Fracture Evolution Characteristics and Permeability of Coal Treated with CO 2 Phase Transition Fracturing

Fracturing pressure serves as a pivotal parameter governing fracture expansion and permeability improvement, which is essential to enhancing coalbed methane (CBM) production via CO 2 phase transition fracturing (CO 2 -PTF). In this study, laboratory CO 2 -PTF experiments on coal were performed under fracturing pressures of 120 MPa, 150 MPa and 185 MPa. Combining scanning electron microscopy (SEM), computed tomography (CT) scanning and permeability testing, the fracture evolution characteristics and permeability enhancement effect of coal samples were quantitatively analyzed. The research results show that CO 2 -PTF can raise the change rate of total fracture volume fraction by an order of magnitude, with absolute permeability being enhanced by one–two orders of magnitude, which further indicates an exponential-enhancement effect of CO 2 -PTF pressure on coal permeability. The exponential-enhancement effect of CO 2 -PTF pressure on coal permeability is closely associated with CO 2 -PTF processes. The early high-pressure CO 2 jet stage main influences the fracture generation effect, and the subsequent quasi-static high-pressure gas stage induces the fracture expansion–transformation effect; collectively, these processes cause the exponential enhancement in coal permeability. The above analysis can offer theoretical guidance for the technical improvement and field application optimization of CO 2 -PTF and efficient enhancement in CBM recovery.