Large-Scale Geological Carbon Sequestration - Introduction to Carbon Capture and Sequestration - page 475

bulk liquid water. This assumption is likely to be incorrect in the case of
seals and well cements, where most of the pore space consists of very
small pores. Figure 10.2.11 highlights several studies of the Opalinus
clay in Switzerland that illustrate the diffi culty of characterizing this nano-
porosity, both because of the multiscale heterogeneity of sedimentary
rocks and because the smallest pores in clay formations (for example,
the interlayer nanopores of smectite clay minerals described in Figure
9.2.3 ) are not detected by most experimental techniques. Detailed char-
acterization of micrometer-scale samples of the Opalinus clay shows that
most of the pore space in these samples is located in pores narrower
than 10 nm, and a signifi cant fraction is located in pores narrower than
2 nm [10.20, 10.21, 10.22, 10.23].
Confi nement in porous media with pore sizes on the order of nanom-
eters to tens of nanometers (such as clays, zeolites, nanoporous silica,
or carbon nanotubes) is well known to infl uence fl uid properties such as
freezing temperatures, dielectric constants, and self-diffusion coeffi -
cients. In fact, this infl uence of confi nement on fl uid properties is used in
the design of nanofl uidic devices [10.24]. The potential implications of
this “confi nement effect” for carbon sequestration are suggested by sev-
eral studies. For example, Figure 10.2.12 shows that CO 2 may have a
much greater solubility in clay interlayer nanopores than in bulk liquid
water [10.25]. This enhanced solubility could be viewed as an adsorption
phenomenon driven by CO 2 -clay interactions or as an absorption phe-
nomenon driven by the confi nement-induced changes in the properties
of water. One implication of this fi nding is that shale formations may act
as barriers to CO 2 leakage, not only because of their low permeability and
high capillary entry pressure, but also because of their ability to absorb
(or adsorb) CO 2 .
The nanoporosity of rocks may also infl uence mineral weathering
reactions, as revealed by the synchrotron X-ray experiments described in
Figure 10.2.13 . This study showed that the precipitation of calcium car-
bonate is inhibited in the 7.5nm diameter nanopores of a nanoporous
glass. This result is unexpected, because silica surfaces are known to
promote the nucleation of solid carbonates [10.26]. Geochemical models
generally predict that, when mineral precipitation occurs in a porous
medium, the smallest pores become fi lled by the precipitating solid more
rapidly than the larger ones, because they have a higher ratio of surface
area to pore volume. Figure 10.2.13 clearly shows the opposite effect.