Mechanical and transport properties of rock salt under simulated cavern wall conditions in the framework of cyclic hydrogen storage

Abstract

Large-scale, temporary and seasonal storage of excess renewable energy using hydrogen fuel has received increased attention in recent years. Caverns in natural salt are widely recognized as a viable and safe storage option, due to the ductile nature, low porosity, and low permeability of rock salt. The rheology of salt has been studied in depth, including the impact of pressure cycling frequency and amplitude on weakening and dilatancy under (lab)dry conditions. Only few studies have investigated the impact of brine on the development and permeability evolution of the damage zone lining the cavern walls, despite the fact that salt caverns contain some brine even when being used for gas storage. We conducted triaxial experiments on pre-damaged (permeable), natural Leine-Steinsalz samples to investigate the creep and permeability evolution of damaged rock salt in response to stress changes. Load-hold (48 h)-unload-hold (24 h) tests were performed at differential stresses relevant for cavern storage, under lab-dry (argon) and wet (saturated brine) conditions. We observed limited permeability reduction under lab-dry conditions, while the introduction of brine led to more axial strain and enhanced permeability reduction over time. Furthermore, initial permeability and stress history played a key role in controlling the dilatancy evolution.