Storing hydrogen: the risks and control measures

The area from England to eastern Poland was once a sea. The water evaporated and a thick layer of salt remained in the deep underground, which over millions of years came to lie under an increasingly thick layer of earth. The weight of that layer of earth pushed the salt layer up in some places, creating mountains of rock salt that are sometimes several thousand meters high. 

One of these mountains is located at Zuidwending in Groningen, from which salt has been extracted for many decades. Extraction takes place through ‘solution mining , in which rock salt is dissolved in water and then pumped away. This creates a large cavity, or cavern, at a depth of about 1,200 meters. Six of these caverns have long been used by EnergyStock, subsidiary of Gasunie, as storage facilitiy for natural gas. The underground rock salt is impermeable and plastic. This makes it excellent for storing gases.

Gasunie is currently developing four salt caverns near Zuidwending for hydrogen storage. The first cavern is expected to be commissioned around 2031. Eddy Kuperus, Hydrogen Storage Engineer at Gasunie, regularly receives questions about the safety of hydrogen storage. Will local residents experience subsidence or earthquakes? 

The caverns are carefully designed, Kuperus explains. Stability of the subsurface is the starting point. The salt caverns are similar to large, architectural domes, such as St. Peter's in Rome. 'In that kind of domes, the stones are bricked together, which gives a solid structure. St. Peter's has also been there for a while,' he winks. 'The roof of the cavern is also very sturdy, and that helps determine the stability of the cavity.'

One of the concerns of local residents is whether saltmining and the creation of caverns will lead to subsidence. According to Kuperus, the answer is unequivocally 'yes', but he immediately notes that it is a small and very gradual subsidence. Because salt is plastic, a cavern will slowly shrink due to the pressure of the surrounding rock.

Before the storage of natural gas in Zuidwending, the decrease in the period from 2011 to 2020 was 1.4 centimeters, and based on various calculations, the total land subsidence in 2050 is expected to be about 3.6 centimeters. Viewed from above, that decline will occur in an area with a radius of 2.4 kilometers from the storage site. At the edge of the area, the decrease will be zero, and at the center, therefore, that 3.6 centimeters. Someone who has a plot 100 meters long in this area will find that the difference in subsidence at both ends of the plot in 2050 totals about 2 millimeters. 'Such a subsidence and skew will not lead to damage to homes', Kuperus states. He bases this on various studies conducted by external agencies and insitutes.

According to the Gasunie engineer, this subsidence is accurately predictable through extensive laboratory tests conducted on the salt. In addition, the area is monitored via satellites, GPS antennas and surveyors. The results are shared publicly on platforms such as bodemdalingskaart.nl and monitored closely by government agencies. Still, there are concerns about damage to buildings. 'If local residents suffer damage from our activities, we will compensate them. There is no question about that,' Kuperus says. 'The Mining Damage Commission (Commissie Mijnbouwschade), as an independent body, will assess the damage in the event of a claim and hold the perpetrator liable.'

Another question is whether hydrogen storage could cause earthquakes. Kuperus' answer to that is 'No'. Very occasionally, a small vibration may occur when a chunk of salt falls down the inside of the cavity. But this vibration is microseismic in nature and not felt at the surface. The vibrations, however, are recorded by a highly sensitive measurement system. This measurement data is reported quarterly and is available through the website Energiebuffer Zuidwending.

Another concern is whether the salt could crack due to the high pressure of the stored hydrogen. Kuperus says the caverns are designed with strict safety factors and that the pressure in the cavity will never exceed the pressure of the surrounding rock. This prevents the salt from cracking.

Leaks are also a potential concern. Although hydrogen particles are small, they cannot penetrate the salt and escape. In theory, however, leaks could occur at the surface around the borehole. To prevent these leaks, double barriers are installed at that location: if the first one fails, there is always a second barrier to prevent hydrogen from escaping, and there is continuous monitoring. 'We design for zero leaks, but you can never completely eliminate the risk. Hence the double safety barrier.' During a demonstration project, according to Kuperus, Gasunie demonstrated that the design prevents hydrogen leaks and that hydrogen can be stored safely in caverns. 

When hydrogen is delivered from the cavern to the grid, the pressure in the cavern drops. If hydrogen is stored, then the pressure rises. This so-called sending in and out creates varying pressure in the cavern and stress on the salt wall. Based on what Kuperus calls 'geomechanical calculations', Gasunie determines the limits within which the pressure in the cavern is allowed to vary. 'We do everything we can to stay within the safety margins and thus continue to guarantee the stability of the cavern', Kuperus says. 

Extensive precautions have also been taken to prevent calamities. The caverns are continuously monitored and controlled by a wide range of measuring instruments. Gasunie regularly conducts tests and collaborates in various research programs to develop new insights and technologies.

The supervision of our storage activities is carried out by the State Supervision of Mines (SodM). This government agency ensures that everything takes place safely and within the granted license. Gasunie reports periodically to SodM and actively involves the regulator throughout the development of the hydrogen caverns. This makes it possible to jointly identify what is going well and where additions are still needed. SodM concluded in October 2024 that the storage of hydrogen in salt caverns is technically feasible, but that 'additional research' is needed to properly manage the risks. Gasunie endorses this conclusion and sees SodM's response as an encouragement to proceed with plans for underground hydrogen storage at Zuidwending.

Although legitimate and understandable questions are raised about the safety of hydrogen storage in underground caverns, Gasunie believes the risks are well manageable. Through extensive prior research, advanced technology, double safety barriers, continuous monitoring and strict safety measures, it will be possible to store hydrogen safely in the future. With the right precautions, hydrogen storage will play a crucial role in the energy transition, protecting both the climate and local residents.