Forsterite Carbonation in Zones with Transport Limited by Diffusion

Fractures in rocks could provide substantial surface area for reactions that lead to carbonate mineral precipitation in geologic carbon sequestration. Diffusion-limited transport of solutes in such zones affects the spatial and temporal distribution of mineral dissolution rates and carbonation products, thus influencing the overall carbon sequestration process. A tube with a packed bed of forsterite and exposed at one end to a solution in equilibrium with 100 bar of CO ₂ at 100 °C was used to explore the timing and spatial localization of carbonate precipitation along a one-dimensional diffusion-limited zone. The identity and quantity of carbonate minerals as a function of depth were determined using Raman spectroscopy and total carbon analysis. Carbonate was observed within the packed bed as early as day 1. Hydromagnesite formed in the bed first and was replaced by magnesite within 5 days. Carbonate was spatially localized with the largest amount formed 0.5 cm into the packed bed. The overall carbonation rate in the bed did not decline until day 30.