The mechanism of inward and outward expansion of multiscale dynamic permeability of coalbed methane at different temperatures

The multiscale micro-nano pores in coal can result in the ultra-low permeability of coal, which restricts the efficiency of gas extraction. It is difficult for the conventional seepage-enhancement measures to affect the nanoscale pores within the coal matrix. Thermal stimulation can reach deep into the micro-nano pores within coal matrix to improve the permeability. Therefore, it is important to study the diffusivity and permeability of the multiscale micro-nano pores at different temperatures. In this study, the experiments of diffusion-seepage measured by the methods of GRI (Gas Research Institution) and steady-state were conducted using a cylindrical coal at different temperatures and pressures. The experimental results show that the apparent diffusion coefficient of cylindrical coal is not constant but variable dynamically; and the classical diffusion model fails to describe the full-time process of gas flow accurately. On this basis, a model of multiscale dynamic apparent diffusion-seepage that can accurately describe the full-time flow process was proposed. As is observed, the apparent permeability attenuates dynamically with time without stress loading, and the initial apparent permeability and the attenuation coefficient increase monotonically with the rise of temperature. Under the stress constraint, the steady-state permeability increases after a decrease as the temperature rises, displaying a “U-shaped” pattern. Without stress constraint, the increasing temperature causes the exterior multiscale pores to expand outward by different degrees so as to increase permeability, while the interior micro-nano pores show three inward and outward expansion mechanisms. Under stress constraint, at low temperature and high effective stress, the increasing temperature causes pores to expand inward and the permeability decreases accordingly. When temperature continues to increase, coal expands outward because the effective stress is counteracted by the thermal stress, leading to an increase in permeability. This study is of significance for thermal gas extraction engineering.