Carbon dioxide storage in deep geological structures is a strategic technology to mitigate climate change and to promote green development. However, despite continuous efforts to develop cost-effective capture processes to clean the stream before transportation and injection, traces of accessory gases cannot be entirely removed. Consequently, before any injection of these gas mixtures, the impact of impurities on the geochemical reactivity of the system must be evaluated. This paper describes numerical simulations done with TOUGHREACT that focus on the chemical reactivity of deep reservoir rock impacted by an injection of and associated reactive impurities (mainly and . A simplified two-dimensional radial geo-model of the near-wellbore domain of a saline reservoir enabled us to predict its global geochemical behavior. Two ratios were studied. The results show the high reactivity of the near-well zone when ancillary gases ( and are co-injected with , which leads to the dissolution of carbonates and the precipitation of sulfate minerals. Major reactions occur in the reservoir formation, whereas clays in the caprock are only slightly affected by the injection of and associated reactive impurities.