Investigating the Phase Behavior of Methane, Ethane, Propane, and Their Ternary Mixture Confined in Nanopores: Equations of State and Monte Carlo Simulation

Tight and shale gas reservoirs contain a wide range of pore sizes on the order of nanometers, which create notable fluid–fluid and fluid–wall interactions inside the nanopores. These interactions have a direct effect on the fluid's properties, especially on the phase behavior and critical properties, that leads to unexpected behavior of the confined fluids in comparison with the bulk system. In this work, Monte Carlo (MC) simulations have been carried out in a variation of the Gibbs ensemble (NVT) that was used to study the phase behavior of methane, ethane, propane, and their ternary mixture in slit-like pores confined between graphite walls. The results were compared with the general trend and the results of modified Peng–Robinson, Soave–Redlich–Kwong, and van der Waals equations of state; the results show that for pure components and ternary mixture, the vapor density increases and liquid density decreases with the reduction of the pore width, and the critical temperature in nanopores of 5 nm decreased by 3.7, 7.9, and 11.2% for pure gases of CH4, C2H6, and C3H8, respectively. The critical pressure decreased by 22% for CH4, 54% for C2H6, and 17% for C3H8 when they are confined in 5 nm pores, and the critical density decreased with decreasing the pore width, while the critical density of the ternary mixture (containing 80% CH4, 10% C2H6, and 10% C3H8) increased from 178.28 to 184.24 kg/m3 when it is confined in nanopores with 5 nm width. Moreover, it increased from 178.28 to 219.56 kg/m3 when the mixture was confined in nanopores with 3 nm width; in addition, the phase diagram of the ternary mixture confined in pores with a size of 5 nm tends to move down compared to the bulk case; on the other hand, the m-PR-EOS gives perfect agreement with the simulation and previous results in bulk and confinement systems for pure components and ternary mixtures, but the m-SRK-EOS and m-vdW-EOS need more modifications to represent correct results for confined fluids.