Three-dimensional particle topological cellular model for characterizing seepage channel evolution in large-scale fractured coal

After coal fragmentation, multi-scale fracture networks develop, where large-scale fractures dominate gas transport within the coal matrix. A three-dimensional particle topological cellular model was established to characterize the flow channel structures within large-scale fractures among fragmented coal particles. The cubic and pyramidal models were employed to represent individual channel structures under the assumptions of the loosest and tightest particle arrangements, respectively. The channel structure models incorporating the synergistic effects of stress and fractal dimension was proposed. Furthermore, a composite cellular model integrating a flow channel correction coefficient was developed to describe the dynamic evolution of seepage channels under varying total stress and pore pressure conditions. Triaxial seepage experiments under various stress and gradation conditions were conducted to validate the model. The results indicate that the permeability evolution of fractured coal is synergistically governed by the area of seepage channels, tortuosity, and porosity. Under varying stress conditions, the permeability variation range of fractured coal approximates the pyramidal model predictions, exhibiting compact internal fissure structures and high seepage pore tortuosity. With the increase in both the Talbot power index (which quantifies the proportion of aggregates of different particle sizes) and the stress, the particle topological structure transitions toward the pyramidal model. Among the three models, the composite cellular model provides more accurate predictions of permeability. The results of the study offer novel insight for coal matrix modeling containing large-scale fractures and establish a theoretical basis for precise prediction of effective gas drainage radii within complex fracture networks in coal.