Environmental Engineering Reference
In-Depth Information
The membrane separation method for capturing CO 2 is furthest from realization compared to
the other two methods. Preliminary estimates predict a thermal efficiency of a power plant with
coal gasification and CO 2 capture by membrane separation of 25-30%, along with an increase of
electricity production cost over 100% compared to coal gasification combined cycle without CO 2
capture. 15
10.4.4
CO 2 Sequestration
After capture, the CO 2 needs to be sequestered in a reservoir for an indefinite period, so it will
not reemerge into the atmosphere. The following reservoirs are being investigated, and in some
instances already employed, for sequestering CO 2 :
Depleted oil and gas reservoirs
Deep ocean
Deep aquifers
10.4.4.1 Depleted Oil and Gas Reservoirs
Oil and gas reservoirs are usually covered by an impenetrable layer of rock, so that CO 2 deposited
into the reservoirs would not reemerge into the atmosphere. In respect to sequestering CO 2 , oil and
gas reservoirs behave differently. Whereas CO 2 can be injected into oil reservoirs while the oil is
being pumped out of it, it can be injected into gas reservoirs only after depletion of the gas (because
of the miscibility of CO 2 and methane). In fact, injecting CO 2 into semi-depleted oil reservoirs
is a well-established technology. It is not done for sequestering CO 2 , but rather for enhanced oil
recovery (EOR).
Worldwide there are some 71 oil fields where carbon-dioxide-enhanced oil recovery (CO 2 -
EOR) is used. The majority of these fields are in the United States, in Texas and Colorado, whereas
others are in the North Sea. In the United States, total production from CO 2 -EOR wells is not
large compared to total oil production (about 2%). Other oil wells use “water flooding”—that is,
pressurized water injection. Water-EOR accounts for roughly 50% of U.S. oil production.
For EOR, carbon dioxide is injected above its critical point (31 C, 73 atm). Its viscosity
is lower than the oil it displaces. This means that CO 2 flows more easily through the reservoir
rock, causing “fingers” of the injectant to move through the oil and to occasionally bypass the
oil altogether. Therefore, CO 2 must be recovered at the well head and reinjected. On the average,
0.1-0.3 standard cubic meters of CO 2 are injected for the recovery of one barrel of oil.
In the United States, most of the injected CO 2 comes from natural sources, rather than from
flue gas capture. The natural sources are subterranean aquifers saturated with CO 2 . At two fields in
Texas, CO 2 comes from flue gas capture using the solvent absorption method. Supercritical CO 2 is
transported from a gas-fired power plant overland a distance of 350 km in a 40-cm-diameter pipe.
The exploitation of CO 2 for enhanced oil recovery is largely dependent on the price of crude oil.
Carbon dioxide injection costs about $5 to $8 per barrel of oil produced, so it is only worth it if the
price of crude oil is above $25/bbl.
The potential storage capacity in depleted oil and gas reservoirs is about 40 Gt carbon. This
compares with an annual worldwide emission of 6.8 Gt carbon per year. Thus, depleted oil and gas
15 Riemer, P., 1993. The Capture of CO 2 from Fossil Fuel Fired Power Stations. Cheltenham: IEA Publications.
 
 
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