Experimental Study on Supercritical CO2 Shock Fracturing of Deep, High-Inclination Coal Rock

Abstract Supercritical CO2 Shock (SCS) fracturing is a waterless fracturing technology that combines supercritical CO2 (SC-CO2) fracturing with dynamic shock effects, offering advantages such as impact-induced fracturing of reservoirs, easy formation of complex fracture networks, safety and efficiency, and reduced reservoir contamination. Uniaxial compression tests were conducted on coal with different inclinations (0°, 30°, 45°, 60°, and 90°), and the rock mechanics properties of coal were analyzed. Additionally, the physical and chemical properties of coal were studied through coal XRD and industrial component analysis. Using a self-developed true triaxial SCS fracturing experimental system, SCS fracturing experiments, hydraulic fracturing experiments, and SC-CO2 fracturing experiments were conducted on coal with different inclinations. The fracture formation characteristics and fracture propagation patterns of coal-rock under different parameters were compared and analyzed. The results indicate that: (1) As the coal inclination angle increases from 0° to 90°, the compressive strength and Young's modulus of coal generally decrease, and the mechanical strength of coal continues to decline. The inclination angle significantly influences the mechanical properties of coal. (2) The cycle time of SCS fracturing experiments is shorter than that of hydraulic fracturing and SC-CO2 fracturing. In SCS fracturing experiments, the formation of primary fractures in coal is controlled by in-situ stress and propagates along directions parallel to the maximum horizontal principal stress. As the shock pressure increases, the complexity of coal fractures gradually increases, and larger crush zones form in the coal. The natural fractures, bedding planes, and other weak surfaces in the coal, as well as the in-situ stress state and shock pressure, significantly influence the propagation of fracturing fractures. (3) Under SCS fracturing, coal can form a more complex fracture network and fragmentation zones with interconnected main fractures, microfractures, and bedding fractures, which facilitates increasing the flow area of the coal seam, promoting methane desorption, and enhancing coalbed methane production. (4) Under the same shock pressure (12 MPa), the complexity of the fracture networks formed after SCS fracturing in coal with different inclination angles is ranked as follows: 0° > 90° > 30° > 45° > 60°. Coal with inclination angles of 0° to 45° produces main fractures that penetrate the entire coal seam, while coal with inclination angles of 60° to 90° forms shorter main fractures and generates secondary fractures that propagate along bedding planes. The research findings have preliminarily revealed the dynamic fracture initiation and propagation mechanisms of SCS fracturing in coal-rock with different inclination angles, providing theoretical references for the integrated development of fracturing and CO2 geological sequestration in high-inclination deep coalbed methane reservoirs.