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Abstract Carbon capture and storage (CCS) projects increasingly involve the injection of CO2 streams that contain trace impurities originating from industrial capture processes. Among these impurities, mercury (Hg) poses potential operational, environmental, and regulatory challenges due to its toxicity, volatility, and complex phase behavior. The presence of mercury as an impurity in CO2 streams poses significant challenges for the efficiency, safety, and environmental sustainability of Carbon Capture and Storage (CCS) operations. In storage reservoirs, mercury may adsorb onto rock surfaces or precipitate as insoluble compounds, leading to reduced porosity and permeability, which adversely affect CO2 injectivity and storage capacity. Furthermore, mercury contamination introduces environmental risks, as leaks or migration can pollute groundwater and ecosystems, posing toxicity concerns for humans and wildlife. This paper presents an integrated numerical modeling workflow to evaluate the transport, phase partitioning, and long-term fate of mercury as an impurity during CO2 injection and geological storage. A compositional multiphase reservoir simulator (CMG GEM) is used to model CO2 injection under supercritical conditions, while geochemical reactions governing mercury partitioning, adsorption, and immobilization are incorporated through coupled geochemical calculations. The modeling framework captures pressure- and temperature-dependent solubility, multiphase transport, and interactions between mercury, formation brine, and reservoir rock. Simulation results indicate that mercury rapidly transfers from the CO2 phase into the aqueous phase after injection and is subsequently immobilized through adsorption and mineral interactions, resulting in limited long-term mobility. Sensitivity analyses highlight the influence of reservoir mineralogy, water saturation, and injection strategy on mercury retention. The results show the mercury impurities significantly influence reducing injectivity, and potential reservoir damage. Sensitivity analyses highlighted that impurity concentration was the most important factor, with impacts on injectivity ranging from 20% to 50%, depending on concentration levels and reservoir mineralogy. Variations in reaction kinetics and mineralogy further modified the outcomes, emphasizing the need to account for these parameters in CCS system design. The proposed workflow provides a practical tool for CCS project screening, impurity risk assessment, and regulatory compliance.