Objective and Method The diffusivity of methane (CH₄) in coal seams significantly influences the surface extraction of coalbed methane (CBM) and underground gas drainage. However, the visualized characterization of the diffusion and migration processes of CH₄ in coals remains challenging. To address this issue, this study systematically demonstrated the similarity between krypton (Kr) and CH₄ in their adsorption and diffusion behavior in coals. Utilizing Kr—a gas with high X-ray attenuation—as a tracer substitute for CH₄, this study achieved the transparent observation of the dynamic gas migration process in low-porosity, low-permeability, and heterogeneous coals using a self-developed experimental system of 4D visualization via in situ computed tomography (CT). Based on the three-dimensional nanopore structure of the coal matrix, reconstructed using focused ion beam-scanning electron microscopy (FIB-SEM), this study conducted numerical simulations of CH₄ diffusion within actual nanopores. Accordingly, the key role of nano-scale pore-throat structures in controlling gas migration was revealed.

Results and Conclusions Low-permeability matrix zones were widely distributed in the low-porosity, low-permeability coals, significantly hindering CH₄ diffusion. Pore fracture in the low-permeability matrix zones occurred as microfractures and micro- and nano-scale pore clusters. Consequently, Kr diffused and migrated from fractures into the low-permeability matrix at extremely low rates, exhibiting heterogeneous, step-wise diffusion. The low-permeability matrix featured narrow nano-scale pore throats (diameters: <30 nm mostly) and poor interpore connectivity (coordination numbers: 3-5). These narrow pore throats led to abrupt drops in the CH₄ concentration curves. As the pore diameter decreased, the Knudsen diffusion effect intensified, resulting in significantly decreasing effective diffusion coefficients and notably increasing contribution rates of surface diffusion. Under a pore diameter of 7 nm, the ratio of surface to bulk diffusion coefficients reached up to 0.24, with surface diffusion gradually predominating in gas migration within micropores. Within the complex micropore clusters in the low-permeability matrix, the movement of gas molecules was strongly constrained by the interaction potential between them and pore walls. This phenomenon further increased the diffusion resistance, leading to a step-wise and sluggish CH₄ diffusion behavior. In future research, it is recommended to develop an in situ CT experimental platform that incorporates multi-process coupling of adsorption, desorption, seepage, and displacement by integrating advanced dynamic 4D visualization via dynamic CT. This will enable transparent visualization and precise characterization of dynamic CH₄ diffusion patterns under varying stresses induced by engineering disturbance, thereby providing deep insights into the trans-scale migration mechanisms of CH₄ during CBM recovery and gas drainage.