Abstract
The geological storage of carbon dioxide
(CO2) is a well-studied technology, and a number of
demonstration projects around the world have proven
its feasibility and challenges. Storage conformance
and seal integrity are among the most important
aspects, as they determine risk of leakage as well as
limits for storage capacity and injectivity. Furthermore,
providing evidence for safe storage is critical
for improving public acceptance. Most caprocks are
composed of clays as dominant mineral type which
can typically be illite, kaolinite, chlorite or smectite.
A number of recent studies addressed the interaction
between CO2 and these different clays and it was
shown that clay minerals adsorb considerable quantities
of CO2. For smectite this uptake can lead to
volumetric expansion followed by the generation of
swelling pressures. On the one hand CO2 adsorption
traps CO2, on the other hand swelling pressures can
potentially change local stress regimes and in
unfavourable situations shear-type failure is assumed
to occur. For storage in a reservoir having high clay
contents the CO2 uptake can add to storage capacity
which is widely underestimated so far. Smectite-rich
seals in direct contact with a dry CO2 plume at the
interface to the reservoir might dehydrate leading to
dehydration cracks. Such dehydration cracks can
provide pathways for CO2 ingress and further accelerate
dewatering and penetration of the seal by
supercritical CO2. At the same time, swelling may
also lead to the closure of fractures or the reduction
of fracture apertures, thereby improving seal integrity.
The goal of this communication is to theoretically
evaluate and discuss these scenarios in greater
detail in terms of phenomenological mechanisms, but
also in terms of potential risks or benefits for carbon
storage.
