To investigate the long-term CO2 behaviors in geological formations and quantification of possible CO2 leaks needs to focus on the potential mobility of CO2 dissolved in formation brines over a wide range of spatial and temporal scales and requires information of the CO2 concentration and density distribution in the geological media. In this paper, the mass transfer of CO2 in reservoir brines at high pressures is investigated by chemical potential gradient modeling based on nonequilibrium thermodynamics instead of concentration gradient modeling and the experimental kinetics data reported by Yang and Gu. In this model, the Statistical Associating Fluid Theory equation of state (SAFT1-RPM) is used to calculate the fugacity and densities of the aqueous CO2 in the investigated systems and the effects of temperature, initial CO2 pressure and volume on the boundary conditions are considered. The calculation results with this model and modified Fick’s second law are compared, the effects of temperature, pressure, the initial CO2 pressure and initial CO2 volume are analyzed and the density distribution of the CO2-dissolved brines are predicted. This study shows considerable differences between the results by the chemical potential gradient modeling and that by the concentration gradient modeling, which shows the importance of considering the non-ideality correction. The results also show the temperature and initial CO2 pressure and initial CO2 volume have great effects on the concentration distribution with distance and greater effects of temperature on the densities of the CO2-dissolved brine than that of initial CO2 pressure and initial CO2 volume.