Abandoned gas reservoirs are ideal sites for carrying out carbon dioxide（CO₂）storage projects. However，impurities in gas reservoirs can affect the CO₂ dissolution in saline aquifers. To this end，a fugacity-activity thermodynamic model is constructed to capture the features of impurities，CO₂，and saline aquifers. On this basis，the operator-based linearization approach is implemented to solve the mass conservation equations accurately and efficiently. Then the effects of gas mixtures on CO₂ diffusion-convection are studied. With the Pernis gas reservoir in the Netherlands as an example，the storage potential of abandoned gas reservoirs is evaluated. The results show that the CO₂ solubility in saline aquifers increases with the rising pressure while decreases with higher temperature and salinity. At the same pressure，the CO₂ solubility at 25 ℃ is about twice that at 90 ℃. In addition，different impurities have different effects on CO₂ dissolution，and CH4 reduces the CO₂ solubility by around 12%. 2D simulation results show that the fingering rate of CO₂ plume in saline aquifers is limited by the impurities. Meanwhile，the onset time of convection is extended due to the impurities. The onset time of CO₂ mixture with 5% methane is around 2.5 times that of pure CO₂. Considering the long timescale，the final dissolution is not affected by the impurities significantly. Through the studies of the Pernis gas reservoir，abandoned gas reservoirs have a large potential for CO₂ storage. In the short run，CO₂ exists in porous media as free gas and residual gas，while the amount of dissolved gas is relatively small. In 30 years after shutting the wells，the free gas accounts for 74.2%，and residual gas is 19.1%，while only 6.7% gas is dissolved into the aquifer.