Molecular Insight into CO₂ Improving Oil Mobility in Shale Inorganic Nanopores Containing Water Films

CO₂ injection into shale reservoirs has been recognized as one of the most promising techniques for enhanced oil recovery and carbon capture, utilization, and storage. However, the omnipresent nanopores and the water films formed near the pore walls affect the understanding of mechanisms of CO₂ regulating crude oil mobility in shale nanopores. In this work, we employ molecular dynamics simulations to study the occurrence and flow of CO₂ and octane (nC₈) mixtures in quartz nanopores containing water films, to illustrate the impact mechanisms of CO₂ on nC₈ mobility. The results indicate that nC₈ exists between water films, and CO₂ is mainly miscible with nC₈ in the pore center, and a small portion of it accumulates at the interface between nC₈ and the water film. CO₂ significantly decreases the apparent viscosity of nC₈ in both the bulk nC₈ region and the nC₈-water interface region, improving nC₈ fluidity. As the percentage of CO₂ in the CO₂ and nC₈ mixtures increases from 0 to 50%, the mean flow velocities of nC₈ in the bulk phase region and the nC₈-water interface region increase by 92.85 and 60.64%, respectively. Three major microscopic mechanisms of CO₂ improving nC₈ fluidity in quartz nanopores with water films are summarized: (i) CO₂ reduces friction between nC₈ and the water film by increasing the angle between nC₈ molecules and the plane of the water film; (ii) CO₂ widens the distance between nC₈ molecules, causing the volume expansion of nC₈ and its viscosity reduction; (iii) CO₂ significantly increases the most probable and average velocities of nC₈ molecules, thus improving their mobility. Our results enhance the comprehension of how CO₂ facilitates oil flow in water-bearing shale reservoirs and the exploitation of unconventional oil resources.