Limitations for brine acidification due to SO_2 co-injection in geologic carbon sequestration

摘要

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 occursin 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.