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 (<i>n</i>C₈) mixtures in quartz nanopores containing water films, to illustrate the impact mechanisms of CO₂ on <i>n</i>C₈ mobility. The results indicate that <i>n</i>C₈ exists between water films, and CO₂ is mainly miscible with <i>n</i>C₈ in the pore center, and a small portion of it accumulates at the interface between <i>n</i>C₈ and the water film. CO₂ significantly decreases the apparent viscosity of <i>n</i>C₈ in both the bulk <i>n</i>C₈ region and the <i>n</i>C₈-water interface region, improving <i>n</i>C₈ fluidity. As the percentage of CO₂ in the CO₂ and <i>n</i>C₈ mixtures increases from 0 to 50%, the mean flow velocities of <i>n</i>C₈ in the bulk phase region and the <i>n</i>C₈-water interface region increase by 92.85 and 60.64%, respectively. Three major microscopic mechanisms of CO₂ improving <i>n</i>C₈ fluidity in quartz nanopores with water films are summarized: (i) CO₂ reduces friction between <i>n</i>C₈ and the water film by increasing the angle between <i>n</i>C₈ molecules and the plane of the water film; (ii) CO₂ widens the distance between <i>n</i>C₈ molecules, causing the volume expansion of <i>n</i>C₈ and its viscosity reduction; (iii) CO₂ significantly increases the most probable and average velocities of <i>n</i>C₈ 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.