Impact of Different CO2/N2 Mixing Ratios on Anthracite Pore Structure Evolution

Injecting mixed gas (CO₂/N₂) into coal seams is an effective method to realize a win-win situation of CO₂ sequestration and enhanced coalbed methane (ECBM) recovery. The ratio of gas mixtures is a critical factor in pore structure evolution. In this study, we used high-pressure saturated systems to examine the effects of different gas mixture ratios on anthracite. The pore structure and mineral content of the CO₂/N₂-treated coal samples were analyzed by LP-N₂ (low-pressure N₂ adsorption), NMR (nuclear magnetic resonance), SEM (scanning electron microscopy), and XRD (X-ray diffractometry). The results of NMR and LP-N₂ showed that the coal samples’ pore volume, specific surface area, porosity increased after CO₂/N₂ treatment. The XRD analysis revealed that mineral consumption was dependent on CO₂ partial pressure and phase state (especially supercritical state). N₂ on the micropore and mesopore was mainly for high-pressure compression, prompting the closure of micropore and transforming mesopores to micropores; on the macropores and microfracture, it was mainly dilatation. This significantly alters pore roughness and complexity and leads to a shift in pore morphology from ink-bottle to slit type. Mineral dissolution, high-pressure compression, and pore throat unblocking were mainly responsible for the pore structure evolution under CO₂ and N₂ synergistic injection. The highest porosity and micropore volume were obtained when treating coal samples with CO₂: N₂ ratio of 8:2. Therefore, this ratio is expected to be optimal for implementing long-term gas mixture-ECBM and geologic CO₂ sequestration.