Fissures can have a significant impact on caprock integrity in the context of carbon capture and storage (CCS). In this campaign, the sealing capacity of a pre-fissured caprock material, the Opalinus Clay shale, is studied with real time x-ray imaging during CO2 injection. 3D image analysis of the acquired x-ray tomographies reveals that the pre-existing fissure drives the hydromechanical response of the material until a certain overpressure level after which, new breakthrough pathways are created though the matrix of the material.