Large releases from CO2 storage reservoirs: analogs, scenarios,and modeling needs Page: 4 of 6
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4. There is rather inconclusive evidence from natural analogs that CO2 presence could lead to
pneumatic eruptions, i.e., self-enhancing, violent CO2 releases driven by high-pressure gas [3, 4,
5]. As opposed to the well-understood hydrothermal eruptions, where depressurization of a hot
water reservoir may cause buoyant runaway of steam, pneumatic eruptions would not require
substantial contributions of thermal energy. Pneumatic eruptions would be particularly harmful
if occurring close to the land surface, which requires the accumulation and sudden release of a
large CO2 volume pressurized in a shallow storage reservoir. It is not clear at present whether
such pneumatic eruptions are physically possible under thermodynamic and hydrogeologic
conditions representative of CO2 injection sites. A thorough evaluation of the possibility of such
high-energy discharges would be useful for demonstrating the technical feasibility of storing
CO2 in geologic reservoirs, and achieving public acceptance of the technology . Numerical
simulators that can handle the complex processes involved in pneumatic eruptions need to be
developed for that purpose, and numerical modeling studies should be conducted covering a
wide range of realistic to extreme scenarios, with the goal of gaining assurance that high-
energy-eruptive releases are not possible.
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 reservoir-caprock system.
CO2 Migration along a Fault
Figure 2 shows a schematic model of a fault zone, along with simulation results for CO2 discharge
through this fault. The fault initially contains water in a normal geothermal gradient of 30 C/km
with a land surface temperature of 15 C, in hydrostatic equilibrium. CO2 discharge is initiated by
injecting CO2 at an overpressure of approximately 10 bar in a portion of the fault at 710 m depth.
The numerical simulation includes two- and three-phase flow of an aqueous phase and liquid and
gaseous CO2 phases in the fault, as well as conductive heat transfer with the wall rocks that are
assumed impermeable [5, 6].
x= 1 m x= 175 m Time (years)
0 1 2 3 4 5 6 7 8 9
land surface 20X10-3
fixed ternerature .12
710 m -8
CO2 h=1000m _ 0
three-phase 2 j
i volume S
w =200 m 0 50 100 150 200 250 300x1f
Figure 2. CO2 leakage along a fault zone [5, 6]. A schematic model of a fault zone is shown on the left. The right panel
gives temporal variation of CO2 leakage fluxes at two different positions at the land surface. Total flow system
volume with three-phase conditions is also shown.
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Birkholzer, Jens; Pruess, Karsten; Lewicki, Jennifer; Rutqvist,Jonny; Tsang, Chin-Fu & Karimjee, Anhar. Large releases from CO2 storage reservoirs: analogs, scenarios,and modeling needs, article, September 1, 2005; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc897395/m1/4/: accessed April 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.