Environmental Engineering Reference
In-Depth Information
Section 5
Monitoring of GCS sites
One way of reducing the possibility of unexpected HSE impacts is to
keep a close watch on the CO 2 injection and storage processes so that
irregularities can be found and any hazardous conditions mitigated. Of
course other good reasons for monitoring also exist, including the need
to quantify storage and trapping to meet the verifi cation needs of cap
and trade and/or carbon tax policies, to optimize storage effi ciency, to
quantify and map the various evolving storage processes, and to assure
the public that the sequestration site is not leaking. In this section, we
discuss the main approaches to sequestration monitoring of the subsur-
face and surface and we provide examples of their use.
Subsurface monitoring
Subsurface monitoring of fl uids in the pore space of deep geological
formations is very challenging due to the large thickness of overlying
rock. The most direct method is to drill a well to the depth desired and
sample the fl uid directly. This approach is expensive and does not pro-
vide spatial coverage. Indirect geophysical methods provide good spatial
coverage but are also expensive and often do not provide high spatial
resolution (i.e., the distance between grid points for which we have data
is large). Figure 10.5.1 illustrates the tradeoff between spatial resolution
and scale of monitoring. As shown, the highest resolution methods are
limited in the scale of spatial coverage, while the methods that can cover
the large areas needed for sequestration monitoring tend to yield low-
resolution data.
In Table 10.5.1 we list several different subsurface monitoring
approaches [10.47]. These can be subdivided on the basis of whether
they are intermittently or continuously applied. Because of the density
and opacity of rock, most of the methods rely on indirect geophysical
approaches whereby acoustic or electromagnetic energy is applied to
the rock and the response is recorded and interpreted. The hydrocarbon
exploration and extraction industries have motivated a great deal of
experience and long history of development of these methods.
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