Free Gas Diffusivity in Coal Matrix: Novel Experiments and Anisotropic Characteristics

Understanding the transport characteristics of free gas in coal seams is crucial for coalbed methane recovery and gas drainage in coal mines. Coal matrix gas diffusion coefficient controls gas migration response from the storage in the matrix to transmission through the fractures. Given the limitations of current testing methods, this study uses bulk coal with associated mesoscale structure to determine impacts on mesoscale diffusion. We use the injection of nonsorbing helium and measure matrix strains to determine the diffusion coefficients in orthogonal directions. This approach enables rapid and accurate evaluation of the anisotropy of free gas diffusion coefficients in the coal matrix system. Experimental results show that, after helium injection, the strain evolution curve can be divided into three stages: a rapid decrease, followed by a gradual rebound, ultimately reaching a steady state. The diffusion coefficient of the coal matrix system exhibits significant anisotropy, with the vertical diffusion coefficient (Dz) being smaller than the horizontal diffusion coefficients (Dx and Dy). The effective diffusion coefficient of the coal matrix system can be expressed as a function of dynamic strain. The strain curve fitting-based diffusion coefficient measurement method enables accurate determination of free gas diffusion coefficients within the coal matrix while preserving the original pore structure and mitigating the influence of fractures on the accuracy of diffusion distance measurements. This study provides insights into free gas diffusion anisotropy in natural coal samples, offering accurate reservoir parameter characterization for methane extraction and gas drainage.