Numerical modelling of the cyclic response of a caprock material with an elasto-viscoplastic constititutive model accounting for debonding

The energy transition is a paradigm shift that affects the entire economic, social and industrial trajectory of our country. In this context, large-scale underground storage of hydrogen is seen as an effective way of overcoming the drawback associated with the seasonal nature of renewable energy sources in meeting the needs of energy demand. However, the injection/production cycles will produce a seasonal cyclic pressure of the stored gas, which in turn will induce stress changes in both the host formation and the caprock material. Such geomechanical variations can seriously compromise the sealing capacity of the caprock, increasing the risk of leakage. It is then of paramount importance to appropriately model the fatigue behaviour of the caprock material. This work investigates the ability of an existing elasto-plastic framework developed for structured soils to model the experimental response of an Italian stiff clay, which represents a typical caprock material. The existing framework is enhanced by introducing a viscous strain component to reproduce the observed time-dependency of the caprock response, which is shown to be crucial in modelling the material behaviour under different cyclic loading conditions.