Environmental Engineering Reference
In-Depth Information
Infl uence of chemical reactions on
fracture permeability
If fracture fl ow is important, seal k v and p c,b values may vary relatively
rapidly because acidic fl uids fl owing through a fracture will react with the
fracture surfaces, potentially modifying the fracture aperture. A key ques-
tion in the long-term security of geological sequestration, then, is whether
fl uid (brine or CO 2 ) fl ow in seal fractures is self-enhancing or self-limiting.
The answer to this question may depend on the composition of the fl uid
that fl ows in the fracture and on the rates of geochemical and geome-
chanical interactions between the fracture fl uid and the seal rock matrix
( Figure 9.5.5 ) [9.20].
At the base of a seal formation, the fl uid fl owing through fractures
may be either pristine reservoir water that has been displaced by the CO 2
injection or, close to the CO 2 plume, acidic fl uids: CO 2 -enriched water or
Figure 9.5.5 Flow and geochemistry in an artifi cially fractured caprock
Experimental results on the evolution of an artifi cially fractured caprock (the top seal of
the Amhersburg formation, a carbon sequestration pilot site in Northern Michigan). The
left side of the fi gure shows a cross-section of the core sample at the beginning of the
experiment (the initial state of the fracture is shown in white) and after one week of expo-
sure to acidic brine (the fi nal state of the fracture is shown in black) as reconstructed from
micro-CT scans. The right side of the fi gure (a back-scattered electron scanning electron
microscope (BSE-SEM) image of a portion of the fracture after exposure to acidic brine)
shows that the dissolution of fracture surfaces occurs preferentially in regions occupied
by calcite grains (“Cal”) rather than in regions occupied by dolomite (“Dol”) or silicate
minerals. Images reproduced from Ellis et al. [9.20], with permission from Elsevier .
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