Mineral saturation state and the implications for reaction rates in reactive transport modeling of CO₂ storage

The injection of CO₂ results in a decrease in the pH and a corresponding change from conditions at or near saturation between the aqueous and mineral phases to one of under-saturation. With growing evidence that many mineral dissolution reaction rates are dependent on the proximity to equilibrium, we seek to address the extent to which a fluid becomes under-saturated with respect to the mineral phases present through the addition of CO₂. It turns out, for many minerals with a dissolution reaction that is pH dependent, that the saturation index changes little from the initially saturated condition. This indicates that reaction rates commonly used in reaction path and reactive transport models of carbon storage are not sufficiently representative of the dissolution mechanism that dominates and tend to overestimate the amount of reaction with time. (authors)