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

CO2 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 CO2-EOR mechanisms at the nanoscale is of critical importance. In this work, we study the structural and dynamic properties of CO2/nC8 systems in organic kerogen and inorganic quartz nanopores by molecular dynamic simulation and clarify the dominant EOR mechanisms of CO2 injection in various mineral nanopores. We find a large positive slip velocity occurs when single-phase nC8 flows in quartz nanopore while it is no-slip boundary condition in kerogen nanopore. The CO2-regulated nC8 in quartz nanopore is the joint effect of slip, competitive adsorption and viscosity reduction. In the first stage, CO2 extracts the first adsorption layer of nC8 by competitive adsorption and forms a CO2 film on the quartz surface. The CO2 film reduces the slip velocity between nC8 and quartz surface and weakens the nC8 flow capacity. After CO2 adsorption is saturated, the CO2 mixes with nC8 in all flow regions and the effective viscosity of nC8 starts decreasing at this stage. The flow capacity of nC8 rises dramatically due to the viscosity reduction mechanism of CO2. In kerogen nanopore, it is no-slip boundary condition and CO2 mixes with nC8 in all flow regions directly instead of preferably adsorbed on the surface. Viscosity reduction is the dominant mechanism to affect nC8 flow behavior and nC8 flow capacity is enhanced monotonically as CO2 injection. Our study advances the understanding of the recovery mechanisms of shale oil by CO2 injection on nanoscale and provides the theoretical foundation for the optimization of CO2-EOR in shale oil reservoirs.