| Abstract |
Hot dry rock (HDR) geothermal is a clean and renewable resource which presents a promising prospect in meeting the growing demand for energy and achieving low-carbon solutions. Creating an inter-connected and highly conductive fracture network is the foundation for obtaining enough heat transfer spaces and high flow flux of working fluid in HDR geothermal. However, the simple 2-D planar fracture pattern created by conventional water-based hydraulic fracturing could result in fast thermal breakthrough and shorten the life of Enhanced Geothermal System (EGS) significantly, and poses challenges for the environment due to the chemical additives and massive water consumption. Herein, a novel non-aqueous stimulation method is investigated, i.e. supercritical carbon dioxide shock (SCS) fracturing, which combines the advantages of supercritical CO2 (SC) fracturing and dynamic shock effect, providing potential solutions to the challenges above. To determine its stimulation performance in EGS, 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. 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. As compared to water fracturing and SC fracturing, the fracture conductivity of SCS fracturing is 3.4~7.0 times and 4.5~21.2 times higher, respectively. 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 carbon storage and utilization. |