SAFT2 equation of state for the CH4–CO2–H2S–H2O–NaCl quinary system

Understanding the phase equilibria and physicochemical properties of the CH₄–CO₂–H₂S–H₂O–NaCl quinary system and its subsystems is essential for assessing fluid migration and changes in geological formations following CO₂ or acid gas injection. Moreover, this system is closely associated with a large percentage of geological fluids responsible for transport, mass transfer, and the formation of critical mineral ore deposits. In this study, a statistical associating fluid theory (SAFT)-based equation of state (EOS) was developed to investigate phase equilibria and thermodynamic properties of the system over temperatures from 298 to 423 K, pressures up to 600 bar, and NaCl concentration up to 6 mol/kgH₂O. The model incorporated pure component and cross-interaction parameters from previous studies, along with CH₄–H₂S cross-interactions derived from experimental data in this work. The SAFT EOS reliably predicted the phase behavior of the CH₄–CO₂–H₂S and CH₄–CO₂–H₂S–H₂O systems, as validated against experimental data and other thermodynamic models. It also successfully predicted phase equilibria and densities for the CH₄–H₂S–H₂O–NaCl and CH₄–CO₂–H₂S–H₂O–NaCl systems across a NaCl concentration range of 0–6 mol/kgH₂O. This study provides the first systematic development of a SAFT-based model for the CH₄–CO₂–H₂S–H₂O–NaCl system, demonstrating reliable performance in characterizing their phase behavior and thermodynamic properties.