A preliminary investigation on the mechanical behaviour of a stiff Italian clay in the context of hydrogen storage

The large-scale use of renewable energy sources is closely linked to the ability to store excess energy generated during periods of overproduction for use when demand is at a peak. Storing green energy is therefore a key component in the move towards a carbon-neutral economy. Underground hydrogen storage in depleted oil and gas reservoirs may provide an efficient long-term solution. Cyclic injection and production of hydrogen alter the chemo-hydro-mechanical conditions of the reservoir and caprocks, and possible geomechanical consequences of such alterations must be preliminarily assessed for safe storage operations. This study aims at exploring the possible effects of cyclic mechanical loads, such as those that might be induced by hydrogen storage and production, on the mechanical behaviour of a clayey caprock. A series of triaxial tests, both monotonic and cyclic, were carried out on undisturbed samples of a stiff Italian clay cored from a caprock formation overlying a hydrocarbon reservoir. The results show that the material response is characterized by the distinctive stress-strain behaviour of stiff clays, with a rather high fragility, which was found to be highly dependent on the loading strain rate. During laboratory experiments conducted at frequencies larger than in situ ones, cyclic loading under stress control causes a gradual degradation of the material structure leading to the formation of a clear shear band followed by a reduction in shear strength. Eventually, failure occurs as the peak shear strength approaches the applied load. The progressive destructuration also implies a reduction in P- and S-wave propagation velocities and a significant change in the signal shape, which is therefore a promising parameter for monitoring the material degradation process.