Large Releases from CO2 Storage Reservoirs: A Discussion ofNatural Analogs, FEPS, and Modeling Needs Page: 2 of 5
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studies would serve to gain a better understanding of the fluid flow and heat transfer processes
that would accompany CO2 migration away from the primary storage reservoir, towards shallow
depths and ultimately to the land surface. Some of these processes may be beneficial in that they
prevent or retard upward migration (e.g., attenuation of CO2 in multi-layer systems; energy losses
through adiabatic cooling), while others may enhance CO2 upflow (e.g., stress-induced increases
in fault permeability; reduced phase interference).
2. Many natural releases of CO2 have been correlated with a specific event that has triggered the
release, such as seismic activity leading to geomechanical damage in sealing caprocks (e.g.,
hydraulic fracturing, fault slip and reactivation). The potential for processes to cause such
damage and trigger the release of CO2 from a storage reservoir should be evaluated with
appropriate geomechanical modeling tools (see "Geomechanical Failure Analysis in a Multi-
3. CO2 can both accumulate beneath and be released from primary (deep) and secondary (shallow)
reservoirs with caprock units located at a wide range of depths. In general, a sequence of caprock
units above the intended storage formation would be a desirable feature for a geological
sequestration site, because it provides additional barriers for leaking CO2. However, when
considering the possibility of large, sudden discharges at the land surface, accumulation of CO2 in
a secondary formation in the shallow subsurface could actually have detrimental effects. Consider
for example a shallow anticlinal structure with a low-permeability caprock, where CO2 leaking
from depth would slowly accumulate. If the accumulated CO2 would then be released, as
triggered, for example, by sudden geomechanical damage of the sealing caprock, the pathway to
the land surface might be too short to allow for significant mitigation or retardation of the plume.
CO2 present as a separate gas phase would move upward by buoyancy forces and pressure
differences (Figure la). CO2 dissolved in water would be subject to rapid degassing following
depressurization, which could result in fast-rising expanding bubbles (Figure lb). The potential
for shallow CO2 accumulation should be considered in site evaluation efforts and, if applicable,
the possible release of CO2 from such secondary accumulation should be evaluated in numerical
Ground Surface Ground Surface
Fracture Zone Fracture Zone
Damage Induced by - Damage Induced by
Earthquake or C02 Exsolution Due To Seismic Event or
Mechanical Failure Sudden Pressure Release Mechanical Failure
(Dissolved in Water)
Figure 1. Sudden CO2 leakage from secondary accumulation in shallow reservoirs. (a) CO2 accumulates as
a separate phase. (b) CO2 is dissolved in water. A fracture or fault zone may develop as a result
of geomechanical damage or in response to seismic activity.
Here we briefly summarize results of numerical simulation studies as examples of the type of scenario
modeling needed to further our understanding of CO2 storage and its related risks. The first example is
on CO2 leakage along a continuous fault zone from depth to surface. The focus is here on capturing
the complex thermodynamics in detail to see whether self-limiting and self-enhancing features would
tend to slow or accelerate the upward migration of CO2. The second example involves geomechanical
modeling to determine the potential for fault reactivation and hydraulic fracturing in a multi-layered
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Birkholzer, J.; Pruess, K.; Lewicki, J.L.; Rutqvist, J.; Tsang,C-F. & Karimjee, A. Large Releases from CO2 Storage Reservoirs: A Discussion ofNatural Analogs, FEPS, and Modeling Needs, article, November 1, 2005; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc896141/m1/2/: accessed November 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.