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382 12 Carbon Capture and Storage

12.8.1 Enhanced Oil Recovery and Enhanced Gas Recovery

Carbon dioxide (CO 2 ) can be employed for EOR by CO 2 flooding for reservoirs
deeper than 800 m. This depth allows CO 2 to be stored as a dense supercritical
fluid. In the EnCana Weyburn field project that started September 2000, for
example, over 2.2 Mt supercritical CO 2 per year is injected to a depth of
1,400–1,500 m for OER [22].
A schematic diagram of an EOR process is shown in Fig. 12.8.CO 2 is injected
into the deep reservoir from the injection well, and it forms a CO 2 bank. A CO 2
miscible oil bank is formed as a result of the miscible CO 2 -oil mixing. The mixture
of CO 2 and crude oil is delivered to the ground surface through the production well.
They are separated at the ground facilities, producing liquid and gaseous fuels with
recovered CO 2 .
For immiscible CO 2 operation, CO 2 bank pushes oil forward from the reservoir
to the production well. Depending on the reservoir, the incremental oil recovery by
EOR ranges from 5 to 15 %. This additional economic benefit makes EOR com-
petitive over other options.
Eventually, a significant amount of CO 2 injected for EOR is trapped under-
ground by residual oil, water, or mineral. It is expected that this part of CO 2 can
remain underground for thousands of years. However, the supercritical CO 2 is
mixed with water may be transported underground out of the reservoirs. Long term
CO 2 distribution in the reservoirs can be predicted by numerical simulations for
long-term risk assessment.
Similar to EOR, CO 2 can also be injected into depleting gas reservoir for
enhanced gas recovery. However, the economical return is not as high as EOR in
that most primary gas recovery rate is pretty high already, and current gas price is
low.

Fig. 12.8 EOR by CO 2
injection