Multiscale measurements of gas diffusion coefficient of coal using counter-diffusion and image-based methods

Abstract

Coal seam gas (CSG), also known as Coalbed Methane (CBM), is an important natural gas resource owing to its abundance and high energy density. During production, the gas desorbs from the coal matrix owing to pressure depletion. It then diffuses primarily within the micropores of the coal matrix toward the coal fractures and then migrates to the wellbore. Therefore, diffusivity plays a significant role in the gas production process, and this process needs to be explicitly characterized. However, experimental gas diffusion measurements are commonly based on coal particles rather than bulk coal samples, and thus, the impacts of coal fractures are typically neglected. This study aims to test the diffusion coefficients of fractured and tight bulk coal samples to provide insights into gas diffusion within the different pores and fractured structures. Diffusion coefficients of bulk coal samples were measured at different diffusion times, confining pressures, and gas pressures using counter-diffusion experiments. The results show that the bulk diffusion coefficient varies with confining pressure and gas pressure, which demonstrates the impacts of internal pore/fracture structure of coal on the diffusivity. The time-dependent diffusion coefficient results show that the diffusion coefficients became constant after 24 h, ranging from 1 × 10−10 m2/s to 2 × 10−10 m2/s. Local diffusion coefficients were obtained using X-ray micro-computed tomography images of krypton gas diffusing through a coal sample. These were higher than the diffusion coefficients of bulk samples, which were in the order of ∼10−9 m2/s. The variations in local diffusion coefficients indicated the heterogeneity of coal, thereby validating the importance of both local and bulk diffusion measurements in coal characterization.