PHASE-EQUILIBRIA MODELING APPLIED TO FLUID INCLUSIONS - LIQUID-VAPOR EQUILIBRIA AND CALCULATION OF THE MOLAR VOLUME IN THE CO2-CH4-N2 SYSTEM

Quantitative use of fluid inclusions requires the determination of com position and molar volume. The molar volume can be calculated in the C O2-CH4-N2 system from both the determination of the temperature of a L + V --> L (or V) equilibrium if the composition is known independentl y, provided an equation of state (EOS) reproduces the P-V-T-X properti es of each phase at equilibrium. This study is applicable to fluids fo r which the sequence of phase transition is S(CO2) + L + V --> L + V - -> L (or V) at increasing temperature. The molar volume is determined by following a two-step algorithm: (1) the.pressure is calculated from a two-parameter cubic EOS with interaction parameters optimised along experimental L-V isotherms; (2) the molar volume is then calculated b y the correlation of LEE and KESLER ( 1975) with the pressure calculat ed in the first step. Projections of polybaric L-V isotherms in upsilo n-X diagrams of the CO2-CH4, CO2-N2, and CH4-N2 systems can be directl y applied to fluid inclusion studies. In addition, it is shown that th e molar volume Of CO2-rich fluids (X(CO2) > 0.6) in the ternary system can be estimated with sufficient accuracy using empirical formulae re lating to the upsilon-X diagrams of the CO2-CH4 and CO2-N2 binary syst ems.