Modelling of CO2 diffusion and related poro-elastic effects in a smectite-rich cap rock above a reservoir used for CO2 storage
Hendrik M. Wentinck, Andreas Busch, 2017. "Modelling of CO2 diffusion and related poro-elastic effects in a smectite-rich cap rock above a reservoir used for CO2 storage", Geomechanical and Petrophysical Properties of Mudrocks, E. H. Rutter, J. Mecklenburgh, K. Taylor
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The transport of dissolved CO2 in brine through a smectite-rich shale-type cap rock above a CO2 storage reservoir may lead to the adsorption of CO2 in the smectite and the associated swelling of this material. These effects on the cap-rock permeability and on the stress in the cap rock have been modelled by combining single-phase two-species convective–diffusive flow with poro-elastic effects. We assume that the cap rock behaves as a poro-elastic, uniform and isotropic rock with two intermingled networks of macropores and of interlayer space between the clay layers. The empirical expressions for the chemical potentials and partial molar volumes of water and CO2 in the macropores and in the interlayer space have been derived from experimental data.
With an emphasis on the physics underlying clay swelling, we have applied the model for uniaxial deformation in a cylindical symmetrical geometry. Considering that this geometry is, to some extent, only representative of the geometry at a reservoir edge, and that anisotropy, plasticity and a possible permeability increase when the stress in the rock is close to shear-type failure have not been included in this work and recognizing the present uncertainties in the experimental clay and shale data, the results are indicative. The model predicts that the stresses following CO2 adsorption in a smectite-containing cap rock are substantial at typical subsurface conditions for a carbon capture and storage (CCS) project. When the rock is under an unfavourable stress condition, local shear-type failure may occur in the cap rock exposed to CO2 over a period of 100–10 000 years, despite the fact that the permeability of the rock may reduce under the increasing compressive stress. For this reason, we recommend including the possibility of swelling cap rock into a containment risk assessment of a CCS project.
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A surge of interest in the geomechanical and petrophysical properties of mudrocks (shales) has taken place in recent years following the development of a shale gas industry in the United States and elsewhere, and with the prospect of similar developments in the UK. Also, these rocks are of particular importance in excavation and construction geotechnics and other rock engineering applications, such as underground natural gas storage, carbon dioxide disposal and radioactive waste storage. They may greatly influence the stability of natural and engineered slopes. Mudrocks, which make up almost three-quarters of all the sedimentary rocks on Earth, therefore impact on many areas of applied geoscience.
This volume focuses on the mechanical behaviour and various physical properties of mudrocks. The 15 chapters are grouped into three themes: (i) physical properties such as porosity, permeability, fluid flow through cracks, strength and geotechnical behaviour; (ii) mineralogy and microstructure, which control geomechanical behaviour; and (iii) fracture, both in laboratory studies and in the field.