Large-Scale Geological Carbon Sequestration - Introduction to Carbon Capture and Sequestration - page 469

and k is an ardu-
ous task because of the large number of variables that might infl uence
this relationship (for example, pore size distribution, fl ow regime, and the
type of reaction that causes the porosity change). Modern imaging tech-
niques provide a promising avenue towards elucidating this relationship
by allowing measurements of permeability, porosity, and pore network
structure, both before and after provoking a porosity change in a porous
medium. This approach is illustrated in Figure 10.2.9 and Movie 10.2.1
[10.8]. Figure 10.2.9 shows that biologically induced precipitation of cal-
cite in a network of glass beads causes a large permeability decrease
(note the logarithmic scale of the permeability axis). Movie 10.2.1 shows
that calcite forms a roughly uniform coating on the entire surface of the
silica grains. A uniform coating will infl uence the aperture of the pore
throats much more strongly than the size of the pore bodies, which
explains the strong permeability decrease.
Another illustration of the strong infl uence of porosity on permeability
is the relationship between fracture permeability and fracture aperture. As
discussed in Section 9.6, for fl uid fl ow in a planar fracture, hydrodynamic
theory predicts that the fracture permeability k has a cubic-law depend-
ence on fracture aperture b (i.e., k

Characterization of the full relationship between φ

∝ b 3 ). Several studies have applied the
imaging approach described above to determine fracture aperture and
permeability both before and after reaction with a CO 2 -acidifi ed brine
[10.9, 10.10]. What they found is that calcite dissolution causes a rapid
increase in the average fracture aperture if the invading brine is under-
saturated with respect to calcite ( Figure 9.5.5 ). However, because of the
heterogeneity of the rock, the preferential dissolution of calcite relative to
other minerals can also cause a signifi cant increase in the roughness of
the fracture surface, which tends to moderate the infl uence of fracture
aperture on permeability [10.9].
In short, in order to predict the relationship between porosity (or
aperture, in the case of a fracture) and permeability, we need to under-
stand where precipitation and dissolution occur in the pore space,
because the constrictions of major fl ow pathways have a much greater
infl uence on permeability than other parts of the pore network. This
requires understanding the feedbacks between hydrodynamics and
geochemistry (in carbonate-rich rocks, these feedbacks can result in
the formation of high-permeability channels or “wormholes” [10.11]),