ACTIVITY-COMPOSITION RELATIONS IN THE SYSTEM H2O-CO2-NACL AT 500 °C, 50 MPA

Experiments in the system H₂O–CO₂–NaCl at 500 °C, 50 MPa, have delimited the solubility of halite in H₂O–CO₂ fluids and have determined the tie lines between fluid and halite at constant values of a_H2O. The phase diagram shows a one-phase vapor field, a one-phase brine field, two-phase vapor + halite, vapor + brine, and brine + halite fields, and a three-phase vapor + brine + halite field. The halite-saturated vapor ranges in composition from x_NaCl ≈ 0 in CO₂ to 0.08 at the vapor corner of the three-phase field, where x_H2O ≈ 0.6. The a_H2O was fixed during the experiment, and the a_NaCl in the vapor was fixed by saturation with halite. The a_CO2 was determined by integrating the Gibbs–Duhem equation along the saturation curve, ∑x_id lna_i=0. The effect of adding NaCl to H₂O–CO₂ vapor is represented by values of ∂ln(γ_H2O)/∂x_NaCl ranging from −7 in CO₂-rich vapor to 3 in H₂O-rich vapor. The value of ∂ln(γ_CO2)/∂x_NaCl shows an even larger range from 10 to −18 over the same compositional range. The measured values of the activities in the three-component system are combined with models for the activities in the H₂O–CO₂ and H₂O–NaCl binaries to calculate the complete phase diagram.