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 (nC(8)) mixtures in quartz nanopores containing water films, to illustrate the impact mechanisms of CO2 on nC(8) mobility. The results indicate that nC(8) exists between water films, and CO2 is mainly miscible with nC(8) in the pore center, and a small portion of it accumulates at the interface between nC(8) and the water film. CO2 significantly decreases the apparent viscosity of nC(8) in both the bulk nC(8) region and the nC(8)-water interface region, improving nC(8) fluidity. As the percentage of CO2 in the CO2 and nC(8) mixtures increases from 0 to 50%, the mean flow velocities of nC(8) in the bulk phase region and the nC(8)-water interface region increase by 92.85 and 60.64%, respectively. Three major microscopic mechanisms of CO2 improving nC(8) fluidity in quartz nanopores with water films are summarized: (i) CO2 reduces friction between nC(8) and the water film by increasing the angle between nC(8) molecules and the plane of the water film; (ii) CO2 widens the distance between nC(8) molecules, causing the volume expansion of nC(8) and its viscosity reduction; (iii) CO2 significantly increases the most probable and average velocities of nC(8) 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.