Thermodynamic Models For Accurate Calculations And Applications Of Vapor-liquid Equilibrium And PVTX Properties Of CO₂?CH₄?-H₂O System Inside The Critical Region

Fluids play an important role in most geological processes.Besides halide,the major components of most water-rich geological fluid systems are H2O,CO2 and CH4.PVTx properties,activity,chemical potential,enthalpy,phase equilibrium and other thermodynamic properties are fundamental physical and chemical parameters of fluids.Therefore,it is essential for fluid geology study to accurately calculate the PVTx and vapor-liquid equilibrium in the CO2(CH4)-H2O system with a wide range of temperature and pressure Meanwhile,it is helpful for interpreting measurement data of fluid inclusions and determining physical and chemical conditions of various metallogenic processesFor fluid systems,there is a strong density fluctuation in the critical region,which leads to the singularity of thermodynamic properties of fluids near the critical point.The study of singularity not only has positive effect for correctly understanding and simulating of many geological processes within the Earth and other planets,but also has practical value for developing related industries,promoting environmental engineering,and studying the physics of high temperature and high pressureThe equation of state is a powerful tool to calculate thermodynamic properties of fluids However,classical equation of state do not work well in the critical region in that they do not consider the density fluctuation in the critical region.This research group has established a model which can calculate vapor-liquid equilibrium and PVTx properties of the H2O-CO2?H2O-CH4 and CO2-CH4 systems over a wide range of temperature and pressure by improving and extending the SAFT-LJ equation of state.But this model does not work well in the near-critical region as well.This study developed the SAFT-LJ+RG crossover equation of state by combining the renormalization group theory proposed by Wilson with the SAFT-LJ equation of state proposed by our group.Specifically,the phase space microcell approximation method is used to deal with the strong density fluctuation near the critical point,the zero-order Helmholtz free energy is calculated by using the SAFT-LJ equation of state,so that the model can work well in both near-critical region and far-critical region.The parameters of this model,including molecular interaction parameters of pure components,mixing parameters between two different components,and renormalization parameters were optimized by regressing the vapor-liquid equilibrium as well as PVT data of pure components and binary systemsComparison of calculation results of this model with experimental data indicates that this model can accurately represent the vapor-liquid equilibrium and PVTx properties of the H2O-CO2,H2O-CH4 and CO2-CH4 systems near the critical region.Major results include the following(1)The SAFT-LJ+RG equation of state was used to calculate the critical point of pure H2O system,pure CO2 and pure CH4 system.The calculated critical temperature,critical pressure,critical density agree well with the experimental data.The average absolute deviation(AAD%)is 0.0145%,0.0549%and 2.86%respectively.The average absolute deviation(AAD%)of calculated PVT properties of three pure componet system near the critical point(including supercritical region and near-critical region)is just 0.957%.SAFT-LJ+RG state equation can also accurately calculate the vapor-liquid equilibrium curves of three single-species systems in the near-critical region(H2O:600-647.1 K,CO2:270-304.1 K,CH4:160-190.5 K).The deviations of saturated vapor pressure,saturated gas density and saturated liquid density are 1.18%,3.16%and 2.04%,respectively.It proves that the renormalization group theory can perfectly deal with the singularity near the critical region The new model can not only accurately calculate PVT properties of the single-species fluid,but also can calculate the vapor-liquid equilibrium of these systems(2)Calculations of this model for vapor-liquid equilibria of the CH4-CO2,H2O-CH4 and H2O-CO2 systems in near-critical region were compared with the experimental data of nine groups under near-critical temperatures.The result indicates that the calculated critical pressure and critical composition agree well with with the experimental data,and the average absolute deviation(AAD%)is 2.84%and 10.9%respectively.The average absolute deviations of the saturated CH4 mole fraction of the CH4-CO2 system,the saturated CO2 mole fraction of H2O-CH4 system,as well as the saturated CH4 mole fraction of H2O-CH4 system from experimental value were 7.47%,13.7%and 16.6%respectively,generally within the uncertainty range of the experimental data(3)Using SAFT-LJ+RG equation of state to calculate the PVTx properties of the H2O-CO2 system,the average absolute deviation from the experimental results is only 1.69%.compared with the original SAFT-LJ equation,there is obvious progress,which proves that the new model can calculate the PVTx properties of the system in the near critical region more accurately.the introduction of renormalization group theory effectively corrects the strong density fluctuations near the critical pointBased on the SAFT-LJ+RG state equation established in this study,application computer program is designed to calculate phys-chemical parameters of H2O-CO2 or H2O-CH4 fluid inclusions.Compared with previous computersoftware,the new software can give more accurate the pressure of H2O-CO2 and H2O-CH4 inclusions homogenized in near-critical region.