Co-injection of sulfur dioxide during geologic carbon sequestration can cause enhanced brine acidification. The magnitude and timescale of this acidification will depend, in part, on the reactions that control acid production and on the extent and rate of SO
2 dissolution from the injected CO
2 phase. Here, brine pH changes were predicted for three possible SO
2 reactions: hydrolysis, oxidation, or disproportionation. Also, three different model scenarios were considered, including models that account for diffusion-limited release of SO
2 from the CO
2 phase. In order to predict the most extreme acidification potential, mineral buffering reactions were not modeled. Predictions were compared to the case of CO
2 alone which would cause a brine pH of 4.6 under typical pressure, temperature, and alkalinity conditions in an injection formation. In the unrealistic model scenario of SO
2 phase equilibrium between the CO
2 and brine phases, co-injection of 1% SO
2 is predicted to lead to a pH close to 1 with SO
2 oxidation or disproportionation, and close to 2 with SO
2 hydrolysis. For a scenario in which SO
2 dissolution is diffusion-limited and SO
2 is uniformly distributed in a slowly advecting brine phase, SO
2 oxidation would lead to pH values near 2.5 but not until almost 400 years after injection. In this scenario, SO
2 hydrolysis would lead to pH values only slightly less than those due to CO
2 alone. When SO
2 transport is limited by diffusion in both phases, enhanced brine acidification occurs in a zone extending only 5 m proximal to the CO
2 plume, and the effect is even less if the only possible reaction is SO
2 hydrolysis. In conclusion, the extent to which co-injected SO
2 can impact brine acidity is limited by diffusion-limited dissolution from the CO
2 phase, and may also be limited by the availability of oxidants to produce sulfuric acid.
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