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Base gas replacement by a cheap gas is one of the approved methods to reduce the cost of investment in underground gas storage process. Maximizing the amount of replaceable base gas is an attractive issue. However, the amount of base gas replacement can be reduced due to some limiting factors. By changing the composition of the candidate base gas, it is possible to obtain a smart gas with an optimized composition through which all limiting factors are simultaneously satisfied. In the present study, the feasibility of this idea is investigated in a real depleted gas reservoir by flue gas as the candidate gas with the aid of GEM compositional simulator and proxy model. The proxy model and Excel Solver optimization tools are used to improve the speed and accuracy in determining the optimum composition of flue gas composition. In addition, the effect of flue gas composition on the reservoir pressure and gas deviation factor is analyzed during gas storage cycles. The results show that when the CO2 content of the flue gas exceeds a specific value, carbon dioxide effect is dominant, which increases the wetting effect of the flue gas. In this case, the simultaneous injection of flue gas and storage gas in odd cycles reduces the reservoir pressure and the reservoir gas deviation factor. The results indicate that it is possible to replace 28.4% of the base gas and reach an enhanced gas recovery of about 18.55% in the reservoir under study using the optimized replacing gas composition of 17.24% CO2 and 82.76% N2. [ABSTRACT FROM AUTHOR]