Characterization of the Multiscale Evolution of Coal Pore Structure Under Liquid CO₂ Phase Change Fracturing

As a green and highly efficient coal seam penetration enhancement technology, liquid CO₂ phase change fracturing (LCPCF) technology has a great deal of promise to advance gas extraction and CO₂ geological storage. A series of investigations were conducted using low-temperature liquid N₂ adsorption (LT-N₂) adsorption, CO₂ adsorption, CT scanning, and mercury intrusion porosimetry (MIP) to elucidate further the effect of LCPCF on the coal's pore structure. These experiments aimed to characterize coal's internal pore structure parameters and investigate 3D pore structure changes before and after LCPCF treatment at varying fracturing displacements. The results indicate that the macropores' (>50 nm) and mesopores' (2-50 nm) porosity, pore volume, and pore-specific surface area (PSA) in the fractured coal showed notable enhancements in comparison to the raw coal. However, the micropores' (<2 nm) pore structural characteristics mainly stayed unaltered. In contrast to the generally stable fractal dimension of mesopores and micropores, the macropores' fractal dimension is exhibiting a declining trend. The connectivity porosity of coal samples Z1, Z2, and Z3 with fracturing distances of 0.5, 2.5, and 5 m increased by 160.33, 108.26, and 64.05%. On the contrary, the closed porosity of fractured coals decreased to different degrees. The shorter the fracturing distance, the more impact it has on the coal pore structure. The impact range of a single fracture hole in the LCPCF process is approximately 5 m. The results are instructive for further revealing the mechanism of LCPCF and optimizing the fracturing process parameters.