CO2-enhanced methane recovery in deep coalbeds: Displacement and diffusion/pressure-driven behaviors

Abstract

With the development and production breakthrough of deep coalbed methane (CBM), research on CO₂ enhanced methane (CO₂-ECBM) under high temperature and pressure in deep coal seams is gaining increasing attention. In this study, molecular dynamic simulation (MD) are employed to investigate the occurrence of methane and CO₂ in pores of various sizes of deep coal bed, considering both adsorption and pore-bound states. Furthermore, CO₂-ECBM CH₄ is simulated, with three distinct mechanisms-displacement, diffusion-driven displacement, and pressure-driven displacement-analyzed at different burial depths. The results show that, the adsorption density of CH₄ increases and then stabilizes with burial depth, while the adsorption density of CO₂ first increases and then slightly decreases. The self-diffusion of methane decreases and eventually stabilizes, while the self-diffusion coefficient of CO₂ initially decreases and then increases. The turning depth for both processes is at 1200 m. The displacement of methane by CO₂ increases with CO₂ pressure and decreases with smaller pore sizes. For CO₂ diffusion-driven methane displacement, elevated temperatures promote CO₂ diffusion, enhancing CO₂-ECBM below 1200 m. The greater the injected CO₂ pressure, the stronger the pressure-driven displacement. However, in small pores, the displacement process is less sensitive to pressure differences. Therefore, in deep coal seams, high temperatures favor CO₂-ECBM, meanwhile, coal beds with well-developed micropores are not conducive to CO₂-ECBM.