Underground hydrogen storage in salt caverns: Laboratory experiments to determine integrity of rock salt and wellbore through effective permeability measurements

Underground hydrogen storage in salt caverns is a promising solution for short-term storage, allowing multiple cycles per year. This study experimentally investigates the integrity of such caverns and their wellbores under operating conditions typical of German salt caverns. Specifically, we measure the permeability of rock salt, cement (API Class G and High Magnesium Resistant (HMR+)), rock salt-anhydrite composites, cement-salt composites, and casing-cement composites. Rock salt demonstrates extremely low permeability (10(-23) m(2)), while casing-cement composites (HMR+) exhibit permeabilities similar to pure cement (10(-20) m(2) or lower). Both salt-cement (HMR+) and casing-cement (HMR+) composites meet the strict tightness requirements for hydrogen storage (10(-19) m(2) or less). While thin anhydrite layers in rock salt can increase permeability, compaction can reduce it to levels comparable to rock salt. This study's novelty lies in evaluating the feasibility of a real German cavern for hydrogen storage, using a custom-built transient permeability setup capable of testing casing-cement composites at a 1:1 wellbore scale.