To curb global CO2 emissions, renewable energy (wind, solar) is increasingly becoming part of the energy mix, though its intermittent nature requires a form of energy storage and energy buffering. Large-scale, cyclic or seasonal, storage of excess renewable energy is best done in the form of hydrogen fuel, with salt caverns providing a reliable storage complex. The ductile nature of rock salt causes it to have excellent sealing qualities, and the capacity to heal damage due to creep, even at upper crustal conditions. However, damage of the cavern wall, possibly further enhanced by hydrogen pressure cycling, could impact the storage integrity and efficiency. Furthermore, farther field intact rock salt, outside of the original damage zone, may over time also become impacted by the cyclic injection and production of hydrogen. Although the rheology of rock salt was studied in depth over the last decades, the impact of cyclic pressure, stress and temperature variations, and the chemical changes caused by the hydrogen itself, have not.
Therefore, we performed cyclic deformation experiments to study the underlying grain-scale mechanisms controlling salt deformation under hydrogen storage conditions, to provide input for numerical models aimed at predicting storage integrity and operational safety (e.g., operational magnitude and frequency). Using a triaxial machine, combined with permeametry, constant stress and pressure injection-depletion tests on natural rock salt were performed, simulating the in-situ conditions for salt caverns in NW Europe. The first series of experiments particularly focused on understanding the creep and permeability evolution of the developing damage zone in the cavern walls during hydrogen storage cycling. Experiments were performed under dry and wet conditions to evaluate the impact of fluid on damage evolution during cycling and possible (mechanical or chemical) healing during stages of constant stress. Combined experimental, microstructural and microphysical modelling will be used to develop mechanism-based constitutive models suitable for modelling of cavern-scale behaviour and integrity over operational timescales.
