Evolution of Pore-Fracture Structure and Coupled Damage–Seepage Characteristics of Coal Induced by LCO2 Phase-Transition Treatment: A 3D Digital Core Study

Abstract

This study combines micro-CT scanning with Avizo-based three-dimensional reconstruction technology to quantitatively characterize the evolution of pore-fracture structures in coal during LCO2 phase-transition treatment and systematically investigates the coupled response of damage evolution and seepage behavior under different stress conditions based on digital coal core models. Results show that LCO2 phase-transition treatment significantly reconstructs the coal pore-fracture network, promoting the development of large pores and preferential seepage pathways while increasing porosity from 6.27% to 10.42% and markedly enhancing pore-fracture connectivity. The dominant peak of the shape factor increases from 2 to 7, accompanied by a 1.41% increase in the average fractal dimension and a 19.2% reduction in fluctuation, indicating an enhanced structural complexity and heterogeneity of the pore-fracture system. Meanwhile, the proportions of large throats and highly coordinated pores both increase significantly, further optimizing the pore-fracture topology. After fracturing, the throat tortuosity distribution converges toward the low-value range, suggesting straighter seepage pathways and a reduced flow resistance. Numerical simulations reveal that the reconstructed pore-fracture network alters the internal stress distribution and induces pronounced matrix-pore heterogeneous deformation, with the pore volumetric strain reaching 3–5 times that of the matrix. LCO2 treatment lowers the damage initiation stress to 4 MPa and accelerates structural failure at 12 MPa while increasing the mean seepage velocity by 1.5–1.9 times. These findings demonstrate that the LCO2 phase-transition treatment effectively enhances pore connectivity and gas transport capacity in low-permeability coal seams.