Molecular simulation of CO2 foam injection to reduce water lock effect and enhance CH4 replacement in coal

Abstract

CO₂ foam fracturing technology has gradually been utilized to fracture coalbed methane (CBM) wells over recent years with the advantages of low water consumption, high fracturing fluid return rate and the ability to displace CH₄ in the coal reservoir. To explore the CH₄ desorption mechanism affected by CO₂ foam fracturing fluid injection into coal seam, the interaction microscopic mechanism between CH₄/CO₂/H₂O as well as interaction with coal molecules was examined employing density functional theory (DFT). By injecting varying quality of CO₂ foam (0% pure water, 20%, 40%, 60%, and 80%) into coal containing CH₄, the water lock formation and the replacement and diffusion of CH₄ following CO₂ foam injection into coal were examined. The findings indicate that hydrogen bond facilitates the formation of H₂O clusters, which attract CO₂ and repel CH₄. The greater adsorption and coexistence of CO₂ and H₂O encourage them to occupy adsorption sites on coal molecules and replace CH₄. As the foam quality increased, the H₂O cluster volume in the slit pores and the water film effect decreased, consequently weakening the water lock effect. The effect of CO₂ and H₂O co-adsorption on the coal matrix surface to replace adsorbed CH₄ was gradually enhanced. The co-adsorption system for CO₂/H₂O with coal matrix was more stable after the injection of higher-quality CO₂ foam, facilitating CH₄ desorption. The CO₂ adsorption depth within the coal matrix exceeded that of H₂O, which could further replace adsorbed CH₄. The diffusivity of adsorbed and free H₂O was predominantly affected by clusters and hydrogen bonds. The diffusion coefficients of adsorbed CO₂ and free CH₄ and CO₂ declined with enhanced CO₂ foam quality, while the diffusion coefficient of adsorbed CH₄ initially rose and then decreased. The research findings furnished a theoretical basis for CO₂ foam to replace CH₄ in coal seams to enhance the CBM recovery (CO₂ foam-ECBM).