Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 413

a two-phase mixture of water and CO 2 , depending on whether CO 2 can
overcome the capillary entry pressure of the fracture. Acidic fl uids would
tend to dissolve the fracture walls and increase the fracture aperture, as
shown by experimental studies with artifi cially fractured seal rocks
( Figure 9.5.5 ). As the acidic fl uid or fl uid mixture travels upward through
the fracture, weathering reactions may promote the precipitation of car-
bonate minerals farther up in the fracture. The overall geochemical pro-
cess is likely to be analogous with the mineral trapping reactions that
occur within the reservoir (which will be discussed in Section 9.8), but the
dissolution and precipitation reactions will occur at different locations
along the fracture. The hypothesis that carbonate mineral precipitation
can seal fractures is supported by studies of natural CO 2 seeps [9.20].

Section 6
Permeability

In contrast to caprock, a good reservoir sandstone will have, in addition to
suffi cient porosity (i.e., storage capacity), a high permeability (i.e., ease with
which fl uids can be transported through the reservoir). In these reservoirs,
the permeability is determined by the degree of connectedness of the pores.
The permeability is measured in units of m 2 or equivalently, darcys
(d), where 1 darcy equals 10 − 12 m 2 and 1 md

10 − 15 m 2 (see Box 9.6.1
for the difference between this defi nition and the permeability of a mem-
brane, which we introduced in Chapter 7). A good reservoir rock will have
permeability on the order of 100 md or higher. For comparison, a good
caprock has very low permeability of 0.1 md or less.
Permeability is a property that varies over many orders of magnitude
for common rocks. Porosity and permeability are generally correlated,
but the relation between porosity and permeability is complex and no
universally applicable correlation exists. Even in the same geological
formation, the relation between porosity and permeability is non-unique
and dependent on the characteristics pertaining to the interconnected-
ness of the pores. Figure 9.6.1 illustrates the permeability as a function

=