(683e) Phase Equilibria of CO2-Hexane Mixtures in Organic and Inorganic Nanopores from Wang-Landau Transition-Matrix Monte Carlo (WL-TMMC) Method

The phase behaviors of CO₂-alkane mixtures under nanoconfinements are crucial for enhanced oil recovery and geological CO₂ sequestration. It is widely accepted that confinement-induced phase behavior deviations depend strongly on pore size as well as surface characteristics.¹ However, due to computational complexity and limitations of common simulation methods, current research has primarily focused on CO₂-alkane binary systems in graphite pores² and on comparisons of single-component alkane phase behavior between organic (carbon, kerogen) and inorganic (quartz, calcite, clay, etc.) pores.³ To the best of our knowledge, no studies have investigated CO₂-alkane binary systems using full atomistic models of inorganic pores, nor systematically compared the differential impacts of organic and inorganic pores on CO₂-alkane phase behavior.

The Wang−Landau Transition-Matrix Monte Carlo (WL-TMMC) method provides a robust approach for simulating vapor-liquid equilibrium (VLE) in binary systems by uniformly sampling all relevant macrostates. By integrating histogram reweighting, this method allows the precise determination of a pair of VLE points from a single simulation. In this study, we employed the WL-TMMC algorithm within the FEASST simulation toolkit⁴ to investigate the VLE behavior of CO₂-hexane mixtures in organic pores represented by a 10-4-3 potential and inorganic pores constructed using a full-atomistic calcite structure. Furthermore, we qualitatively analyzed the differences in phase diagrams arising from competitive adsorption between CO₂ and hexane on the pore walls.

At a representative pore size (3 nm) and reservoir temperature (313.15 K), our analysis revealed a significant reduction in the corresponding bulk-phase apparent critical pressure in both organic and calcite pores. For the x/y-p phase diagrams, due to differences in selectivity, the CO₂ mole fraction in the equilibrium vapor phase decreased substantially in organic pores, while the CO₂ fraction in the equilibrium liquid phase increased slightly. In contrast, calcite pores exhibited a marked increase in the CO₂ fraction of the equilibrium liquid phase, with no significant change in the vapor phase. For the ρ-p phase diagrams, confinement effects led to increased vapor-phase density and decreased liquid-phase density in organic pores, whereas both vapor and liquid densities increased in calcite pores—a phenomenon not observed in previous single-component simulations.

Our work provides a preliminary qualitative comparison of CO₂-alkane phase behavior between organic and inorganic pores, highlighting the importance of distinguishing pore surface properties in shale phase behavior research and provides valuable insights for applying WL-TMMC in complex mineral pores.

References:

(1) Lowry, E.; Piri, M. Effects of chemical and physical heterogeneity on confined phase behavior in nanopores. Microporous and Mesoporous Materials 2018, 263, 53-61.

(2) Xing, X.; Feng, Q.; Zhang, W.; Wang, S. Vapor-liquid equilibrium and criticality of CO2 and n-heptane in shale organic pores by the Monte Carlo simulation. Fuel 2021, 299, 120909.

(3) Zhou, W.; Zhu, J.; Fang, J.; Dou, R.; Liu, X.; Chen, C. Phase behaviors of hydrocarbons in confined shale nanopores: Insights from molecular simulations. Fuel 2025, 392, 134965.

(4) Hatch, H. W.; Siderius, D. W.; Shen, V. K. Monte Carlo molecular simulations with FEASST version 0.25. 1. The Journal of Chemical Physics 2024, 161 (9), 092501.