Vapor-liquid and liquid-liquid phase equilibria in the CO2-CH4-N2 system: A Gibbs ensemble Monte Carlo simulation

Abstract

CO₂, CH₄, and N₂ are the most common gaseous species in geological fluids, originating from diverse sedimentary, metamorphic, igneous, and hydrothermal processes. Analyzing relic paleo-geofluids trapped in minerals as fluid inclusions is crucial for understanding their roles in these geological processes, which depends on the established phase and volumetric behavior of the fluids as a function of pressure (P), temperature (T), and composition (x). However, collecting comprehensive data for such a ternary system across all potential PTx variables is a significant experimental challenge. Therefore, this study simulated the fluid-phase equilibrium in the unary, binary, and ternary systems of CO₂-CH₄-N₂ using the Gibbs Ensemble Monte Carlo (GEMC) method, a powerful molecular simulation technique for studying systems with multiple fluid phases. The considered PT range encompasses liquid/liquid, liquid/vapor, and liquid/liquid/vapor equilibrium regions. Molecular interactions in the system are described using two-body Lennard-Jones potentials, requiring only two temperature-independent parameters for similar molecules. The Berthelot-Lorentz rules are applied to define the Lennard-Jones interactions for dissimilar molecules, with an additional temperature-independent mixing parameter. The equilibrium compositions and molar volumes of the coexisting phases in all mixtures are predicted with an accuracy comparable to that of the experimental data. The PT dependence of liquid-liquid-vapor coexistence at very low temperatures and the potential for gas-gas coexistence at high temperatures are discussed for the binary subsystems. It can be concluded that the GEMC simulation is an important “computational experiment” to investigate thermodynamic properties of natural fluids if molecular interaction potential is well described, providing appreciable supplementation to experimental findings.