CO Capture and Storage - Energy and Climate: How to Achieve a Successful Energy Transition - page 111

Storage in depleted oil and gas fields

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This option is particularly interesting, as it involves technologies which
are already well known and geological formations which are generally
well characterised.
Carbon dioxide injection in oil fields is used also for enhancing oil
recovery. If CO 2 is injected before the field is completely depleted, in
particular with the aim of EOR, it is clearly essential to separate CO 2
present in the production stream in order to recycle it into the reservoir.
EOR already consumes 32 million tons of CO 2 in the USA.
It provides an added value to the CO 2 which is injected and it is
therefore probably the option which will be developed most rapidly in
the near future.
As a further advantage, CO 2 is stored in geological formations which
have trapped hydrocarbons over millions of years: natural confinement
offered by such structures is certainly one of the most attractive aspects of
this option.

Storage in deep saline aquifers

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Storage in deep saline aquifers has the greatest potential in terms of storage
capacity. Such aquifers are distributed in many regions of the world. They
are located at depths between 1000m and 3000m. Due to the depth of
these formations and their high salt content, they cannot be used as
sources of drinking or irrigation water.
These aquifers can be open or closed. The configuration of closed
aquifers is identical to that of oil and gas reservoirs, which ensures
effective confinement vertically and laterally.
This is the type of aquifer used to store natural gas. It certainly provides
a safe option in terms of confinement, but capacity is limited.
Open aquifers lie on a horizontal or slightly inclined plane. Since they
do not ensure lateral confinement this would enable CO 2 to migrate. This
being said, their large size and low flow velocity can ensure a satisfactory
confinement, assuming sufficient CO 2 is present. In this case, the main
trapping mechanism is the dissolution of gas in water.
The increase in densitywhich results from this dissolution tends to carry
CO 2 towards the bottom of the aquifer. In the longer term, CO 2 reacts
with the surrounding rock and as a result of this mineralisation process
the safety of CO 2 trapping increases. The weak point of this solution is
that little is known about the subject. This type of formation does not