Introduction to Geological Sequestration - Introduction to Carbon Capture and Sequestration - page 360

Section 2
Trapping mechanisms
The aim of injecting CO 2 into geological formations is to keep it from
entering the atmosphere. We inject the CO 2 as a supercritical fl uid. The
density of this fl uid is lower than the density of most of the fl uids that
exist in these formations, and elementary laws of physics tell us that
buoyancy effects will tend to drive the CO 2 plume upward. We therefore
need to select a geological formation capable of trapping the CO 2 , and
we need to examine the likelihood of escape of the injected CO 2 . There
is much experimental evidence that we indeed can trap gasses in geo-
logical formations. For example, we have confi dence in the feasibility of
trapping CO 2 in depleted natural gas reservoirs, because natural gas was
trapped in these geological formations for millions of years by some of
the very same mechanisms that are known to trap CO 2 . In addition, there
are places on earth where CO 2 is produced by decarbonation of carbon-
ate rocks (e.g., limestone and dolostone) and becomes trapped in geo-
logical formations in the form of natural CO 2 reservoirs (e.g., Bravo
Dome, NM; Jackson Dome, MS; and Sheep Mountain, CO). These CO 2
sources are used for most of the CO 2 -enhanced oil recovery that is car-
ried out in the USA.
Let us visualize what happens when we inject CO 2 into a geologi-
cal formation. Figure 8.2.1 shows the plume of injected CO 2 . As the
injection proceeds, the brine that occupied the pore space before the
injection will be displaced and initially we will see a large plume of
CO 2 around the injection site. Because the density of supercritical
CO 2 is lower than that of the surrounding fl uids, buoyancy effects will
cause the plume to move upward. As long as the CO 2 remains in this
large plume, we rely on the geological formation above the injection
layer, a so-called caprock or top seal, to prevent the CO 2 from escap-
ing the storage formation. Caprock is a fi ne-textured rock that has a
very limited permeability. Hence, during the initial phase we rely on
structural trapping or stratigraphic trapping of the CO 2 by this cap-
rock. If, because of buoyancy effects, the plume moves through the
storage formation, some of the CO 2 will become immobilized as small
disconnected bubbles in the pores of the formation (see Movie 8.2.1 ).