The safe storage of CO2 in deep reservoir units requires an efficient sealing of the overlaying caprock. The acidic environment caused by the dissolution of CO2 into the pore fluid can induce changes in the microstructure of the material over the long term and might consequently impact its retention capabilities through changes in the pore size and pore connectivity. In the first part of this paper, the impact of a low pH environment on some of the physical properties, such as grain density, void ratio and dominant entrance pore size, and on the retention capacity of a shaly caprock representative material, i.e., Opalinus Clay shale, is investigated by using an HCl solution inducing a pH = 3. The results show that grain density, dominant entrance pore size and void ratio are not significantly affected by the contact with a low pH environment in the considered period. Similar conclusions can be drawn for the retention capacity because the air entry value appears to be the same for treated and non-treated material. 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.