Recovery mechanisms of shale oil by CO2 injection in organic and inorganic nanopores from molecular perspective.

• The static and flow behavior of CO 2 -nC 8 in kerogen and quartz nanopores are studied. • Oil viscosity reduction of CO 2 is the dominant EOR mechanism in kerogen nanopores. • CO 2 extracts adsorbed nC 8 and reduces the slip by competitive adsorption on quartz. • Oil viscosity reduction occurs in quartz nanopore after CO 2 adsorption is saturated. CO 2 injection is the feasible method to enhance shale oil recovery and has attracted extensive attention in recent years. Since shale reservoir has omnipresent nanoscale pores, understanding the underlying CO 2 -EOR mechanisms at the nanoscale is of critical importance. In this work, we study the structural and dynamic properties of CO 2 /nC 8 systems in organic kerogen and inorganic quartz nanopores by molecular dynamic simulation and clarify the dominant EOR mechanisms of CO 2 injection in various mineral nanopores. We find a large positive slip velocity occurs when single-phase nC 8 flows in quartz nanopore while it is no-slip boundary condition in kerogen nanopore. The CO 2 -regulated nC 8 in quartz nanopore is the joint effect of slip, competitive adsorption and viscosity reduction. In the first stage, CO 2 extracts the first adsorption layer of nC 8 by competitive adsorption and forms a CO 2 film on the quartz surface. The CO 2 film reduces the slip velocity between nC 8 and quartz surface and weakens the nC 8 flow capacity. After CO 2 adsorption is saturated, the CO 2 mixes with nC 8 in all flow regions and the effective viscosity of nC 8 starts decreasing at this stage. The flow capacity of nC 8 rises dramatically due to the viscosity reduction mechanism of CO 2. In kerogen nanopore, it is no-slip boundary condition and CO 2 mixes with nC 8 in all flow regions directly instead of preferably adsorbed on the surface. Viscosity reduction is the dominant mechanism to affect nC 8 flow behavior and nC 8 flow capacity is enhanced monotonically as CO 2 injection. Our study advances the understanding of the recovery mechanisms of shale oil by CO 2 injection on nanoscale and provides the theoretical foundation for the optimization of CO 2 -EOR in shale oil reservoirs. [ABSTRACT FROM AUTHOR]