An Accurate Model Of Interfacial Tension Of CO₂/NaCl Aqueous Solution

Fluids play a key role in mass and heat transfer processes within the Earth.Extensive studies of fluid inclusions have shown that,in addition to chloride salts,CO₂ is the predominant uncharged molecular solute of geological fluids in various crustal environments.The composition of fluids in metamorphic rocks,pegmatites,carbonates and various hydrothermal deposits can be approximated as the H₂O-NaCl system or the CO₂-H₂O-NaCl system.Understanding the various physical and chemical properties of geological fluids is an important prerequisite for the study of the role of fluids played in geological processes.Among them,interfacial tension is one of the most important physical and chemical parameters of two-phase flow in porous media.Understanding the interfacial tension between CO₂ and aqueous NaCl solution is of great significance for the geological sequestration of carbon dioxide,the application of inclusions,and the simulation of various hydrothermal deposits and metamorphisms.However,there is no model allows of calculations of the interfacial tension between CO₂ and aqueous NaCl solution in a wide temperature and pressure range with high accuracy.In this paper,by combining the SAFT-LJ equation of state,which considers the intermolecular interaction force,and the density gradient theory,which considers the density gradient change of the fluid interface and the inhomogeneity of the molecular distribution,a calculation model for interfacial tension of CO₂/H₂O and CO₂/NaCl aqueous solution was established in a wide range of temperature and pressure.Comparison with experimental data shows that the model proposed in this paper can accurately calculate the interfacial tension of CO₂/H₂O and CO₂/NaCl aqueous solution in a wide range of temperature and pressure.For the interfacial tension between CO₂ and H₂O,the accuracy of this model is generally within the uncertainty of experimental measurements.For the interfacial tension between CO₂ and aqueous NaCl solution,there are large discrepancies among experimental data.Calculations of this model are comparble to to measurements.Based on the new model this study developed,a computer program was written to calculate the interfacial tension between CO₂ and brine under conditions of CO₂ geological sequestration.This model can correctly predict the variations of the interfacial tension between CO₂and aqueous NaCl solution with temperature,pressure and composition.That is to say,（1）the interfacial tension decreases with the increase of pressure under constant temperature condition and the decrease rate decrease with the increasing pressure.Lower the temperature,more rapid the decrease rate at low pressures but slower the decease rate at high pressures.（2）the interfacial tension increases with the increase of salinity at constant temperature and pressure.