MRCSP Phase I Geologic Characterization Report - Midwest ...

CO 2-SEQUESTRATION STORAGE CAPACITY FOR THE MRCSP PROJECT
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CO 2 -SEQUESTRATION STORAGE CAPACITY FOR THE MRCSP PROJECT
CO 2-STORAGE MECHANISMS IN
GEOLOGIC FORMATIONS
Carbon dioxide sequestration in geologic strata relies upon a
number of different storage mechanisms that are based on sitespecific
geologic conditions. Based on the geologic sequestration
research conducted over the last decade by a number of researchers,
these mechanisms are now fairly well described in published
papers and proceedings of conferences such as the Greenhouse
Gas Control Technology (GHGT) series organized by the International
Energy Agency Greenhouse Gas R&D Programme (see
www.ieagreen.org.uk for conference proceedings information) or
in the Special Report on Carbon Dioxide Capture and Storage prepared
by the Intergovernmental Panel on Climate Change (IPCC)
(e.g. Houghton and others, 1996; 2001). The commonly discussed
storage mechanisms are volumetric storage, solubility storage,
adsorption storage, and mineral storage. Volumetric storage refers
to the amount of CO 2 that is retained in the pore space of a geologic
unit, generally as a supercritical phase retained by structural
or stratigraphic traps or by the overlying cap-rock layers. Solubility
storage involves dissolution of a part or all of the CO 2 into the
formation waters of the geologic unit. Adsorption storage involves
the holding of CO 2 molecules onto the fracture faces and into the
matrix of organic-rich rock units, such as coal or black shale. Mineral
storage involves the chemical reaction of CO 2 with the minerals
and brine in the geologic unit. Under appropriate conditions, some
chemical reactions may form a solid precipitate, permanently binding
the carbon to the geologic unit. Mineral storage is not investigated
as part of this report because the complex nature of the reactions
and the uncertainty in reaction rates makes it difficult to determine
the storage volumes on a regional scale. In addition to the types of
formations and storage mechanisms evaluated in this report, basalt
layers and salt caverns are also potential repositories for CO 2-storage;
however, due to the early state of research for these options,
they were not evaluated at this time for MRCSP region.
CO 2 PROPERTIES
Before the description of the calculation methods used for CO 2-
storage capacity determinations can begin, it is important to briefly
review the physical properties of CO 2, since these physical properties
affect how much CO 2 can be placed into storage. The phase behavior
of CO 2 is well understood and can be found in general chemical
references such as Lemmon and others (2003) or in literature on enhanced
oil recovery (e.g., Jarrell and others, 2002). Carbon dioxide
can exist as four different phases (Figure 21), as a solid, liquid, gas,
or as a super-critical gas. The triple point for solid, liquid, and gas
is at -69.826º F (-56.57º C) and 75.2020672 psia (0.5185 MPa). At
temperatures greater than 87.8º F (31.1º C) and pressures greater than
1,071 psia (7.38 MPa), CO 2 is in a super-critical state, behaving similar
to a gas by filling all available space, while having the density of a
liquid. Using typical parameters for the MRCSP area, such as a geothermal
gradient of 0.01º F/ft (0.0182º C/m), a surface temperature of
56º F (13.33º C), and a pressure gradient of 0.433 psia/ft (9,792.112
Pa/m), a line representing the typical pressures and temperatures with
depth can be superimposed on the phase diagram (Figure 21). This
line shows that at shallow depths (less than ~2,500 ft), CO 2 would be
stored in a gaseous phase, while at deeper depths (greater than ~2,500
ft), most of the CO 2 will be in the super-critical gas phase, with some
storage as a liquid. The recognition of the super-critical gas phase is
important since, under most geologic storage scenarios being evaluated,
CO 2-storage will occur as a super-critical gas.
Figure 21.—CO 2 phase diagram. The triple point for CO 2 occurs at -69.826°F (-56.57°C)
and 75.202 psia (0.518 MPa) (Lemmon and others, 2003). The super-critical gas phase
occurs at 87.8°F (31.1°C) and 1,071 psia (7.38 MPa). The dashed line represents typical
reservoir conditions in the MRCSP area.