Thermodynamic properties and fluid phase equilibrium of natural gas containing CO2 and H2O at extreme pressures typically found in pre-salt reservoirs

Abstract

This paper presents an assessment on the use of Monte Carlo simulations and equations of state to calculate fluid phase equilibrium of natural gas containing CO2 and H2O at extreme pressures. Standard Gibbs ensemble Monte Carlo simulations in the canonical ensemble were used to study the coexistence properties of carbon dioxide and methane at temperatures ranging from the triple point up to the surroundings of the critical point. Vapor-liquid equilibrium of binary mixtures was investigated at temperatures of 230 K and 270 K, over a broad range of pressures. Simulation results obtained for binary mixtures showed that the different force fields and combining rules yield equivalent results, providing good accuracy at 230 K, whereas larger deviations were found at 270 K. Due to the highly non-ideal behavior and the individual characteristics of each components, vapor-liquid equilibrium of ternary mixtures was calculated using an equation of state based on the associating fluid theory (viz., PC-SAFT). The PC-SAFT equation with adjusted binary interaction parameters resulted in consistently low absolute average relative deviation of the water content, mostly within experimental uncertainty.