Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 392

Porosity in rocks can also be provided by fractures or faults in the
rock; this is referred to as fracture porosity, defi ned as the void space in
the apertures of the fractures and faults of the rock divided by the bulk
rock volume.
Section 3
Fluids
Now that we have considered some of the salient properties of rocks, we
turn to understanding the properties of fl uids confi ned in these rocks.
This is critical to geological CO 2 sequestration, because we ultimately
seek to displace those fl uids naturally present in rocks with our super-
critical CO 2 from the capture process.
Deep groundwater environment
The vast majority of deep subsurface pore space in sedimentary basins is
fi lled with saline groundwater. Some geological formations, of rare occur-
rence but great economic value, have pores that are fi lled predominantly
with hydrocarbon fl uids (oil and/or natural gas) and various other gasses
(e.g., He or CO 2 ). Regardless of the composition, fl uids at the depths rel-
evant to geological carbon sequestration are at high pressure (6-25 MPa).
The hydrostatic pressure is the equilibrium pressure due to gravity
alone, and most fl uids in sedimentary basins are at this pressure.
However, the degree of interconnectedness of the pores and changes in
crustal loading can give rise to notable exceptions. For example, in sedi-
mentary basins that were overlain by ice during the last ice age (e.g., in
the upper midwestern USA), reservoirs isolated from surrounding aquifers
by caprock may be underpressured, i.e., at less than hydrostatic pres-
sure, because the rock has physically expanded due to the unloading
of ice [9.9]. Similarly, in the Gulf of Mexico, there are deep overpressured
reservoirs caused by the loading and compression of the overlying rock
caused by deposition of sediment carried by the Mississippi River [9.10].