal stress field on fracture propagation was unclear, it was necessary to understand the interaction mechanism between hydraulic fracture and bedding plane in layered formation. Combined with the experiments, the numerical simulation model was proposed based on the damage mechanics theory to illustrate the fracture propagation mechanism under high temperature. The simulations indicated that the reservoir temperature was a predominant factor affecting the fracture propagation when the temperature was over 200 ℃. Under high reservoir temperature, the breakdown pressure and extension pressure reduced by 7.61 % and 8.44 % compared with the reservoir temperature was 25 ℃, and the average fracture width increased by 23 %. Combined with the triaxial rock mechanics test and CT scanning, it indicated that the increased temperature difference between reservoir and fracturing fluid contributed the degradation mechanism of rock mechanical properties, the main reason was that the higher temperature difference induced the growth of micro fracture system in the reservoir, making the opening of micro fracture larger and the number of micro fracture increased, so the mechanical strength of rock experienced a dramatically decrease. With the degradation mechanism of rock mechanical properties, the mechanical strength difference between rock and bedding plane was reduced, thus the hydraulic fracture was more easily cross the bedding plane.