On sorption and swelling of CO2 in clays
Geomech. Geophys. Geo-energ. Geo-resour.
On sorption and swelling of CO2 in clays
0 Y. Gensterblum School of Earth, Energy and Environmenal Sciences, Stanford University , Stanford, CA , USA
1 P. Bertier Energy and Mineral Resources Group, Clay and Interface Mineralogy, RWTH Aachen University , Bunsenstr. 8, 52072 Aachen , Germany
2 A. Busch (&) H. M. Wentinck Shell Global Solutions International B.V. , Kessler Park 1, 2288 GS Rijswijk , The Netherlands
3 C. J. Spiers M. Zhang Department of Earth Sciences (HPT Lab), Utrecht University , Budapestlaan 4, 3584 CD Utrecht , The Netherlands
4 G. Rother Chemical Sciences Division, Oak Ridge National Laboratory , Oak Ridge, TN 37830-6110 , USA
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
CCS; CO2 storage; Clay swelling; Carbon dioxide; Leakage; Containment; Smectite
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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.
1 Introduction
For the characterization of geological CO2 storage
reservoirs, a number of critical parameters need to be
assessed. From existing knowledge and experience,
especially collected in the oil and gas industry, the
storage capacity and injection rate are generally well
understood for specific reservoirs. Critical parameters
are reservoir size and reservoir heterogeneity, i.e.
porosity and (relative) permeability, fluid saturation,
the reservoir stress field, in particular the minimum
horizontal stress, as well as pressure and temperature
conditions.
In addition, the identification and risk assessment of
potential leakage pathways, reservoir depletion rate in
case of leakage and reservoir pressure at which leakage
is initiated or inhibited, are unique to a CCS project and
thus need to be considered. Pressure is a key parameter
in any leakage scenario, and will decrease in typical
fluid extraction processes but increase in storage
applications. The CO2 injected into a reservoir may,
sometimes significantly, increase the average reservoir
pressure. Initially, pressure builds up only locally, i.e. in
the vicinity of the injection well. Imperfections in
cementation of injection, monitoring or abandoned
wells, can result in the formation of micro-annuli
between cement and caprock or cement and casing,
potentially acting as pathways for gas leakage. With
continuing injection, the pressure pulse will eventually
be transmitted to the far field. The resulting rise of the
reservoir pore pressure reduces the effective stress on
existing fractures and faults, potentially causing their
(re)activation.
The main trapping mechanisms in CO2 storage are
structural and residual trapping. In structural trapping, a
continuous, connected gas column will form underneath
a sealing formation. Buoyancy results in fluid pressure
acting on the reservoir-caprock interface, which must be
lower than the capillary entry pressure of the seal to
prevent capillary leakage. Hence, this maximum
pressure or gas column height is a key parameter in the
assessment of a storage scenario. In residual trapping,
gas resides in disconnected bubbles in pores and is
therefore not contributing to a buoyancy pressure.
The factors controlling leakage from a gas storage
reservoir have been addressed earlier for a range of
applications. Yet, the specific properties of CO2 add to
the complexity of the as (...truncated)