Experimental investigation on the diffusion property of different form coal: Implication for the selection of CO2 storage reservoir

Abstract

Gas diffusion is an important migration behavior for coal, and the diffusion ability serves as an important parameter to evaluate whether CH4 extraction in a coal reservoir is effective and whether the reservoir is suitable for CO2 storage. Granular coal, whose morphology differs greatly from that of coal mass in actual gas extraction, remains the research object of most current studies on the diffusion characteristics of coal. Consequently, problems occur in the application of the experimental results to engineering practice. In this study, first, different shape units and gas diffusion paths were proposed for coal matrix. Subsequently, an improved Uni-pore spherical diffusion model considering the gas loss at the beginning of the experiment was established based on the Uni-pore spherical diffusion model of granular coal. Then, a gas diffusion model suitable for coal mass was established based on Fick’s Second Law of Diffusion and the cylindrical matrix shape. In addition, a contrastive analysis was conducted on the desorption laws of coal with different forms under different adsorption equilibrium pressures. It is found that granular coal corresponds to a great gas desorption amount than coal mass. The gas diffusion coefficients of coal under different conditions were calculated based on the gas desorption curves and the corresponding diffusion models. To better grasp the diffusion coefficient law of coal with different forms, coal mass and 0.2–0.25 mm granular coal at the same matrix scale were contrasted. The results suggest that they are in the same order of magnitude and differ insignificantly. Finally, the primary reason why the desorption amounts of coal mass and 0.2–0.25 mm granular coal differ notably was revealed: The shape factor of spherical matrix of granular coal is larger than that of coal mass matrix. Furthermore, considering that different matrix shape factors caused by the structures of granular coal and coal mass lead to notably different gas adsorption capacities, this study held that tectonic coal seams may be more suitable for CO2 storage than intact coal seams under the same condition.