Impact of CO2 injection on the hydro-mechanical behaviour of a clay-rich caprock

Subsidence and mechanical failure of the caprock are among the issues related to CO2 storage technology. The second part of the paper is dedicated to the analysis of the impact of CO2 injection on the mechanical behaviour of the Opalinus Clay shale. CO2 injection under constant stress conditions with consideration of different overconsolidation ratios is conducted to take into account different loading paths that could be experienced in situ by the caprock. The results show that the injection of CO2 induces the development of volumetric strains of less than 0.1%. Lower strains are measured when the material is overconsolidated; this result can be related to the fact that the material structure is more prone to collapse when it is found in normally consolidated conditions. The observed vertical displacements can be partially caused by desaturation effects induced by the differential pressure between CO2 and pore water, together with double layer effects induced by the CO2 diffusing along the height of the specimen. The impact of CO2 injection on the hydro-mechanical properties of the material is also analysed. The findings suggest that the diffusion of CO2 into the shale does not impact the hydro-mechanical properties of the material because no significant change in oedometric modulus, coefficient of consolidation, secondary compression coefficient, poromechanical parameters and hydraulic conductivity are highlighted after the injection of CO2.