Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 383

injection wells. We also need to ensure that the aquifer is insulated from
the atmosphere by a seal (so-called caprock). This seal needs to prevent
CO 2 from moving upward over an area that can span a hundred square
kilometers. Finally, we need to be able to access the storage formation,
e.g., through deep injection wells.

Geology

Before starting our geology review section, let us think a few moments
about our requirements. We need an aquifer with a large pore volume in
which CO 2 can fl ow easily, and on top of this aquifer we need a seal in
which CO 2 does not fl ow at all. How is it possible that nature created at
the same location such different types of materials?

Clastic sedimentary rocks

Aquifers are often found in clastic sedimentary rocks . Clastic sedimen-
tary rocks are formed from eroded fragments of older rocks. These rock
fragments are transported by water or wind, then deposited (in the pro-
cess of sedimentation). The grains of these sediments are subsequently
consolidated into a rock (in the process of lithifi cation).
The different clastic sedimentary rocks are classifi ed according to
their grain size (see Table 9.2.1 ). If the rock were composed only of grains
stacked together against one another, it would not have any strength and
would disaggregate like sand on a sand dune. Many sandstones are in
fact poorly consolidated; they readily disintegrate into constituent grains
and cannot be quarried or used as building or paving stones. Other sand-
stones are very hard by virtue of being held together with other natural

Table 9.2.1 Clastic rocks

Grain size Sediment name Rock name

Coarse (>2 mm) Gravel Conglomerate/ breccia

Medium (0.06-2 mm) Sand Sandstone

Fine (0.008-0.06 mm) Mud Siltstone

Very fi ne (<0.008 mm) Mud Shale