Molecular simulation of CH4 replacement and coal structure by CO2 foam injection in slit pores

Abstract

CO₂ foam fracturing technology in the coal seam offers the dual benefits of enhancing coalbed methane extraction and promoting carbon neutrality. However, the water lock effect and structural response mechanism following CO₂ foam injection into coal remain unclear. The occurrence and interaction mechanisms among CH₄, CO₂, and H₂O in coal slit pores were analyzed following CO₂ foam injection using molecular dynamics approach. The mechanism by which CO₂ foam injection influences CH₄ desorption, diffusion, and coal structure deformation was elucidated. The results indicate that with 25 % quality foam injection, water clusters nearly filled the coal slit pores, forming a water film that inhibited CH₄ desorption, and adsorbed CH₄ increased to 77 n/cell. At 85 % quality, the co-adsorption displacement effect of smaller clusters with CO₂ was significant, adsorbed CH₄ was sharply reduced to 26 n/cell, and CO₂ adsorption weakened the water film effect; The adsorption and occurrence of H₂O molecules induced shrinkage deformation of coal, whereas adsorption and diffusion collisions of CO₂ led to expansion deformation. High-quality CO₂ foam injection increased the coal matrix's pore volume and surface area but reduced the stability, facilitating CO₂ and H₂O adsorption and CH₄ displacement; The water clusters presence after CO₂ foam injection significantly altered the gas diffusion form, and the H₂O molecule diffusion coefficient was mainly correlated with volume and aggregation of clusters; As foam quality enhanced, the diffusion coefficients of adsorbed CO₂, free CH₄, and CO₂ rose gradually, while those for adsorbed CH₄ initially increased, then declined, and ultimately increased due to displacement effects.