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Underground gas storage reservoirs (UGSRs) are used to keep the natural gas supply smooth. Native natural gas is commonly used as cushion gas to maintain the reservoir pressure and cannot be extracted in the depleted gas reservoir transformed UGSR, which leads to wasting huge amounts of this natural energy resource. CO₂ is an alternative gas to avoid this particular issue. However, the mixing of CO₂ and CH₄ in the UGSR challenges the application of CO₂ as cushion gas. In this work, the Donghae gas reservoir is used to investigate the suitability of using CO₂ as cushion gas in depleted gas reservoir transformed UGSR. The impact of the geological and engineering parameters, including the CO₂ fraction for cushion gas, reservoir temperature, reservoir permeability, residual water and production rate, on the reservoir pressure, gas mixing behavior, and CO₂ production are analyzed detailly based on the 15 years cyclic gas injection and production. The results showed that the maximum accepted CO₂ concentration for cushion gas is 9% under the condition of production and injection for 120 d and 180 d in a production cycle at a rate of 4.05 kg/s and 2.7 kg/s, respectively. The typical curve of the mixing zone thickness can be divided into four stages, which include the increasing stage, the smooth stage, the suddenly increasing stage, and the periodic change stage. In the periodic change stage, the mixed zone increases with the increasing of CO₂ fraction, temperature, production rate, and the decreasing of permeability and water saturation. The CO₂ fraction in cushion gas, reservoir permeability, and production rate have a significant effect on the breakthrough of CO₂ in the production well, while the effect of water saturation and temperature is limited.