Molecular Insights on Competitive Adsorption of CH₄, CO₂ and Flue Gas in Shallow and Deep Coals for Gas Injection Technology

Gas injection for enhancing gas recovery (GI–EGR) is a multifaceted process that requires a solid theoretical foundation to be implemented orderly. However, there are limited reports on the micro-mechanisms of GI–EGR technology applied to coalbed methane reservoirs, especially for deep strata. To address this gap, this study utilized molecular simulation techniques to construct the organic pore models of anthracite with varying sizes and morphologies, and explored the micro-dynamic behaviors of CH₄ and various gas injected components including N₂, CO₂ and flue gas confined in nanopores. The aim was to reveal the competitive adsorption mechanisms of gases in multi-component systems under shallow and deep geological conditions. The results demonstrated that the isosteric heats of CH₄, N₂ and CO₂ all increased after the transition from shallow to deep, with rising amplitudes of 18.8%, 22.8% and 17.8%, respectively, in the respective single-component systems. In multi-component adsorption models, the isosteric heats remained higher than those under shallow conditions, but there were some small fluctuations due to the interference between various gases. On the other hand, the self-diffusion coefficients of single CH₄, N₂ and CO₂ in the deep condition decreased by 37.6%, 27.2% and 23.1%, respectively, compared to those in conventional shallow conditions. As a consequence, the difference in diffusivity among various gases would get narrowed. The molecular-level observations herein have the potential to improve the understanding of gas occurrence and lay a solid foundation for the GI–EGR technology.