revue_geosciences16

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a worldwide overview of carbon dioxide storage
Injecting and storing CO2
into deep geological formations
Injection of CO2 into the Utsira Formation in the
North Sea began in 1996 as part of Statoil’s Sleipner
Project (Norway) and has now stored over 16 Mt CO2.
In Algeria, the In Salah project has been injecting CO2
since 2004 with over 4 Mt CO2 stored. The Snøhvit
project in Norway started injection in 2008 and has
stored 1.6 Mt of CO2 to date. These projects have used
various monitoring techniques and have demonstrated
that it is possible to safely manage the injection
of CO2 into geological reservoirs. Oil companies have
been injecting CO2 into depleting oil fields to enhance
oil recovery (CO2 EOR) since the 1970s and there are
now about 130 such operations, counting active projects
in the United States alone. On a smaller scale, acid gas
(mixtures of CO2 and H2S separated from natural gas
production facilities) has been injected in depleted oil
and gas reservoirs and in aquifers since the mid 1980’s.
Similar in process to CO2 storage, there have been no
issues with the safe injection of these fluids [Wichert
& Royan (1997); Davidson et al. (1999)].
Optimal operation of CO2 EOR fields results in the
production of a significant amount of the injected
CO2, which is then re-injected as part of the injection
stream. During this process, a significant amount of
CO2 is trapped in the reservoir and is stored. The Weyburn
Oil Field in Canada, for example, has now stored in
excess of 20 Mt CO2. Because of the vast experience
in CO2 EOR, the technologies and operational aspects
of injecting and storing CO2 in geological formations
are well established (figure 2). However, storing CO2
captured from industrial processes in geological
formations is also the component in the CCS chain that
presents some of the greatest project uncertainties.
Each geological storage site is unique and must be
screened and extensively characterised, taking years
and millions of dollars before a decision can be made
to proceed with a commercial project. Geological
storage can also represent the most important public
perception challenge and will be the greatest long-term
liability associated with a CCS project.
A brief review of CO2 behaviour
Depending on pressure and temperature, CO2 can
exist as a gas, liquid, solid or a supercritical fluid (a
supercritical phase refers to the physical state of the
phase at conditions above the critical temperature
1
2
3
4
Storage
Overview
Site Options
1 Saline
formations
2 Injection
into deep
unmineable
coal seams
or ECBM
3 Use of CO2
in enhanced
oil recovery
4 Depleted oil
and gas
reservoirs
Fig. 2: Geological storage options for CO2. Several types of rock formations
can be suitable for CO2 storage, including depleted oil and gas fields, deep
saline formations and deep unmineable coal seams. Other types of
formations such as basalts and oil shales are being examined by scientists
for possible future use.
Fig. 2: Options de stockage géologique du CO2. Plusieurs types de formations
rocheuses peuvent être favorables au stockage de CO2, y compris des
réservoirs pétroliers épuisés, des formations salines profondes et des veines de
charbon profondes inexploitables. D’autres types de formations telles que les
basaltes et les schistes bitumineux sont examinées par des scientifiques pour
une utilisation future éventuelle. © GCCSI.
and critical pressure), as shown in the phase diagram
presented in figure 3. As pressure and temperature
increase with depth of burial due to hydrostatic
pressures and geothermal gradients, the state of
CO2 will be a function of its depth in the subsurface.
Below about 800 m, pressures and temperatures in
reservoirs will generally exceed the critical point of
CO2 [Bachu (2000)] so that CO2 will be maintained
in a liquid or supercritical state. This makes it desirable
to target or screen reservoirs for storage sites where
depths generally exceed 800 m.