Geoscience Reference
In-Depth Information
(Sminchak and Gupta, 2003), whereas the density of most formation fluids within potential
reservoirs is higher, typically 1.05-1.30 g/cm
3
. Supercritical CO
2
is also less viscous than
saline formation fluids. These differences in density and viscosity mean that the liquid CO
2
will behave buoyantly within the reservoir. This buoyancy is what makes CO
2
an effective
fluid for EOR (Szulczewski et al., 2012).
For CCS, however, the buoyancy of CO
2
means that the geologic reservoir must have
a covering of impermeable rock (a “seal”) to ensure that the CO
2
will not escape upward
(Szulczewski et al., 2012). Depending on the composition of the geologic reservoir for
the injected CO
2
, some potential exists for supercritical CO
2
either to dissolve, weaken,
or transform existing minerals or to precipitate new minerals in the geologic reservoir. For
these reasons, selection of a suitable reservoir in which to inject and store CO
2
is critical.
The effects of supercritical CO
2
on geologic materials and the potential impacts of geo-
chemical reactions with brines, cements, casing materials in injection wells, and materials
that may seal faults and fractures in the reservoir have been topics of research supported by
the Department of Energy (DOE) at academic institutions and national laboratories, and
also by the petroleum industry. For example, in 2009 DOE supported 11 projects to con-
duct site characterization of promising geological formations for CO
2
storage.
12
Research at
DOE's National Energy Technology Laboratory (NETL) is based on developing efficient
injection techniques, protocols that assess and minimize the impacts of CO
2
on geophysi-
cal processes, and remediation technologies to prevent or reduce CO
2
leakage. Currently
NETL lists 37 active projects that address the critical geologic barrier for CO
2
storage.
13
The volumes of supercritical CO
2
discussed for CCS are extremely large. An Inter-
governmental Panel on Climate Change special report on CO
2
capture and storage sug-
gests that between approximately 97 and 306 million m
3
per year (converted from 73 and
183 million metric tonnes)
14
of CO
2
could be captured and stored worldwide from coal
and a similar amount from natural gas energy plants (Metz et al., 2005). This amount is
equivalent to approximately 40,000 to 120,000 Olympic size swimming pools. For com-
parison, over 300 million m
3
of crude oil were produced in the United States in 2010 (over
4 billion m
3
were produced worldwide) (see Table 3.3). It is anticipated that CCS would
take place at a number of locations, ideally places near power plants that produce CO
2
so
as to avoid long transportation distances. Many of the facilities would be expected to inject
CO
2
volumes on the order of several million tonnes (equivalent to several million cubic
meters) or more into the ground each year (e.g., Szulczewski et al., 2012). Globally, only
a few small-scale commercial CCS projects (the committee defines small-scale as about
12
See www.fossil.energy.gov/recovery/projects/site_characterization.html.
13
See www.netl.doe.gov/technologies/carbon_seq/corerd/storage.html.
14
As the density of supercritical CO
2
ranges from 600 to 750 kg/m
3
, the volume of 1 million metric tonnes (~1.1 mil-
lion tons) of supercritical CO
2
ranges from 1.33 to 1.67 million m
3
. In-ground storage volume will depend on the effective
porosity (i.e., the porosity times the storage efficiency).
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