Analysis of Sorption-Induced Permeability Reduction Considering Gas Diffusion Phenomenon in Coal Seam Reservoir

Abstract

The sorbed gas diffuses in a coal matrix, where diffusion varies considerably with pore structure and pressure. The diffusion is an essential factor due to high sorption affinity of $\hbox {CO}_{2}$ on coal grain surface. The sorption within the matrix causes swelling or shrinkage of matrix, in which the amount of sorbed gas affects gas diffusion. In this study, in order to observe permeability change occurred when the sorbed gas diffuses in a coal matrix, experiments were conducted with a bituminous coal sample. Firstly, gas partial pressure was measured through a diffusion experiment with a transient flow method. Secondly, flowing experiments were conducted to examine the permeability reduction yielded by sorption phenomena according to the various pore pressures. The results were then analyzed for the effect of diffusion coefficient on permeability change. The diffusion coefficient of $\hbox {CH}_{4}$ in low-pressure section was greater than that of $\hbox {CO}_{2}$ owing to the difference in their molecular weight. However, as pressure increased, the diffusion coefficient of $\hbox {CO}_{2}$ became greater because of higher sorption affinity of $\hbox {CO}_{2}$ compared to $\hbox {CH}_{4}$. Finally, it was revealed that the gas diffused instantaneously in the case of crushed coal in which its diffusion coefficient had infinite value due to the destroyed pore structure. Meanwhile, in the coal core sample in which pore structure was preserved as original state, it was ascertained that diffusional flow occurred gradually corresponding to concentration gradient, and hence, the permeability was reduced accordingly. This phenomenon was also confirmed through pore images obtained from X-ray CT.