In the light of growing concerns for the climate change, it is of particular interest for governments to encourage efficient capture and safe storage of large amounts of carbon dioxide in the subsurface. In this perspective and in order to accurately predict the short and long-term response of the reservoir, a precise characterization of the geomechanical properties has to be carried out. In addition to the classical poroelastic properties, time-dependent deformation, such as viscous creep should also be considered. Storage capacity of a caprock may be seriously affected by local creep deformation allowing fast vertical fluid flow through an a priori very impermeable formation. Within this study, we investigate the ease to creep of Opalinus clay (Jurassic shale) under shallow geological storage conditions and predict the propagation of high porosity channels at operational time scales. The effective poroviscoelastic parameters of rock are inferred from the novel laboratory experiments that allow evaluation of time-dependent deformation. The bulk viscosity of the shale is found to be ∼ 1014 - 1015 Pa·s and it decreases with rise of temperature and pore fluid pressure to total mean stress ratio. Furthermore, the propagation speed of high porosity channels (porosity waves) is calculated to be on the order of the centimeters per year.