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

Abstract

Supercritical carbon dioxide (ScCO2) fracturing is an important stimulation technology for enhancing coalbed methane production, and microfracture evolution directly affects the seepage capacity of coal reservoirs. To reveal the evolution and transformation mechanism of ScCO2-induced microfractures under true triaxial stress conditions, fracturing experiments were conducted under two horizontal stress-difference conditions and different injection flow rates. Image processing was used to analyze the distribution, structure, connectivity, and complexity of microfractures. The results showed that microfractures exhibited clear anisotropy after ScCO2 fracturing. The maximum horizontal stress direction (σH) had the highest fracture connectivity and complexity and dominated fracture network development, whereas the minimum horizontal stress direction (σh) was mainly characterized by simple and isolated microfractures, and the vertical principal stress direction (σv) showed transitional characteristics. Under the higher horizontal stress-difference condition, fracture directionality became stronger, and complex microfractures were mainly concentrated in the σH 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 σH 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.