Effects of Supercritical CO₂ Immersion Time on CO₂/CH₄ Gas Seepage Characteristics in Coal

Low permeability has always limited the efficient extraction of coalbed methane (CBM) in China. To investigate the permeability enhancement effect of supercritical CO₂ on coal seams, experiments were conducted using a self-developed supercritical CO₂ immersion system and a single-component gas (CO₂ and CH₄) seepage experimental apparatus, considering different immersion times and injection pressures. The gas seepage characteristics of CO₂ and CH₄ in coal seams were studied under various conditions. Additionally, nuclear magnetic resonance (NMR) was used to obtain the porosity components of the coal samples at different immersion times. The changes in permeability before and after the experiment were compared to analyze the permeability enhancement effect of supercritical CO₂ on the coal samples. The results show that the original porosity of the coal sample was 2.06%. After 5, 10, 15, and 20 days of immersion, the porosity of the coal samples increased by 2.78%, 3.26%, 3.22%, and 2.86%, respectively. After immersion in supercritical CO₂, the porosity exhibited a trend of initially increasing and then decreasing. During the single-component gas seepage experiment following supercritical CO₂ immersion, the outlet flow rates of both CO₂ and CH₄ reached their maximum on the 10th day of immersion. Compared with the 0-day immersion, the outlet flow rates of CO₂ and CH₄ increased by 4.49 times and 3.23 times, respectively. After immersion, the CH₄ permeability within the coal sample was stronger than that of CO₂.