Thermodynamic Models For Accurate Calculations Of Densities Of Brines And Solubilities Of Gas Mixtures In Brines

Geological fluid is the most active media for material migration and energy transfer in the Earth. CH₄, CO₂, H₂S, C₂H₆ and N₂ are important gas components of geological fluids while Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, SO₄²- are major ions of geological fluids. It is important to model solubilities of these gases in electrolyte aqueous solutions and density of brine solution containing above ions for various geochemical studies such as those related to the analysis of fluid inclusions from hydrothermal deposits and geology sequestration of CO₂.Previous solubility models can represent single gas solubility in aqueous solution with high accuracy over a wide P-T range. However, they cannot predict solubilities of gas mixtures over a wide P-T range needed by various geochemical studies. This study extended the solubility model developed by Duan and coworkers for pure CO₂, CH₄, C₂H₆, and H₂S to the CO₂-CH₄-N₂-C₂H₆-H₂S gas mixture. The DMW92 EOS was extended to gas mixtures and was used to calculate fugacity coefficients of components in gas mixtures. The Pitzer model was used to calculate activity coefficients of gas dissolved in aqueous solutions. Solubility parameters determined by Duan and coworkers for pure CO₂, CH₄, C₂H₆ and H₂S were followed and solubility parameters for pure N₂ were evaluated by this study. Because this model contains no parameters evaluated from solubility data of gas mixtures, it is predictive for solubilities of gas mixtures. Comparison with experimental data available confirms the accuracy of this model. Calculations of this model indicate that the solubility of CO₂ is decreased by small percentage of CH₄, N₂ or H₂S added. The homogenization pressure of CCh-IHbO-NaCl fluid inclusions will be increased by CH₄ or N₂ added. Models for calculations of densities of mixed electrolyte solutions established by previous studies only works well at atmospheric pressure. This study proposed a new model for calculations of densities of brine solutions containing Na⁺, K⁺, Ca²⁺, Mg²⁺ and Cl^- based on the relation between apparent molar volumes of electrolytes with density of mixed salts solutions derived from Pitzer model. The density of pure water is calculated by using IAPWS95 equation. Equations for apparent molar volumes of single electrolyte determined by Mao and Duan were adopted and this model contains no parameters evaluated from density data of mixed electrolyte solutions. Comparison with experimental data available shows that this model can predict densities of brines at temperatures from 273 K to 573 K and at pressures from 1 bar to 1000 bar with high accuracy.