Molecular dynamics simulations of two-phase flow of n-alkanes with water in quartz nanopores

Nanopore networks comprising shale reservoirs exhibit special behaviors in two-phase flow. We use molecular dynamics simulations (MD) to explore the single n-alkanes (nC3-nC10)-water and mixed n-alkanes (nC3, nC6, and nC10)-water flow mechanisms in the confined nanopores of quartz. The results reveal that water molecules form four adsorbed layers near the wall, showing greater viscosity than the bulk water. For single n-alkanes cases, in the n-alkanes-water interface regions (AWIR), n-alkanes molecules tend to be parallel to the pore wall, and the tendency is more pronounced for longer n-alkanes, resulting in a decrease of AWIR width. Moreover, we observed typical liquid-liquid slip in AWIR. Ignoring the slip velocity can underestimate the flow rate of n-alkanes more than 20%. Under the driving force, the bulk n-alkanes exhibit distinct different velocity distributions due to viscosity differences. However, the total fluid velocity distributions in AWIR are quite the same, showing an apparent viscosity independent of the type of n-alkanes and pore size. For mixed n-alkanes, the velocity distribution and apparent viscosity are still the same as single n-alkanes cases due to the unchanged structure of the molecules in AWIR. Our study could provide an in-depth understanding of the oil-water two-phase flow in nanopores, especially the influence of carbon chain length on oil-water interface properties.

Automated summary

This study using molecular dynamics simulations found that water molecules form four adsorbed layers near the wall in quartz nanopores, exhibiting greater viscosity than bulk water. In the interface regions between n-alkanes and water, n-alkanes tend to align parallel to the pore wall, with longer n-alkanes showing a more pronounced alignment. Ignoring slip velocity can lead to an underestimation of n-alkanes flow rate by over 20%.