Comparison of transient and pseudo-steady diffusion of methane in coal and implications for coalbed methane control

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Diffusion coefficient is a key parameter characterizing the methane diffusion capacity. Because of the differences between transient diffusion and pseudo-steady diffusion in methane diffusion rate, the diffusion processes and diffusion coefficients are different. In this study, the transient diffusion and pseudo-steady diffusion properties were measured via particle method and counter diffusion method, respectively. The results indicated that the pseudo-steady diffusion coefficient is higher than the transient diffusion coefficient, and the difference may be attributed to the influence of surface diffusion of adsorbed methane. The matrix shape factor is also a key parameter influencing the methane diffusivity, and it has a higher influence on diffusion amount of methane than the diffusion coefficient. It is suggested that for methane extraction project, the pseudo-steady diffusion coefficient and coal matrix shape factor should be considered, while transient diffusion coefficient should be used to calculate the indices for evaluating the risk of coal and gas outburst.