In the framework of selecting potential targets for CO₂ geological storage, depleted gas reservoirs are well positioned because they offer directly reusable platform facilities, proven cap rock, and seal integrity; because these reservoirs contained a highly mobile gas phase for thousands to millions of years; and a very detailed reservoir characterization developed for gas production purposes. Furthermore, coupled with enhanced gas recovery, CO₂ sequestration in gas reservoirs is also a means to reduce costs for CO₂ injection by producing the remaining gas. This chapter presents a numerical simulation study of CO₂ injection into the nearly depleted gas reservoir at the K12-B field, North Sea, selected as a demonstration site for the Offshore Reinjection of CO₂ project. Simulations have been conducted using two different codes: TOUGHREACT, for characterizing the geochemical fluid-rock interactions that may occur during the injection period, and TOUGH2/EOS7C, for simulating the CO₂ sequestration coupled with enhanced methane production. Simulation results show that considering the injection of about 3 million tons of CO₂ for 10 yr, (1) a very low geochemical impact is expected to occur, which is favorable in terms of cap rock and reservoir integrity, and (2) the enhanced gas recovery efficiency remains limited when considering the full-scale CO₂ injection rate.