Modeling the Properties of H2S/CO2/Salt/Water Systems in Wide Ranges of Temperature and Pressure

Abstract

SummaryTo address the need to predict the properties of fluids in severe environments in the oil and gas industry, a comprehensive thermodynamic model has been developed for mixtures containing hydrogen sulfide (H2S), carbon dioxide (CO2), H2O, and selected salts. The model is based on the previously developed mixed-solvent electrolyte framework, which combines an equation of state for standard-state properties of individual species, an excess-Gibbs-energy model, and an algorithm for solving phase and chemical equilibria in multiphase systems. The standard-state properties are calculated from the Helgeson-Kirkham-Flowers (Helgeson et al. 1974a, 1974b, 1976, 1981; Tanger and Helgeson 1988) equation, whereas the excess Gibbs energy is expressed as a sum of a long-range electrostatic-interaction term expressed by a Pitzer-Debye-Hückel equation (Pitzer 1980), a virial coefficient-type term for interactions between ions, and a short-range term for interactions involving neutral molecules. The model has been parameterized using critically evaluated phase equilibrium data for various binary and ternary subsystems of the H2S/CO2/H2O/Na/Ca/Cl system and has been validated for temperatures ranging from 0 to 300°C, pressures up to approximately 3,500 atm, and salt concentrations up to solid saturation. The model reproduces chemical speciation in acid gas/brine systems as exemplified by the accurate prediction of pH. Because of its capability of predicting pH and activities of solution species, the model can serve as a foundation for studying metal/environment interactions in severe oil and gas environments.