New Thermodynamic Models To Predict Phase Equilibria Of Water-Gases-Salts Fluid Systems At Low Temperatures

Fluids take part in many geological processes and play important roles in them. Previous researches indicate that, water (H2O), methane (CH4), carbon dioxide (CO2), nitrogen (N2), and sodium Chloride (NaCl) are major components of many Geo-fluids. In H2O-CH4-CO2-N2-NaCl fluid systems (water-gases-salts fluid systems), several phase equilibria may occur at low temperatures (from -20 to 200℃). Among of them, Gas-Liquid Equilibrium and Gas hydrates-Gas-Liquid Equilibrium are the most important. To study phase equilibria of water-gases-salts fluid systems at low temperatures is important both to investigate the role of Geo-fluids play in the geological processes that take place in the surface and upper Crust of the Earth and to guide the prospecting and exploitation of oil and gases and natural gas hydrates resources. The knowledge on phase equilibria of water-gases-salts fluid systems at low temperatures is also essential for designing certain separation equipment in the chemical or oil-related industries.Due to the scarcity of the experimental data, thermodynamic models are the most important tools to study phase equilibria and other thermodynamic properties of fluids. Many thermo- dynamic models have been developed to predict gas-liquid equilibria of water-gases-salts fluid systems by previous researchers. However, none of them can predict gas-liquid equilibria of H2O-CH4 system and H2O-CO2 system at low temperatures between 0 and 100℃ accurately. None of them can predict carbon dioxide solubility and nitrogen solubility in pure water and aqueous NaCl solutions at temperatures below 200℃ accurately. Previous thermodynamic models for clathrate hydrates can't predict phase equilibrium of methane hydrate at pressures more than 500 bar with sufficient accuracy. Furthermore, there still exist some theoretical flaws in these models. The aim of this study is to establish new thermodynamic models to predict gas-liquid equilibrium of water-gases-salts fluid systems and methane hydrate phase equilibrium at low temperatures. The following are the major achievements of this study.(1) A new equation of state (EOS) has been developed to predict Gas-Liquid Equilibrium and volumetric properties of CH4-H2O system at low temperatures. The equation of state consists of a reference part and a perturbation contribution. Following the approach of Anderko and Pitzer (1993), we adopt hard sphere model and dipolar hard sphere model to account for the contribution of "Reference Fluid". A new virial-type expansion was adopted to deal with perturbation contribution. The adjustable parameters of the equation of state were evaluated from experimental PVT data of pure water and methane and phase equilibrium data of CH4-H2O system. Comparison of the model predictions with experimental data demonstrates that this equation of state can predict PVT data of pure water and methane from 273 to 623 K, and from 0 to 3000 bar with an average error about 0.3%. Moreover, the equation of state can predictgas-liquid equilibrium of CH4-H2O system accurately from 273 to 373 K, and from 0 to 1000 bar. The deviation of this model is within experimental uncertainty. The main advantages of this model are: (a) The accuracy of this model is much better than previous models; (b) This model can predict volumetric properties of liquid water and aqueous solutions accurately, while most of models to deal with gas-liquid equilibria of fluids can't predict volumetric properties of liquid water and aqueous solutions quantitatively. (2) A new model to predict phase equilibrium of methane hydrates has been established. This model is based on the Van der Waals-Platteeuw Model (1959). Langmuir constant, the key parameter of Van der Waals-Platteeuw model, is calculated from exp-6 potential. Englezos- Bishnoi model (1988) was adopted to account for the effect of NaCl on the activity of water. The improvements of this study on Van der Waals-Platteeuw model are: (a) this model adopted exp-6 potential to calculate Langmuir constant. The parameters of exp-6 poten...