MODELING OF PHASE-EQUILIBRIA INVOLVING MIXED GAS CLATHRATES - APPLICATION TO THE DETERMINATION OF MOLAR VOLUME OF THE VAPOR-PHASE AND SALINITY OF THE AQUEOUS-SOLUTION IN FLUID INCLUSIONS

Modelling of clathrate stability in single component gases (CO2, CH4, N2, H2S and C2H6) and of mixtures in systems with or without NaCl is described, and then applied to fluid inclusions in which the dissociation of clathrate occurs in the presence of a vapour phase. The chemical potential of the water component in clathrate is calculated from a combination of the models of Parrish & Prausnitz (1972) and of Munck et al. (1988). The Langmuir constants have been recalculated to obtain the best fit of the available experimental data on single gas systems. The dependence of the activity of water on the salt concentration in the aqueous solution is described by the model of Pitzer. Pressure is calculated with an uncertainty of less than 10 % along the dissolution curve of clathrate for single component gas systems and gas mixtures. The bulk salinity, expressed in equivalent NaCl concentration, can be derived from the melting temperature of ice in presence of the assemblage clathrate + vapour phase + aqueous solution, but is shown to be a function of the volume fraction of the vapour phase. The fugacity coefficient of the gas components and the molar volume of the vapour phase are computed from the Soave equation of state.