Depleted gas reservoirs with bottom water show significant potential for long-term CO₂ storage. The residual gas influences mass-transfer dynamics, further affecting CO₂ dissolution and convection in porous media. In this study, we conducted a series of numerical simulations to explore how residual-gas mixtures impact CO₂ dissolution trapping. Moreover, we analyzed the CO₂ dissolution rate at various stages and delineated the initiation and decline of convection in relation to gas composition, thereby quantifying the influence of residual-gas mixtures. The findings elucidate that the temporal evolution of the Sherwood number observed in the synthetic model incorporating CTZ closely parallels that of the single-phase model, but the order of magnitude is markedly higher. The introduction of CTZ serves to augment gravity-induced convection and expedites the dissolution of CO₂, whereas the presence of residual-gas mixtures exerts a deleterious impact on mass transfer. The escalation of residual gas content concomitantly diminishes the partial pressure and solubility of CO₂. Consequently, there is an alleviation of the concentration and density differentials between saturated water and fresh water, resulting in the attenuation of the driving force governing CO₂ diffusion and convection. This leads to a substantial reduction in the rate of CO₂ dissolution, primarily governed by gravity-induced fingering, thereby manifesting as a delay in the onset and decay time of convection, accompanied by a pronounced decrement in the maximum Sherwood number. In the field-scale simulation, the injected CO₂ improves the reservoir pressure, further pushing more gas to the producers. However, due to the presence of CH₄ in the post-injection process, the capacity for CO₂ dissolution is reduced.