Cyclic CO2/N2 injection enhances CO2 storage and CH4 recovery in anthracite by optimizing pore structure and competitive adsorption

Abstract

Synergistically optimizing coalbed methane recovery rate and CO₂ sequestration efficiency remains a core bottleneck in CO₂/N₂-ECBM engineering. The competitive adsorption mechanisms underlying the differential impacts of varying CO₂/N₂ ratios on recovery and sequestration are key factors influencing ECBM project performance. Therefore, this study focuses on simulating competitive adsorption mechanisms driven by CO₂/N₂ ratio variations in deep coal reservoir environments to elucidate their critical influence on ECBM efficiency. This study proposes synergistic injection of high-concentration CO₂ from industrial flue gas with associated N₂ into deep coal seams, leveraging N₂-assisted CO₂ delivery through connected pores to improve total storage capacity, thereby realizing cost-effective CO₂ geological storage coupled with coalbed methane stimulation. Through breakthrough adsorption experiments, low-temperature liquid nitrogen adsorption, and nuclear magnetic resonance (NMR) techniques, we investigate the impacts of varying CO₂/N₂ ratios and injection modes (constant pressure/cyclic) on the pore structure and adsorption characteristics of anthracite. Results demonstrate that cyclic injection mode enhances CO₂ adsorption capacity by 28.74 %–40.01 % compared to constant-pressure injection, with a strong positive correlation between micropore volume and adsorption capacity. Although increased N₂ proportion induces pore compression effects, cyclic pressure fluctuations significantly improve macropore porosity (5.67 % increase) and connectivity, facilitating gas diffusion. A three-stage competitive adsorption model is proposed: CO₂ preferentially occupies high-affinity sites, N₂ optimizes transport pathways through pore network expansion, ultimately forming a storage pattern dominated by micropore adsorption and macropore diffusion under dynamic equilibrium. Our results indicate that a 4:1 CO₂/N₂ ratio combined with cyclic injection mode and enhanced meso-macropore porosity synergistically improves both storage efficiency and methane recovery, providing theoretical guidance for gas proportioning and process optimization in CO₂/N₂-ECBM projects.