Phase equilibria modelling applied to fluid inclusions: Liquid-vapour equilibria and calculation of the molar volume in the CO 2CH 4N 2 system

Abstract

Quantitative use of fluid inclusions requires the determination of composition and molar volume. The molar volume can be calculated in the CO 2CH 4N 2 system from both the determination of the temperature of a L + V → L ( or V) equilibrium if the composition is known independently, provided an equation of state (EOS) reproduces the P- V- T- X properties of each phase at equilibrium. This study is applicable to fluids for which the sequence of phase transition is S co 2 + L + V → L + V → L ( or V) at increasing temperature. The molar volume is determined by following a two-step algorithm: 1. (1) the pressure is calculated from a two-parameter cubic EOS with interaction parameters optimised along experimental L- V isotherms; 2. (2) the molar volume is then calculated by the correlation of Lee and Kesler (1975) with the pressure calculated in the first step. Projections of polybaric L- V isotherms in v- x diagrams of the CO 2CH 4, CO 2N 2, and CH 4N 2 systems can be directly applied to fluid inclusion studies. In addition, it is shown that the molar volume of CO 2-rich fluids ( X co 2 > 0.6) in the ternary system can be estimated with sufficient accuracy using empirical formulae relating to the v- x diagrams of the CO 2CH 4 and CO 2N 2 binary systems.