Study on the cracking mechanism of hydraulic and supercritical CO2 fracturing in hot dry rock under thermal stress

Abstract

Abstract To realize the effective extraction of high-temperature thermal energy in enhanced geothermal system (EGS), it is necessary to explore how to efficiently construct fracture network in hot dry rock (HDR). Considering the cryogenic induced thermal stress, we investigate the cracking mechanism when using supercritical CO2 (SCCO2) and hydraulic fracturing to stimulate HDR. Firstly, the established three-dimensional THMD coupling model is verified by conducting high temperature granite fracturing experiments. Then, characteristics of the heat and mass transfer and fracture propagation when using SCCO2 and H2O in HDR fracturing are explored. Finally, the cracking mechanism of hydraulic and SCCO2 fracturing in HDR under thermal stress is revealed. The results indicate that the cryogenic induced thermal stress can reduce the cracking pressure and tend to form branch fractures under the cooperation of injection pressure. Adopting SCCO2 in HDR fracturing more micro fractures can be generated in near-well zone. Hydraulic fracturing has better cooling efficiency and preferably capability of extending fractures. For HDR fracturing, the scope of fracture network tends to increase first and then decrease with the raise of injection mass flux. The lower viscosity and higher specific heat capacity of fracturing fluid can promote the formation of fracture network.