Abstract: The hydraulic fracturing (HF) behavior under CO2 condition is a key scientific issue in CO2-enhanced geothermal system project. The granite specimen was taken from the quarry in Zhangzhou, Fujian province. HF experiments were conducted to investigate the effects of CO2 and H2O on the HF process and crack propagation by using a new independently developed hollow cylinder. This study reveals that with the decrease of the viscosity of the fracturing fluid, a greater number and more sinuous microcrack branches form in the fracture process. This indicates that CO2 fracturing may be more conducive to the formation of the crack network, which helps to increase the heat exchange efficiency in the CO2-EGS projects. The fracture pressure of the specimen decreases with decreasing the viscosity of the fracturing fluid, which leads to the safe operation of the injection well with a lower value. The experimental results consist with the interaction mechanism of fluid rock from the convective heat transfer perspective, and its accuracy is verified.

Key words: fracture characteristics, heat transfer coefficient, fluid viscosity, hydraulic fracturing, CO2-EGS