Carbon dioxide (CO2) has great utilization potential in the exploitation of deep geothermal resources, especially hot dry rock (HDR). As a clean and renewable resource, HDR geothermal presents a promising prospect in meeting the growing demand for energy and achieving low-carbon solutions. To address the challenges posed in HDR development, such as large water consumption, simple fracture pattern and corresponding fast thermal breakthrough of Enhanced Geothermal System (EGS), a novel non-aqueous stimulation method which combines the advantages of supercritical CO2 (SC) fracturing and dynamic shock effect is proposed and investigated in this paper, i.e. supercritical carbon dioxide shock (SCS) fracturing. To determine its stimulation performance in HDR, we performed controllable lab-scale SCS fracturing experiments on high-temperature granites subjected to true tri-axial stresses. By comparing with conventional water fracturing and SC-CO2 fracturing, the fracture initiation behavior and stimulation performance of SCS fracturing were investigated quantitatively based on CT scanning and reinjection tests, with respect to fracture morphology and conductivity. Effects of critical parameters were analyzed as well, such as shock pressure, in-situ stress and rock temperature. Results indicate that the breakdown pressure of granite is 24.2~57.5% lower than the shock pressure during SCS fracturing, and it decreases with increasing rock temperature. SCS fracturing could create complex fracture network with more interconnected branches and larger seepage spaces. The volume, area and width of fractures by SCS fracturing are 545.3%, 98.4% and 126.3% higher than those of water fracturing, respectively. The fracture conductivity is 3.4~7.0 times and 4.5~21.2 times higher, as compared to water fracturing and SC fracturing. As the rock temperature increases, both the tortuosity and conductivity of fractures improve dramatically, which benefits to extend the flow path of working medium and enhance the heat transfer performance. In-situ stress plays a relatively weak role in controlling fracture propagation of SCS fracturing. At horizontal stress difference coefficient of 0.14~0.60, the fracture propagation behaves more randomly in direction, contributing to forming complex fractures with multi-branches. Higher shock pressure conduces to the stimulation performance enhancement of SCS fracturing, improving the complexity and connectivity of fracture networks, and promote the fracture to get rid of the control of in-situ stress in EGS. The key findings are expected to provide a novel insight into developing HDR geothermal in a more environmentally and more efficient way, and achieving CO2 utilization and storage.
Keywords Hot dry rock, CO2 Utilization, EGS, Supercritical CO2, Shock Fracturing