Equation Of State For The CO₂-CH₄-N₂ Fluid Mixtures And Its Applications

CO2,CH4 and N2 are common components of geofluids and their pressure-volume-temperature-composition(PVTx)and vapor-liquid equilibrium(VLE)properties are used to determine the composition,temperature and pressure of fluid inclusions(paleo-fluids).The solubility of the CO2-CH4-N2 gas mixtures in the electrolyte solutions can provide the quantitative assessment for the storage of CO2 in deep saline aquifers.Because experimental data are limited,thermodynamic models have become a more powerful tool.Based on this purpose,this study mainly does the corresponding work in the applicatio ns of the equation of state(EOS)of CO2,the establishment and applications of the EOS of the CO2-CH4-N2 fluid mixtures,and the prediction of the solubility of the CO2-CH4-N2 gas mixtures in electrolyte solutions,and details are as follows:(1)The SW96 equation(Span and Wagner,1996)explicit in Helmholtz free energy is the most accurate EOS for CO2 fluid at present.The SW96 equation can reproduce all thermodynamic properties from 216.592 to 1100 K and from 0 to 8000 bar.In this study,the SW96 equation is applied to the study of fluid inclusions by thermodynamic relationships and calculation methods based on the phase-transition temperatures:calculation of saturated properties,volume fractions of vapor bubble and isochores.At the same time,the SW96 formulation is used to calculate the equilibrium curve of decarbonization of minerals.At the same time,the SW96 equation is applied to calculate the equilibrium curve of decarburization of minerals.(2)A general Helmholtz free energy form EOS for the CO2-CH4-N2 system has been proposed,which can reproduce experimental PVTx and VLE data of CO2-CH4-N2 fluid mixtures up to 673 K and 1000 bar.This EOS and the Gibbs free energy model of solid CO2 can be combined to determine the compositions,molar volumes and isochores of CO2-N2 fluid inclusions.The EOS of ternary mixtures can also be applied in the study of fluid inclusions:calculations of isochores and homogenization pressures.(3)The EOS of CO2-CH4-N2 system developed in this work and the previous solubility model of pure component gases(CO2,CH4,N2)can be combined to predict the solubility of the CO2-CH4-N2 gas mixtures in electrolyte aqueous solutions based on the activity-fugacity relationship at phase equilibrium.The predicted results are in good agreement with experimental data available.