Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 445

The origin of the discrepancy between the experimental fi eld-scale
weathering rates and the prediction of mathematical models describing
these geochemical processes are caused by the approximations that are
made in these models. First, as noted above, the models assume that the
same rate equations describe both dissolution and precipitation, whereas
experiments show that mineral growth can follow a different rate law than
mineral dissolution ( Figure 9.8.6 ). Since dissolution and precipitation
reactions are coupled, slow growth of M-poor silicates could limit the
overall rate of carbonate mineralization. Second, the reactive surface
area of complex mineral assemblages may differ signifi cantly from the
geometric surface areas of its mineral constituents, because a portion of
the grain surfaces may be occluded by other minerals ( Figure 9.8.8 )
[9.41] and because mineral reactivity may be inhibited by slow fl uid fl ow
( Figure 9.8.9 ) [9.42]. Third, surface coatings and metastable amorphous
phases often form during dissolution in near-equilibrium conditions
( Figure 9.8.10 ) [9.43, 9.44]. Surface coatings can modify the reactivity of
dissolving minerals, and metastable amorphous phases often are poorly
characterized or absent from thermodynamic databases. The infl uence of
surface coatings, inaccessible (or poorly-accessible) surface areas, and
other effects is often approximately accounted for in the geochemical
models by multiplying all mineral surface areas by 10 − 2 . This suggests

that the uncertainty of the overall rate of CO 2 mineral trapping in seques-
tration is quite large (possibly several orders of magnitude). Existing
research tools have the capability to decrease this uncertainty by com-
bining nano-to core-scale imaging and multiphase reactive transport
modeling to reveal the distribution of minerals, brine, and CO 2 as well as
their weathering reaction rates in reservoir rocks. The challenge is that
the characterization and computational demands are considerable
because rocks are inherently multiscale, their scale ranging from less
than a micrometer (the scale of individual pores) to hundreds of meters
(the thickness of geological formations).

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Outlook

In this chapter, we have reviewed the physico-chemical basis for the
sequestration of CO 2 in rocks. We see that we are faced with many
research challenges and opportunities that are needed to further under-
stand and improve our predictions about the transport, equilibria and