Vapor-liquid phase equilibrium of n-pentane in quartz nanopores by grand canonical Monte Carlo calculation

Abstract

As the main basis for calculating other physical property data, the critical temperature and pressure of fluid are important parameters in the process of oil and gas field development. In unconventional reservoirs, crude oil fluid generally exists in nanopores. The interaction between fluid molecules and the pore wall leads to the deviation of fluid properties. Pore size is an important parameter affecting the phase state of reservoir fluid. The phase state laws of alkanes in different nanopore sizes are quite different. We studied the effect of the pore size of quartz nanopores on the vapor–liquid phase equilibrium using the grand canonical Monte Carlo method. The quantitative relationship between the nanopore size and the adsorption amount, density profile, and phase diagram of n-pentane was obtained, and its micromechanism was analyzed. The results showed that the critical temperature and critical pressure of n-pentane increased with an increase in the nanopore size. The critical temperature was consistent with the bulk phase after the pore size reached 10 nm, and the critical pressure could be treated with the bulk phase after the pore size reached 30 nm. The van der Waals force between the fluid and the pore wall was the main factor leading to the phase change of the system. These findings suggest that the aperture of nanopores was significant for alkane phase change, which might provide a reference for understanding the phase transformation law behavior of alkanes in nanopores of tight reservoirs.