Characterising Salt Cores for Underground Gas Storage
Authors: Dasgupta Kanchan , Hujer Wolfgang 
Published: November 2024

Salt deposits are crucial for energy storage solutions, particularly for hydrogen, natural gas, compressed air, and potentially for waste disposal (radioactive waste). These deposits are predicted to play a significant role in solving modern-day energy challenges. The European target of net zero greenhouse gas emissions by 2050 involves a policy towards hydrogen-supported electrification. The use of hydrogen may become indispensable, particularly for long haul transportation which can rely on fuel cells or internal combustion engines to convert chemical power to useful power. Salt caverns should be considered as part of a larger hydrogen distribution network and could provide the necessary energy buffering and thus stability in the energy provision, either by delivering hydrogen directly to the end user, such as trucks and aircraft, or by converting it back into electricity by fuel cells or gas turbines to support the grid. Also, the incentive of governments to emit less greenhouse gases and become less dependent on fossil fuels for grid electricity generation, contributes to a substantial growth in wind and solar energy production. However, the amount of energy produced by these systems relies heavily on favourable meteorological and seasonal conditions, and the energy availability is therefore not necessarily in tune with the power demand. One way to recover this lost potential is by converting the excess available power into hydrogen by electrolysis and store it in rock salt caverns. This technique has the potential to cover the seasonal energy deficits observed during wintertime.

Underground hydrogen storage in salt caverns is currently the most advanced technology option and, from a purely technical point of view, is currently the preferred option in the industry (despite significantly lower storage capacities compared to porous media). Salt caverns are the by-product of solution mining that refers to the mining of various salts by dissolving them in water or undersaturated brine and pumping the resultant brine to the surface. Water or undersaturated brine is injected through a well drilled into a salt layer to etch out a cavern. For salt caverns to be used for underground gas (hydrogen) storage, rather clean, thick homogenous halite in a depth range of ~ 400 – 2500 m TVD are preferred, like those found in the salt domes in northern Germany, the Netherlands and below the North Sea (Zechstein salt deposits). The current deepest salt solution operation is in the northern Netherlands in Zechstein salts at a greater depth of 2900 meters.

As a sedimentary deposit, rock salt is formed by chemical precipitation from a saturated fluid that has undergone solar evaporation, in arid climate systems, in salinas, perennial lakes and sabkha environments. Besides evaporation, additional processes can lead to the formation of rock salt, namely temperature changes, mixing of brines and brine freezing. Upon primary precipitation, the salt sediments commonly undergo diagenetic processes in shallow burial or post uplift settings, resulting in secondary rock salt textures. Moreover, the structure of rock salt varies between domal salt and bedded salt deposits.