Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 407

Section 5
Caprock

We have seen that during injection we form a plume of supercritical CO 2
(see Figure 9.5.1 ). In many cases, we will have to rely on the caprock to
prevent the CO 2 from moving upward. We will now apply what we know
about fl uids in pores to understand the sealing mechanism of caprock.

Mechanism

Recall that caprock is often a shale with relatively small pores. Because of
the capillary effects, these pores prefer to be fi lled with brine relative to
CO 2 . Let us now assume that our caprock can be modeled as a “mem-
brane” with parallel cylindrical holes that are perfectly wetted by the brine
(see Figure 9.5.2 ). According to the Young-Laplace equation, as long as
the pressure of CO 2 is below:

2

pp R =+γ ,

g w gw

the capillary force will prevent the CO 2 from moving upward. We also see
that the smaller the pores, the larger the pressure we can sustain below
our caprock (see also Question 9.5.1 ) .

Figure 9.5.1 Caprock
Because of the caprock, the injected CO 2 does not move upward.