Economic Potential of DACCS and Global CCS Progress
APPENDICES
6.1 CO₂ GEOLOGICAL STORAGE
SUMMARY OF STORAGE MECHANISMS AND SECURITY
Four mechanisms exist for trapping CO₂ in the subsurface. These mechanisms occur
simultaneously upon injection but occur at different rates (Appendix figure 1). The
relative contribution of each trapping mechanism - physical, residual, dissolution,
mineralisation - changes with time and with a CO2 plume's evolution. In the initial
decades of a standard storage operation, physical trapping of free-phase CO2 is the
primary trapping mechanism. Trapping of CO2 is strongly dependent on a site's geology
and local formation conditions (in-situ fluids, pressure, temperature). A portion of the
CO2 plume may always remain in its free phase, but physical trapping is permanent
when the geologic setting is stable and the CO2 plume is behaving in the reservoir as
predicted.
PHYSICAL TRAPPING
Physical trapping occurs when buoyant, free-phase CO₂ migrates into a body of rock
that has been folded or faulted into a subsurface structure (or "trap"), which closes
in three or four directions, and is contained below a low-permeability caprock (or
"seal") (see Appendix figure 2). Physical trapping is the same mechanism that traps
hydrocarbons in the subsurface. Appendix figure 2 illustrates types of physical traps,
including independent folded rock bodies and fault-dependent folds (which rely on
closure against a fault for CO2 containment). In certain geological settings, physical
trapping of CO2 occurs when a reservoir thins laterally and ultimately pinches-out. This
is called a stratigraphic trap and is shown at "E" in Appendix figure 2.
B
A
CONTRIBUTION TO
CO: TRAPPING (%)
100
STRUCTURAL TRAPPING
Caprock
Mmm
10
100
TIME AFTER CO2 INJECTION (YEARS)
RESIDUAL TRAPPING
DISSOLUTION TRAPPING
~10-50μm
Caprock
"0.5mm
10,000
MINERAL TRAPPING
Caprock
"0.5mm
D
C
STORAGE FORMATIONS
FAULTS
✓ INJECTED CO2
SPILL POINTS (FAULT DEPENDENCY
OF STRUCTURAL CLOSURES)
A RESIDUAL TRAPPING (MONOCLINE FOLD)
B FAULT-INDEPENDENT STRUCTURAL TRAP (ANTICLINE FOLD)
CFAULT-DEPENDANT STRUCTURAL TRAP (EXTENSIONAL FAULT)
D FAULT-DEPENDANT STRUCTURAL TRAP (CONTRACTIONAL FAULT)
E STRATIGRAPHIC TRAP (PINCH OUT)
E
APPENDIX FIGURE 1: (LOWER PANEL) THE FOUR TRAPPING MECHANISMS OPERATING IN THE SUBSURFACE TO
PERMANENTLY STORE CO2. (UPPER PANEL) RELATIVE CONTRIBUTION OF THE FOUR TRAPPING MECHANISMS
TO PERMANENT CO₂ STORAGE THROUGH TIME. EACH MECHANISM OPERATES SIMULTANEOUSLY UPON CO₂
INJECTION, BUT THEY OCCUR AT DIFFERENT RATES. SOURCE: IPCC (2005)
APPENDIX FIGURE 2: SCHEMATIC ILLUSTRATION OF PHYSICAL TRAPS IN THE SUBSURFACE. CIRCLES SHOW
"SPILL POINTS" OR FAULT DEPENDENCY OF STRUCTURAL CLOSURES. (A) Residual trapping can be the dominant trapping mechanism
in gently dipping (that is, relatively flat-lying) rock bodies that do not exhibit structural closure. (B) A fault-independent folded rock body (anticline) can trap buoyant
CO2 down to its "spill point", below which CO2 will migrate out of the folded trap. (C) A fault-dependent (extensional fault) folded closure relies on the juxtaposition of
sealing lithologies across the fault plane to prevent CO2 migration out of the trap. (D) A fault-dependent (contractional fault) folded closure relies on the juxtaposition
of sealing lithologies across the fault plane to prevent CO2 migration out of the trap. (E) A stratigraphic trap relies on lateral changes in lithology (often lateral
stratigraphic terminations or "pinch-outs") to prevent CO2 migration out of the trap.
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