Geoscience Reference
In-Depth Information
Technology Background
Geologic formations considered suitable for underground storage of CO
2
include oil
and gas reservoirs, unmineable coal seams, and deep saline rock formations (Kaldi et al.,
2009). Naturally occurring CO
2
has been trapped in geologic formations for millions of
years, which indicates that retaining injected CO
2
in the Earth under the right geological
conditions is possible. Injection of CO
2
for EOR has been used in the oil and gas industry
for many decades with no obvious adverse effects (see the section Conventional Oil and Gas
Production Including Enhanced Oil Recovery, this chapter); CO
2
has also been injected in
small volumes into saline rock formations in the western United States and Canada since
1989 without negative consequences (NETL, 2012; Price and Smith, 2008). Saline rock
formations used for this purpose are sedimentary rocks that are naturally saturated with
highly saline water that is otherwise unsuitable for humans, livestock, or agriculture.
Individual large, coal-fired power plants in the United States produce CO
2
emissions
that amount to up to 25 million metric tonnes (~27 million tons) per year.
8
Capturing
and transporting CO
2
from industrial plants is technologically possible but is currently
expensive, though a significant amount of research is exploring ways to bring costs down
(Melzer, 2011). The United States as a whole accounted for approximately 1.5 billion metric
tonnes (~1.7 billion tons) of CO
2
emissions in 2010 (EIA, 2012). Storing even a portion of
this amount of CO
2
would require capturing the gas at many locations around the country
and transporting it to facilities that could inject the CO
2
into appropriate subsurface rock
formations.
9
Efficient underground storage of CO
2
requires that it be in the supercritical (liquid)
phase to minimize required storage volume.
10
For CO
2
to remain in a supercritical phase,
the confining pressure in the reservoir must be greater than 7.3 MPa (about 73 atm
11
) and
temperatures greater than 31.1°C, which can be achieved at depths greater than about
2,600 feet (790 m) (Buruss et al., 2009). These conditions require that the CO
2
be injected
at high pressures (62-64 bars [6.2-6.4 MPa or 900-930 psig] at the well head) so that the
CO
2
stays as a liquid. The density of supercritical CO
2
is in the range of 0.60-0.75 g/cm
3
8
See Carbon Monitoring for Action, available at carma.org/.
9
EOR operations do pump CO
2
underground. However, EOR operations are designed to roughly balance the natural
pressure in a reservoir from pumping out of hydrocarbons with pumping in of CO
2
. EOR using CO
2
injection currently
accounts for approximately 6 percent of U.S. crude oil production (Koottungal, 2010). Natural CO
2
fields are currently the
dominant source of CO
2
for U.S. EOR and provide approximately 45 million metric tonnes (~50 million tons) per year,
whereas anthropogenic sources, such as CO
2
captured from industrial facilities, account for approximately 10 million metric
tonnes (~11 million tons) per year (Kuuskraa, 2010). One of the biggest challenges for EOR projects that wish to use CO
2
injection is being able to secure enough CO
2
consistently at an acceptable cost (Melzer, 2011).
10
One pound of liquid CO
2
, which is about the volume of a typical fire extinguisher, will expand to approximately 8.8
cubic feet (0.25 m
3
) at normal room temperature and pressure.
11
One unit of atmospheric pressure or 1 atm is equivalent to the pressure exerted by the Earth's atmosphere on a point
at sea level.
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