Modelling of CO₂ diffusion and related poro-elastic effects in a smectite-rich cap rock above a reservoir used for CO₂ storage

The transport of dissolved CO2 in brine through a smectite-rich shale-type caprock 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 caprock permeability and on the stress in the caprock have been modelled by combining single phase two-species convective-diffusive flow with poro-elastic effects. We assume that the caprock 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 symmetric geometry. Considering that this geometry is only to some extend representative for the geometry at a reservoir edge, considering 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 recognising the present uncertainties in the experimental clay and shale data, the results are indicative. The model predicts that the stresses following from CO2 adsorption in a smectite containing caprock are substantial at typical subsurface conditions for a CCS project. When the rock is under an unfavourable stress condition, local shear type failure may occur in caprock exposed to CO2 in a period of 100 - 10000 years despite that the permeability of the rock may reduce under the increasing compressive stress. For this reason, we recommend to include the possibility of swelling caprock into a containment risk assessment of a CCS project.