Experimental Investigation of Fracture Propagation Induced by Supercritical CO<sub><b>2</b></sub> in Deep Shale Reservoirs

Deep shale reservoirs are often associated with extreme geological conditions, including high temperatures, substantial horizontal stress differences, elevated closure stresses, and high breakdown pressures. These factors pose significant challenges to conventional hydraulic fracturing with water-based fluids, which may induce formation damage and fail to generate complex fracture networks. Supercritical carbon dioxide (SC-CO2), with its low viscosity, high diffusivity, low surface tension, and minimal water sensitivity, has attracted growing attention as an alternative fracturing fluid for deep shale stimulation. This study presents a series of true triaxial large-scale physical experiments using shale samples from the Longmaxi Formation in the southern Sichuan Basin to investigate fracture initiation and propagation behavior under different fracturing fluids. The results show that, under identical experimental conditions, SC-CO2 fracturing results in a significantly lower breakdown pressure compared to slick water and promotes the formation of more complex fracture geometries. These advantages are attributed to both the favorable flow characteristics of SC-CO2 and its potential chemical interactions with shale minerals. The findings not only confirm the effectiveness of SC-CO2 as a fracturing fluid in deep shale environments but also provide new insights into its fracture propagation mechanisms.