Influence Mechanism of Gas Pressure on Multiscale Dynamic Apparent Diffusion-Permeability of Coalbed Methane.

The permeability and diffusion coefficient of coal show multiscale characteristics due to the influence of multiscale pore sizes. The gas pressure will continuously decrease during the coalbed methane (CBM) extraction. However, there are contradictory perceptions in the effect of gas pressure on the diffusion coefficient and permeability. Therefore, it is essential to clarify the influence mechanism of gas pressure on multiscale diffusion-seepage. Diffusion-seepage experiments are carried out using particle coal and cylindrical coal without stress loading. Meanwhile, seepage experiments measured by the steady-state method are conducted under stress loading. The results show that the apparent diffusion coefficient is dynamically attenuated with time in the experiments of particle and cylindrical coal. A new model of multiscale dynamic apparent diffusion is proposed. The mechanism of gas flow in multiscale pores is elucidated. The multiscale pores determine the attenuation of the diffusivity and permeability of coal. The initial apparent permeability decreases and then increases with the increase of gas pressure, which is caused by the effect of gas pressure stretching and multiscale flow regime. Three patterns of permeability with gas pressure, monotonically increasing, monotonically decreasing, and U-shaped changes, will occur.