Physical simulation experiments on 4D visualization of dynamic methane diffusion and adsorption in low-porosity and low-permeability coals

Authors

LIU Tong, School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China; State Key Laboratory of Coal Mine Disaster Prevention and Control, China University of Mining and Technology, Xuzhou 221116, China; Key Laboratory of Theory and Technology on Coal and Rock Dynamic Disaster Prevention and Control National Mine Safety Administration, China University of Mining and Technology, Xuzhou 221116, ChinaFollow
JIAO Baizhuo, School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China; State Key Laboratory of Coal Mine Disaster Prevention and Control, China University of Mining and Technology, Xuzhou 221116, China; Key Laboratory of Theory and Technology on Coal and Rock Dynamic Disaster Prevention and Control National Mine Safety Administration, China University of Mining and Technology, Xuzhou 221116, China
LIN Baiquan, School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China; State Key Laboratory of Coal Mine Disaster Prevention and Control, China University of Mining and Technology, Xuzhou 221116, China; Key Laboratory of Theory and Technology on Coal and Rock Dynamic Disaster Prevention and Control National Mine Safety Administration, China University of Mining and Technology, Xuzhou 221116, China
SANG Shuxun, School of Mineral Resource and Geoscience, China University of Mining and Technology, Xuzhou 221116, China
YANG Wei, School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China; State Key Laboratory of Coal Mine Disaster Prevention and Control, China University of Mining and Technology, Xuzhou 221116, China; Key Laboratory of Theory and Technology on Coal and Rock Dynamic Disaster Prevention and Control National Mine Safety Administration, China University of Mining and Technology, Xuzhou 221116, China
LIU Shiqi, School of Mineral Resource and Geoscience, China University of Mining and Technology, Xuzhou 221116, China
LIU Ting, School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China; State Key Laboratory of Coal Mine Disaster Prevention and Control, China University of Mining and Technology, Xuzhou 221116, China; Key Laboratory of Theory and Technology on Coal and Rock Dynamic Disaster Prevention and Control National Mine Safety Administration, China University of Mining and Technology, Xuzhou 221116, China
LI He, School of Resource, Environment and Safety Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
YANG Mingxing, School of Public Administration, Central South University, Changsha 410075, China

Abstract

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.

Keywords

CH4 diffusion, krypton(Kr), low porosity and low permeability, coal matrix, nanopore, in situ computed tomography(CT), 4D visualization

DOI

10.12363/issn.1001-1986.25.12.0896

Recommended Citation

LIU Tong, JIAO Baizhuo, LIN Baiquan, et al. (2025) "Physical simulation experiments on 4D visualization of dynamic methane diffusion and adsorption in low-porosity and low-permeability coals," Coal Geology & Exploration: Vol. 53: Iss. 12, Article 15.
DOI: 10.12363/issn.1001-1986.25.12.0896
Available at: https://cge.researchcommons.org/journal/vol53/iss12/15

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