Vapor-liquid equilibrium and criticality of CO2 and n-heptane in shale organic pores by the Monte Carlo simulation

Abstract

Thanks to the application of CO2 enhanced oil recovery (EOR) technology in shale oil production, phase behavior of CO2 and alkanes in nanopores has received considerable attention. Understanding the confinement effect on the vapor–liquid equilibrium and critical state of CO2 and alkanes is imperative to the calculation of phase behavior and prediction of oil recovery. In this work, the vapor–liquid equilibrium of pure CO2 and n-heptane (nC7) and their mixtures in shale organic pores are simulated using the Monte Carlo simulation. Compared to the bulk phase, the density of equilibrium gas is larger due to adsorption while the liquid phase density is lower because of capillary condensation. The influence of the confinement effect on the equilibrium phase composition results in the lower bubble point pressure and higher dew point pressure of the binary system. Changes in the equilibrium phase lead to a decrease in the critical temperature and pressure of single-component fluids and the minimum miscible pressure of binary system in nanopores. We fitted the critical property shift and analyzed the influence of pore size on fluid properties such as equilibrium pressure, composition, critical parameters, density distribution, etc. This study provides better insight into vapor–liquid equilibrium and critical state of CO2 and alkane in shale organic pores.