Abstract: To address the difficulty in conducting wettability studies at the molecular scale,molecular simulation methods were employed to investigate the wettability behavior of droplets in shale nanopores after interaction with the methane-carbon dioxide-carbon system (wettability was characterized by surface tension and contact angle).The results show that the proportion of CO₂ molecules, X_CO2,significantly affects the surface tension γ of the CH₄-CO₂-H₂O system.The surface tension γ decreases with increasing temperature and X_CO2,with a maximum reduction of approximately 40%.Further analysis revealed the variation of droplet contact angle with changes in CH₄ and CO₂ pressure.It was found that in a CH₄ environment,when the pressure exceeds 78 MPa,the droplet detaches from the solid surface,forming a contact angle of 180°,indicating that the shale pore surface reaches a completely hydrophobic state. In a CO₂ environment,the corresponding pressure for the shale surface to become completely hydrophobic is 12 MPa.The simulation results are consistent with relevant experimental data. Compared to CH₄, CO₂ exhibits stronger interaction with the shale surface, thereby displacing CH₄ attached to the solid surface and enhancing gas recovery.For CH₄-CO₂ mixtures, the contact angle shows a linear positive correlation with the proportion of CO₂ molecules.The research findings provide theoretical guidance on the relationship between wettability and enhanced recovery in shale.

Key words: water-methane-carbon dioxide-carbon system, wettability, contact angle, molecular modeling, enhanced recovery, shale