Evaluation of coal swelling-controlled CO2 diffusion processes

CO2 sequestration in deep and unmineable coalbeds is regarded as a viable option for carbon storage. CO2 diffusion is a key parameter determining the effectiveness of sealing CO2 in coal seams. By conducting laboratory tests of coal CO2 desorption and unsteady gas diffusion theory model calculations, the desorption dynamics of Yaojie coal were studied under the condition of constant temperature and varied balance pressures, and the CO2 diffusion coefficient was obtained using the non-steady diffusion model at different times. The results show that the higher the balance pressure is, the greater the diffusion coefficient is under the same conditions. As the pressure drops, the CO2 diffusion coefficient gradually decreases as time increases, and the mass fraction of desorbed CO2 has a linear relationship with the diffusion coefficient. It was observed that under high pressures, the diffusion coefficient initially increases and then gradually decreases. The non-steady CO2 diffusion in Yaojie coal is related to CO2-induced swelling and to the effect of dissolved CO2 on the coal's structures and properties, especially under high-pressure conditions. The glass transition of coal containing CO2 may be the governing factor leading to the nonlinear diffusion of CO2 in coal.