The global effort to mitigate carbon emissions has led to significant interest in Carbon Capture and Storage (CCS) technologies. Depleted oil and gas reservoirs are one of the prominent options for CO2 storage, given their proven ability to contain fluids over geological timescales. This thesis focuses on the simulation and modeling of CO2 injection into depleted gas reservoirs, emphasizing geochemical interactions and near-wellbore effects. The study investigates key factors such as CO2-brine-rock interactions and caprock integrity in ensuring long-term CO2 sequestration stability. Using the CMG-GEM compositional simulator, various models are developed to explore multiphase flow, phase transitions, and geochemical reactions under different reservoir conditions.