Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 419

Figure 9.6.4 illustrates this scale-dependence of the permeability for
measurements from laboratory to borehole scale [9.23].
The implications of the scale dependence of permeability for carbon
sequestration are profound. First, because injection wells are relatively
small-scale access points to the formation, it is most likely that they will
not intersect rare high-permeability features. But if they do intersect frac-
tures or faults, or once the injected CO 2 encounters a nearby permeable
fracture or fault, these features will take up most of the injection mass,
thereby bypassing nearby pore space. This tendency continues through-
out the injection process. Under these conditions, CO 2 does not fi ll the
pore space effi ciently. Second, at long times following the injection
period, a similar effect occurs as CO 2 buoyancy provides a constant
upward driving force and the injected CO 2 plume underlies a large area of
caprock. In this confi guration, any weakness in the sealing integrity of the
caprock over the large footprint of the CO 2 plume will likely result in leak-
age into or through the caprock. The analogy with rainfall and an old roof
on a house comes to mind; if there is a way for water to leak in, it will.
We will return to some of these ideas related to how permeability and
its variation affect injection and migration of CO 2 in the section on Capacity
Assessment (Chapter 10).

Borehole

− 10

Regional

Laboratory

− 15

− 20

midrange

− 25

− 2 − 1 0 1 2 3 4 5
log 10 scale (m)

Figure 9.6.4 Scale effect of permeability
Scale effect of permeability as measured for crystalline rocks. Sedimentary rocks are
believed to show the same general trends in permeability as a function of measurement
scale. Data from from Clauser [9.23].