Coal–Fluid Interfacial Tension in the Coal–Water–CO2 System: Implications for CO2 Sequestration in Coal Seams

CO2 geological sequestration in coal seams is an effective method for reducing CO2 emissions and enhancing coal-bed methane extraction. In this context, the coal–fluid interfacial tension (γcoal–fluid) is a critical parameter that influences the CO2 injection, fluid distribution, CO2 storage capacity, and containment safety. Especially, the γcoal–fluid cannot be directly measured, and the influencing factors for γcoal–fluid (including γcoal−CO2 and γcoal–water in the coal–water–CO2 system) were not reported before. In this study, we calculated the γcoal–fluid of different rank coals under different pressures (3–8 MPa) and temperatures (40–70 °C) by using the combination of Young's equation and Neumann's equation. The results show that the γcoal−CO2 of all coal samples decreases with increasing pressure and coal rank, while it increases with increasing temperature. The γcoal–water slightly decreases with increasing temperature. Furthermore, we conducted a comprehensive investigation into the influence mechanism underlying these factors on γcoal–fluid and quantitatively established the γcoal−CO2 assessment model. Based on this, we also proposed a methodology for determination of the threshold pore size of caprock (TPSC) and evaluation of sealing security. The calculated TPSC decreases with the increase of burial, and a more water-wetting caprock corresponds to a lower TPSC. This study contributes to a deeper understanding of the wetting phenomenon in the coal–water–CO2 system and provides insight into monitoring the safety of CO2 sequestration in coal reservoirs.