  Caprock integrity is one of the chief concerns in the successful development
of a CO$_2$ storage site. In this chapter, we provide an overview of the
permeability of fractured shale, the potential for mitigation of CO$_2$ leakage
by sorption to shale, and the detection by acoustic methods of CO$_2$
infiltration into shale. Although significant concerns have been raised about
the potential for induced seismicity to damage caprock, relatively little is
known about the permeability of the damaged shale. We present a summary of
recent experimental work that shows profound differences in permeability of up
to three orders of magnitude between brittle and ductile fracture permeability.
In the ductile regime, it is possible that shale caprock could accommodate
deformation without a significant loss of CO$_2$ from the storage reservoir. In
cases where CO$_2$ does migrate through damaged shale caprock, CO$_2$ sorption
onto shale mineralogy may have a mitigating impact. Measured total storage
capacities range from 1 to 45 kg-CO$_2$/tonne-shale. Once CO$_2$ is in the
caprock, changes in the acoustic properties of CO$_2$-saturated shale that are
predicted by Gassmann fluid substitution calculations show a significant
reduction of the bulk modulus of CO$_2$-saturated shale.
