The storage of gas in underground storage sites is in full operation in over 600 sites worldwide. Underground hydrogen storage is relatively new and the experiences are scarce. Storing hydrogen in underground salt caverns requires the fulfillment of several criteria including social, legal, economic, engineering, and geological.

Feasibility studies of underground hydrogen storage in salt caverns have been conducted by several researchers. The geological criteria is the most important concern for the use of salt caverns. Site selection is a vital aspect of geological criteria along with the cost of operation, operational risks, and operational efficiency. In addition to site selection, this article discusses other issues affecting the feasibility of storing hydrogen in underground salt caverns. These issues include injection strategies, hydrodynamic effects, biological, geochemical, and bacterial effects, and withdrawal strategies.

Site selection

Site selection is the first step in choosing an underground hydrogen storage site. While salt caverns are regarded as the best option, the number of salt caverns is limited. Unlike salt caverns, depleted oil reserves are unsuitable for hydrogen storage due to the high potential risk of a reaction between hydrogen and the residual oil.

Due to the limited knowledge and experience of salt caverns, only very little is known about their structure, tightness, cost of operation, and overall efficiency. A lack of appropriate investigation before selecting a cavern may result in loss of hydrogen via leakages, which can affect the overall cost of operation and maintenance.

The location of a salt cavern can impact the economy and feasibility of the storage operation. For example, site availability and distance between the hydrogen source and pipeline. In this case, salt caverns close to the pipeline and hydrogen source are highly recommended.

Another important factor for site selection is available sealing. A seal prevents the gas from moving into overlying formations and adjacent layers. Salt layers are known to be one of the most common cap rock types. Salt layers have good hydraulic integrity and tightness in the presence of hydrogen.

Injection strategies

Hydrogen’s pre-existing fluids contact, avoidance of lateral spreading, and the efficiency of hydrogen storage are controlled by the location of the injection wells. However, locating several injection wells beneath the caprock can save a significant amount of hydrogen from viscous fingering, dissolution, and lateral spreading.

Loss of hydrogen can be prevented by ensuring a low and steady rate of hydrogen injection. Deep geological structures such as salt caverns have attracted several research to examine the injection rate. Many recent studies suggested that the rate of injection (injection pressure) should not exceed the capillary entry pressure and fracturing pressure.

Despite the current challenges of underground storage, several companies have invested heavily in underground hydrogen storage in salt caverns. Linde hydrogen operates the world’s first large-scale hydrogen high-purity salt cavern storage in Texas, U.S. The storage facility can supply up to 600 million cubic feet of hydrogen per day on an average and 700 million cubic feet per day at peak capacity.

Sabic Petroleum stores nearly 1 million cubic feet of pure hydrogen in its underground storage facility in Yorkshire, U.K. The storage facility has a 400 m depth and operates at 50 bar.

The presence of a cushion gas can cause less residual hydrogen saturation and a faster pressurizing process. Nitrogen can be used as a cushion gas due to its higher density and viscosity than hydrogen. Also, nitrogen can displace water more efficiently. However, the intensive mixing of nitrogen and hydrogen during the cyclic operation is a big disadvantage. Carbon dioxide may be used as a cushion gas due to its high density relative to hydrogen.

Study of hydrodynamic effects

An example of hydrodynamic issues is the mixing of water and gases. These issues can be suppressed by optimizing the cyclic duration, injection and withdrawal rates, number of wells, and well configurations. Also, the selection of an appropriate cushion gas with suitable viscosity and density can suppress hydrodynamic issues.

Hydrogen gas rising, fingering, and lateral spreading may result in unstable displacement and uncontrolled gas leakage beyond the geological gap in the salt cavern. The extent of gas fingering and spreading can be reduced and the displacement can be stabilized by gravity forces with reduced dynamic effects.

Study of biological, geochemical, and bacterial effects

Due to the geochemical and microbial reactions, the role of pre-existing fluids and rock types is significant. Unlike salt caverns, it is important to consider the presence of ions, bacteria, and rock types in the water.

The presence of sulfate ions in an underground reservoir can lead to the production of hydrogen sulfide. The higher the molar concentration of sulfide ion, the more would be the production of hydrogen sulfide and vice versa. Ultimately, the presence of sulfide ions results in the loss of hydrogen.

Salt caverns are not easily affected by geochemical and microbial reactions. The presence of carbon dioxide leads to acetogenic and methanogenic reactions. These reactions form methane and acetate ions, resulting in non-negligible hydrogen loss.

The presence of iron III ions can result in the loss of hydrogen due to the production of Fe2H3. The occurrence of this reaction is observed in the presence of excessive water.

The presence of clay minerals, for example, Feldspar, Illite, and Kaolinite, can lead to permeability and porosity changes via dissolution/precipitation reactions. Recent studies have reported the trapping of hydrogen by clay minerals.

Withdrawal strategies

The storage of hydrogen in underground sites is a cyclic operation, which includes many injection and withdrawal cycles depending on the demand for energy. To ensure successful storage operations, efficient withdrawal cycles are essential.

Achieving the best withdrawal efficiency in salt caverns depends on the capacity of surface facilities, well and reservoir deliverability, rate of withdrawal, withdrawal scheme, and well patterns. A recent study proposes an optimization study of the above parameters by using modeling and simulations to improve the efficiency of hydrogen withdrawal from salt caverns with minimal water production.

A viable system to manage the huge amount of water generated, which might be contaminated with toxic chemicals, can be established. Water produced during the periods of withdrawal poses severe environmental issues that can hamper the economic feasibility of the process. Nonetheless, the production of water greatly depends on the storage location.

The duration of each withdrawal cycle and the purity of the extracted hydrogen can increase the storage performance of salt caverns. The duration between every withdrawal cycle segregates the pre-existing fluid and hydrogen due to the force of gravity, which leads to higher efficiency. Impurities may be present in the withdrawn gas stream and can hinder the economy and feasibility of the stages of withdrawal.

Future perspective

Future research should assess salt caverns at the basin scale to study site screening criteria such as cavern properties, fluid properties, and storage capacities. A successful pilot-scale study can be conducted to understand the optimum mechanisms before a commercial-scale implementation. The characteristics of hydrogen molecules and the needed purity of hydrogen limits the available options for underground hydrogen storage in salt caverns. Increased practice and technical amendments are needed to advance the technology in the future.