Predicting dynamic changes in coal's permeability is a critical prerequisite for efficient coalbed methane extraction. This study addressed three major shortcomings of traditional models: neglecting matrix permeability contributions; relying on empirical constants for the slip factor; and ambiguous weighting of multiple mechanisms affecting apparent permeability. A dual-medium apparent permeability model for coal was developed to examine heterogeneity, stress, adsorption, and slip effects. The model quantified regulatory mechanisms relating to heterogeneity, and stress conditions on the slip factor, and analyzed influences on heterogeneity, stress, adsorption, and slip effects on permeability changes, including an evaluation of the relative contributions of each factor. The results indicated: (1) Permeability decreased with increasing roughness, with that reduction amplified under higher effective stress owing to gas's collision dissipation, an expanded adsorption surface area, and local vortex resistance; (2) Effective stress significantly suppressed permeability by compressing pore radii and fracture apertures to increase flow resistance; (3) Heterogeneity and effective stress jointly drive the slip effect: Increased surface roughness amplifies the slip factor by expanding the specific surface area; however, it concurrently reduces permeability. In parallel, a rise in fractal dimension elevates flow path tortuosity, leading to a sharp decline in permeability before it eventually stabilizes. On the other hand, effective stress enhances the slip factor via pore-throat contraction, yet its overall influence remains permeability-reducing, dominated by the compression of principal flow channels; (4) The quantification contribution displayed effective stress as the core influencing factor, whose omission would cause significant predictive errors, with slip effect contribution falling under rising gas pressure while the stress contribution would increases, thus exhibiting dynamic competition where slip would dominates at low pressure, and stress would prevail at high stress. These findings have furthered the understanding regarding coal's permeability changes, and have provided theoretical support for optimizing coalbed methane extraction.
Tang et al. (Fri,) studied this question.