SAFT2 equation of state for the CH4–CO2–H2O–NaCl quaternary system with applications to CO2 storage in depleted gas reservoirs

Understanding the phase equilibria and physical-chemical characteristics of the CH₄–CO₂–H₂O–NaCl quaternary system is important for evaluating costs and risks for the storage of CO₂ in depleted natural gas reservoirs as well as fluid inclusion studies. In this study, phase equilibria and thermodynamic properties of this system were investigated through the utilization of a statistical association fluid theory-based (SAFT) equation of state (EOS) at temperatures from 298 to 513 K (25–240 °C), pressures up to 600 bar (60 MPa) and concentration of NaCl up to 6 mol/kgH₂O. The model parameters were obtained from the fitting of available experimental data of subsystems (i.e., CH₄–H₂O, CH₄–CO₂, and CH₄–H₂O–NaCl) that were judged to be reliable and incorporation of available parameters for the subsystems (i.e., pure component, CO₂–H₂O, and CO₂–H₂O–NaCl). Using the SAFT EOS developed in this study, we predicted the solubility of (CH₄ + CO₂) gas mixtures in pure H₂O and compared it with the available experimental data and the predicted values from four popular numerical simulators. The results indicate that our model can provide reliable predictions for the CH₄–CO₂–H₂O ternary system. Subsequently, we further predicted the phase equilibria and density of the CH₄–CO₂–H₂O–NaCl system with NaCl varying from 0 to 6 mol/kgH₂O. We also employed the SAFT EOS to predict the solubility of CO₂ and CH₄ in the water-alternating-gas process for CO₂-enhanced oil recovery, demonstrating good agreement with the simulation results obtained through the Peng-Robinson EOS for predicting the CO₂ and CH₄ solubility. These predicted thermodynamic properties and phase behaviors in the CH₄–CO₂–H₂O–NaCl system provide quantitative insights into the implications of CO₂ storage in depleted oil and gas reservoirs.