Microscopic fracture distribution characteristics and control mechanism of deep coal body fractured by supercritical CO2 fracturing

Deep coal seams are commonly characterized by high burial depth, dense coal-rock structures and relatively undeveloped natural fractures, which together restrict permeability enhancement and efficient coalbed methane (CBM) extraction. Under such conditions effectiveness of conventional hydraulic fracturing is constrained by its tendency to generate predominantly planar fractures. In contrast, supercritical CO2 (ScCO2), owing to its small molecular size, low viscosity, and strong penetration capability can interact with coal micro-and nano scale pores and fractures in coal, thereby promoting the development of more complex fracture networks which facilitate additional pathways for gas permeation and migration. This study integrates micro-computed tomography (micro-CT), acoustic emission instrument and morphology scanning to investigate the spatial distribution and propagation behavior of micro-fractures in deep coal from the Yan’an block following ScCO2 fracturing. Fracture spatial distribution characteristics under different stress constraints are systematically examined. The controlling mechanism of fracture initiation and propagation associated with coal structure, in-situ stress conditions and ScCO2 characteristics in fracture initiation and propagation were analyzed. In addition, fracture morphology and heterogeneity are quantitatively characterized providing insight into governing mechanisms of fracture propagation and spatial distribution. The results indicate that the vertical stress difference coefficient (Kv) exerts a strong control on fracture initiation pressure. When Kv =0.4 (sample 1-3), fracture pressures are approximately 25 MPa, whereas at is Kv=0.6 (sample 4), the fracture pressure decreased to 16 MPa, indicating the increased Kv facilitates to the fracture initiation. Following ScCO2 fracturing, extensive micro-fractures develop in coal, with complex geometries and enhanced connectivity, which is conducive to gas migration. ScCO2 preferentially accesses coal through relatively large pore–fracture channels and subsequently penetrates micro - nano scale pores and fractures. With increasing injected gas volume, the compressive effect of ScCO2 on the coal intensifies, leading initially to micro-fracturing; as ScCO2 accumulation pressure increases, fractures are progressively opened and coalesce into macroscopic failure. Owing to the ability of ScCO2 to penetrate coal across multiple scales and its relatively uniform distribution, the resulting fractures exhibit a wide spatial distribution.The fractal dimension of fracture surface roughness for samples 1-4 are 1.915, 1.828, 1.814 and 1.797 respectively. This indicates that the higher fracture pressures are associated with increased fracture surface complexity and stronger heterogeneity. The coal structure, in-situ stresses and ScCO2 properties are the primary factors controlling fracture development and distribution in deep coal. These findings provide guidance for permeability enhancement for deep coal seams and coalbed methane reservoir transformation using ScCO2 fracturing.