Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 436

Figure 9.8.1 The fi nal steps in geological sequestration
The fi gure on the left illustrates the dissolution of CO 2 in the brine and the fi gure on the
right the conversion into carbonates.

very slow; the expected time scale for converting CO 2 into minerals is
(very roughly) on the order of 10,000 years.

Mineral weathering

The rate-limiting step in the sequestration of CO 2 as carbonate minerals
is the rock weathering rate, in particular the in-situ dissolution rate of sili-
cate minerals.
Two key classes of mineral weathering reactions that occur in CO 2
storage formations are the dissolution of calcite, which buffers the pH of
water near the plume to ∼ 4.9:

2 + −

CaCO + CO + H O Ca + 2HCO , (I)

()

22 3

3s

and the dissolution of silicate minerals, which leads to CO 2 trapping in
carbonate minerals (MCO 3 , where M

= Ca, Mg, or Fe):

M + CO + H O MCO + M .

()

(II)

rich silicates − 2 2 poor − silicates

3s

Reaction (I) is relatively rapid and often treated as an equilibrium
phenomenon. Reaction (II), on the other hand, occurs on time scales of
hundreds of years. Its inclusion in sequestration models requires knowl-
edge of the rates of silicate dissolution and precipitation reactions at