PERMANENT CARBON DIOXIDE STORAGE AND MINERAL CARBONATION IN GEOLOGIC RESERVOIRS

The most permanent and secure CO₂ storage mechanism is the formation of carbonate minerals. The capacity of reservoirs for solid storage of CO₂ is largely a function of host rock composition. Mineral carbonation involves combing CO₂ with divalent cations including Ca²⁺, Mg²⁺, Fe²⁺. The most abundant geologic sources of these cations are silicate minerals such as olivine, pyroxene, serpentine and plagioclase. These are primarily found in basaltic and peridotitic rocks, which may provide permanent CO₂ storage for billions of tons of CO₂ per year (McGrail et al. 2006; Goldberg et al. 2008; Kelemen and Matter et al. 2008). This is demonstrated by natural analogs. For example, ~10³ tons of CO₂/km³/yr are consumed by peridotite carbonation in Oman (Kelemen and Matter, 2008). Full carbonation of peridotite in Oman, using all the magnesium and iron to form carbonate+quartz, would consume ~40,000 Gigatons of CO₂. The natural uptake of CO₂ may be enhanced by taking advantage of the heat released by the exothermic carbonation reaction. Rapid carbonation at high temperature can achieve a self-heating regime in which heat production is faster than cooling due to injection of cold CO₂ and diffusive heat loss to cold surroundings. The energy released by carbonation can then be harnessed to maintain temperature at the optimal temperature for rapid reaction.

However, in situ carbonation could be a self-limiting process. Fluid-rock reactions that increase solid volume are often self-limiting because they fill porosity, reduce permeability and form reaction rims on un-reacted minerals. However, new data from naturally carbonated peridotites in the Sultanate of Oman suggest that mineral carbonation of peridotite is not always self-limiting. Completely carbonated peridotites and relationships of carbonate veins in partially carbonated peridotites indicate coeval carbonate crystallization, formation of cracks and volume expansion to accommodate carbonate precipitation. Thus, it is vital for CO₂ storage via in situ mineral carbonation to understand coupling of geochemical and geomechanical processes.

Goldberg, D.S et al. (2008), Proc. Nat. Acad. Sci. 105(29), 9920-9925.

Kelemen, P. B., and J. Matter (2008), Proc. Nat. Acad. Sci. 105(45), 17295-17300.

McGrail et. al. (2006), Journal of Geophysical Research 111, B12201.