Analysis of methane recovery through CO2–N2 mixed gas injection considering gas diffusion phenomenon in coal seam

Abstract

When enhancing coalbed methane recovery using CO2 or N2 injection, injected gas flows into coal matrix by diffusion. Gas diffusion velocity varies, depending on gas molecular size and pore geometry which causes different sorption rates of the gas in coal seam. In this aspect, this study provides the fundamental reason for the reduction in gas permeability through cleats and methane recovery during enhanced coalbed methane (ECBM) processes. This reduction occurs not only because of the sorption affinity as reported in previous works, but also because of the characteristics of gas diffusional flow which this study attempted to examine experimentally. From the results obtained by diffusional flow experiment, diffusion coefficient is no longer increased at high pressure. Although CO2 injection rate is very high, a large amount of CO2 moves through cleat instead of adsorbed in matrix, which causes early CO2 breakthrough. In ECBM, N2 mostly acts as a displacing agent of methane, because co-diffusion of N2 with methane is more dominant than counter-diffusion owing to its extremely low adsorption affinity. On the other hand, CO2 is rapidly adsorbed due to its fast increasing rate of diffusion coefficient with pressure increase. Consequently, CO2 permeability is greatly reduced at the beginning of injection and ultimately becomes the lowest value at the maximum adsorption pressure. Also, delayed methane recovery by fast diffusion and high adsorption affinity of CO2 occurs accordingly. This study confirms that the CO2–N2 mixed gas injection is advisable comparing to only injecting CO2 to pursue the prevention of CO2 injectivity reduction and enhanced methane recovery, simultaneously.