Modelling of CO 2 diffusion and related poro-elastic effects in a smectite-rich cap rock above a reservoir used for CO 2 storage

Abstract

AbstractThe transport of dissolved CO2in brine through a smectite-rich shale-type cap rock above a CO2storage reservoir may lead to the adsorption of CO2in 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 CO2in 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 CO2adsorption 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 CO2over 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.