Large-Scale Geological Carbon Sequestration - Introduction to Carbon Capture and Sequestration - page 491

increase. On the other hand, there can also be cases where the tectonics
are inactive, as in ancient faults where no particular built-up stress exists.
Injection into these areas can also lead to induced seismicity, but the
magnitudes of the induced earthquakes will tend to be much smaller
because of the lack of tectonic stress contribution.
The bottom line for induced seismicity related to sequestration is that
it is a very real hazard. In well-chosen sites, the main impact is nuisance:
earthquakes that might be felt but that usually do not do any measurable
damage. In tectonically active areas, it is possible that injection could
trigger a tectonic earthquake that could cause measurable damage.
Recently, some researchers have suggested that caprock integrity
could be compromised by induced seismicity [10.39], but data on perme-
ability of faults in shale caprock to support this assertion are currently not
available.
Near-surface impacts due to leakage
to the atmosphere
The ultimate failure of a geological carbon sequestration site would
involve leakage of CO 2 to the atmosphere. Before entering and mixing in
the atmosphere, the CO 2 would likely have an impact in the near-surface
environment. Figure 10.4.1 indicates that many of these near-surface
HSE impacts can be severe because people and other animals and
plants are present. Figure 10.4.4 shows some of the features and envi-
ronments in which impacts could occur.
Impacts to groundwater have already been discussed in this section.
Above the water table is the unsaturated (so-called vadose ) zone in which
resides the roots of trees and plants and burrowing animals. Elevated
CO 2 concentrations in the vadose zone can cause plant stress or even
plant death. This occurred on a relatively large scale starting in the late
1980's at Horseshoe Lake in the Eastern Sierra mountains of California
due to natural magmatic CO 2 [10.41].
Simulation studies have shown that it does not take a very large CO 2
leakage fl ux to create high concentrations in the vadose zone [10.42].
Figure 10.4.5 shows simulation results [10.43] for a typical geological
sequestration site containing 4 x 10 9 kg CO 2 . Above this site, it is assumed
that there is a cylindrical area (100 m) in which there is a leakage of CO 2 .
Figure 10.4.6 shows simulation results for maximum leakage fl ux and