Microfracture Evolution and Transformation Mechanism of Coal Induced by Supercritical Carbon Dioxide Fracturing under True Triaxial Stress Conditions

Under the higher horizontal stress-difference condition, fracture directionality became stronger, and complex microfractures were mainly concentrated in the σ_H direction, providing more favorable conditions for seepage pathway development. Connectivity and fractal analyses further showed higher connected microfracture area, intersection count, and fractal dimension in the σ_H direction. Injection flow rate and the principal-stress configuration jointly affected microfracture evolution.

Under a low stress difference, increasing injection flow rate enhanced fracture connectivity and complexity. Under a high stress difference, fracture evolution first increased and then became limited, with fluid-driven effects becoming more pronounced at higher injection flow rates. SEM observations showed microfracture formation and propagation, mineralized interface detachment, and local closure.

T-shaped connections promoted the connection between natural fractures and new microfractures, forming a multiscale fracture network. These results provide a basis for microfracture transformation and seepage capacity enhancement in coal reservoirs by ScCO₂ fracturing.