A CFD-DEM Coupled Simulation Study on Mechanical Properties and Seepage Mechanisms of Prefabricated Fractured Coal Under Triaxial Compression

Abstract

In response to the increasing severity of gas-related issues associated with the transition from shallow to deep coal mining, this study systematically investigates the fracture evolution and seepage behavior of artificially fractured coal specimens under triaxial stress conditions through experimental tests. On the other hand, two-dimensional (2D) numerical modeling and multi-physics field coupling simulations of gas flow in coal seams with distinct fracture morphologies were implemented via COMSOL Multiphysics, involving mesh generation with adaptive refinement for 2D fracture networks and parameterized analysis of seepage-mechanics coupling effects, aiming to quantitatively clarify the regulatory mechanisms of mechanical parameters of prefabricated fractured coal on gas seepage characteristics. The results indicate that a significant surge in acoustic emission (AE) activity is observed during the stress-strengthening stage, which manifests the accumulation of internal damage within the coal matrix. At the peak strength, AE characteristic parameters exhibit an abrupt mutation, consistent with the numerical simulation results of 2D crack propagation paths. The Forchheimer equation effectively characterizes the quantitative correlation between pressure gradient and seepage velocity, and the deviation from linear growth behavior confirms that the seepage process conforms to non-Darcy flow mechanisms, as verified by the numerical fitting of 2D seepage curves and sensitivity analysis of flow parameters. Furthermore, it is worth noting that as effective confining pressure increases, the permeability (K) of the internal fracture network shows a monotonic decreasing trend, while the non-Darcy flow coefficient keeps rising steadily. This trend is well captured by the 2D coupled numerical model considering the nonlinear deformation of fractures. As the crack propagation zone expands, both permeability (K) and related seepage parameters exhibit a synchronous decreasing trend, which is quantitatively described by the damage variable derived from 2D numerical simulation. Numerical simulation results further reveal that the anisotropy index of outlet flow velocity corresponding to 15^∘ inclined fractures is conspicuous, and the degree of dominant channel aggregation in multi-fracture systems is notably greater than that in single-fracture systems, as visualized by the 2D flow field cloud maps generated from simulations. Once the fracture length surpasses 25 mm, the pressure gradient field presents evident anisotropic properties, which is consistent with the experimental observations and numerical prediction results of the 2D stress-seepage coupling model.