Molecular dynamics simulation of CO2-oil miscible fluid distribution and flow within nanopores

CO2 Huff-n-Puff is a promising technology for recovering unconventional oil reservoirs nowadays. Studying the structural properties of CO2-oil inside nanoscale pores and the transport mechanism during production can help further improve crude oil recovery. In this work, molecular dynamics simulations are performed to explore the distribution and flow of CO2-decane within a 6 nm SiO2 nanopore under reservoir conditions. CO2 will substitute the decane adsorbed on the surface under the strong electrostatic interactions with the SiO2 surface. The variation of CO2 content in the adsorbed layer was quantitatively described. The flow behavior of CO2-decane on the pore surface does not obey the continuous hydrodynamics theory. The adsorbed CO2 forms an adhesion layer on the pore surface, shifting the flow boundary condition from slip to negative slip. The CO2-decane permeability model is established to investigate the influences of the negative slip on the permeability. The adhesion layer reduces the permeability of CO2-decane in pores. By contrast, the dissolved CO2 reduces the bulk phase viscosity and raises the decane mobility. The flux analysis results indicate that viscosity reduction contributes more than the flux loss due to negative slip, and CO2 improves the decane transport ability within the nanopore. Besides, sensitive factors such as oil content, pore pressure, and pressure gradient are considered. This study provides new insights into the surface flow behavior of CO2-oil in nanopores, which is essential for the accurate prediction of CO2-oil transport behavior under confined conditions.