Pore-Microfracture Structure Alteration of Coal Induced by High-Pressure CO₂ Gas Fracturing

High-pressure CO₂ gas fracturing (CO₂-Frac) is a novel technology to stimulate coal for improving coal-seam permeability and increase gas extraction efficiency. However, the multiscale pore-microfracture evolution characteristics induced by CO₂-Frac and its damage mechanism are still in their nascency. Herein, a high-pressure CO₂-Frac system of 120-150 MPa was newly developed and carried out on anthracite coal to reveal the damage mechanism. The results of mercury intrusion porosimetry and field emission scanning electron microscopy (FESEM) tests on coal before and after the impact showed that the micropores (<10 nm) and transition pores (10-100 nm) were significantly reduced after CO₂-Frac; the volume and specific surface area of seepage pores (large pores, 1000-10,000 nm) increased significantly. The FESEM images show four damage structures including (1) damage marks, (2) tri-wing fractures, (3) pore-microfracture linkage structures, and (4) zigzag fractures. Based on these new findings, a two-stage damage mechanism was proposed as a high-pressure CO₂ damage mechanism and a low-pressure quasi-static CO₂ damage mechanism.