Thermodynamic description of equilibria in mixed fluids (H2O-nonpolar gas) in a wide range of temperatures (25–700°C) and pressures (1–5000 bar)

Abstract

to the presence of solute nonpolar gas may result in a cardinal change in components of the solution and a decrease in mineral solubility by orders of magnitude.Thermodynamic description of mixed fluids such as H 2 O-nonpolar gas at high temperatures and pressures is a challenging problem. Obviously, consideration of the equilibria of individual reactions is insufficient for describing the equilibrium in the system as a whole. The purpose of this work is to propose a general method allowing the study of equilibria in multi-systems over mixed fluid. The method uses the HelgesonKirkham-Flowers (HKF) electrostatic equation of state for aqueous components (Tanger and Helgeson, 1988) and can be regarded as an extension of this equation to the case of mixed fluids H 2 O-volatile. The extensive thermodynamic database SUPCRT98 , used along with the Gibbs universal minimization program BALANCE (Akinfiev, 1986(Akinfiev, , 1993, permits not only consideration of individual reactions but also study of equilibria in such hydrothermal systems as a whole. KEY RELATIONSHIPSTo predict the behavior of components in a solution at high temperatures and pressures, so-called electroAbstract -A new method is proposed for calculating equilibria in mixed multicomponent systems H 2 O-nonpolar gas at high temperatures and pressures. The method is based on calculation of electrostatic interactions between a solute component and surrounding molecules of a solvent ( H 2 O ). The SUPCRT98 database and the Helgeson-Kirkham-Flowers equation of state for components of an aqueous solution were used for computer realization of the method. The thermodynamic properties of solute gases at elevated temperatures and pressures are calculated by the Redlich-Kwong equation. The dielectric properties of a mixed solvent are determined using the modified Kirkwood equation. The proposed method was approved in description of the available set of experimental data on constants of H 2 O and NaCl dissociation, as well as data on solubility of both covalent and ionic crystals [ SiO 2 , AgCl, Ag 2 SO 4 , Ca(OH) 2 , CaCO 3 ] in mixed solvents H 2 O-nonpolar component [1,4-dioxane ( C 4 H 4 O 2 ), Ar, CO 2 ]. The calculation and experimental data agree in a wide range of PTx conditions (temperatures up to 500°C , pressures up to 5000 bar, and mole fractions ( x ) of a nonpolar component in fluid up to 0.3).The proposed approach can be used for assessing the Born parameters of solute components. The calculation algorithm developed allows us not only to consider separate reactions but also to study equilibria in hydrothermal systems as a whole. Hence, the proposed approach can be used for constructing thermodynamic models of evolution of fluids rich in volatile components and vital to natural processes of mineral formation.