Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 425

Question 9.7.1 CO 2 storage capacity by residual trapping
Estimate CO 2 storage capacity (in Mt CO 2 per km 2 of plume footprint) if storage
formation thickness is 150 m, sweep effi ciency is 0.20, porosity is 0.25, resid-
ual CO 2 saturation is 0.25, and CO 2 density is 650 kg/m 3 . How many times the
surface area of the USA would be required to store 40% of the USA's CO 2
emissions (at current emission levels) during the next 50 years, assuming that
CO 2 is not stored offshore or in overlapping geological formations?

(for example, in unsaturated soils), but they remain much less extensively
examined in the case of brine-CO 2 fl uid mixtures at high T and P .
Because of their relative simplicity, Equations (I-III) do not capture the full
complexity of measured characteristic curves. In particular, they are
poorly adapted to porous media where several distinct types of pores
with very different sizes contribute signifi cantly to the porosity.

Hysteresis

The reader may have noticed that in our discussion of the relative perme-
ability and capillary pressure as a function of the brine saturation, we
conveniently started with a sample that was fully saturated with brine.
Let us now do the following experiment (see Figure 9.7.2 ). We start
with our sample for which the pores are fully brine-saturated ( S w 1). We

have seen that we can inject CO 2 if we apply a pressure higher than
the local hydrostatic pressure of the fl uid (see Figure 9.6.3 ). This will
displace the brine (the drainage process) in the largest pores, and by
increasing the CO 2 pressure we can displace brine from increasingly
smaller pores until CO 2 forms a continuous path and we are left with our
residual brine saturation.
The next step in our experiment is to reverse the invasion of CO 2 . We
stop the injection and the pressure of the CO 2 phase will slowly decrease
and brine will fl ow back (the imbibition process). As brine is wetting the
grains, the capillary forces cause it to wet the pore throats fi rst. This pro-
cess traps bubbles of CO 2 . At this point, it is important to note that imbi-
bition and drainage present qualitatively different situations: because of
the trapped bubbles of CO 2 , at the same capillary pressure the water
saturation is lower during imbibition. Hence, imbibition and drainage do
not follow the same capillary pressure curve. The imbibition process will

=