Hydrogen underground storage

Underground gas storage

Storing energy as a gas is efficient but also demanding in space, because gases are low density. Using surface tanks is restricted to small volumes because of tank and land cost. Storing gas underground allows storage of large quantities in a safe manner and over long periods for seasonal balancing.

Underground gas storage is a widespread technology for natural gas storage. It is mainly used to absorb the dynamic differences between production and demand. While natural gas production is fairly constant along the year, demand is largely shifted towards the cold months. Excess natural gas production during the hot months is stored underground until next cold-months period. Such a storage can be done in depleted oil or gas fields or in purpose-built caverns.

While natural gas storage helps absorb demand variation, hydrogen storage would help absorb production variations: excess electricity from periods with strong renewable production could be converted into hydrogen and stored underground. Hydrogen could then be used either directly as a hydrogen-derived energy carrier (hydrogen, ammonia, methanol,…) or converted back into electricity during periods of weak renewable production.

Working principles of underground storage

Any underground gas storage uses a cavern or a porous geological formation to store a gas. Only formations that includes a top layer impermeable to the stored gas are relevant to avoid gas leaks.

The storage volume is made available by removing the material initially present in the cavern or porous layer. Leaving such volume empty would create a mechanical weakness induced by the weight of all above layers. Avoiding cracks and collapse therefore requires to ensure a minimum pressure in the storage volume. Beyond mechanical structure, a minimum pressure is also required to ensure gas extraction by simply using the pressure difference between the storage underground volume and the receiving surface tank. The maximum of these 2 pressures defines the minimum operating pressure of the storage.

Specifically speaking of salt caverns, operating below the structural minimum gas pressure creates salt dilatancy, whereby the difference between gas pressure effect on walls and stress with the material surrounding the cavern is excessive, leading to microcracking and expansion of rock material towards the inner cavern.

The amount of gas ever left in the storage volume to ensure minimum operating pressure is named the cushion gas. To avoid contamination and separation efforts during extraction, the cushion gas is generally identical to the useful gas. The cushion gas is defined as an economic inventory and is therefore part of initial investment in the installation.

When adding gas in the storage volume, pressure increases inducing stress on the volume walls. Any type of exploited underground formation will have a maximum pressure beyond which mechanical damages to the structure are created, either immediately or by fatigue over multiple filling/emptying cycles. That maximum pressure (minus a safety margin) defines the maximum amount of gas that can be present in the storage volume.

Because of the requirement for a cushion gas, the maximum mass of gas in the storage is not the mass storage capability of the formation. Instead, the amount of stored gas, named the working gas, is defined as the difference between the mass at maximum pressure and the mass at minimum pressure (total mass minus cushion gas mass).

Storage capacity is not the only parameter defining the usefulness of a storage volume. Deliverability or delivery capacity is also important depending on the planned used of stored gas. It is defined as the speed at which gas can be extracted/delivered/withdrawn from the storage facility. Deliverability depends both on the characteristics of the storage equipment and on its state of charge. A nearly full storage volume, i.e. filled at high pressure, will deliver gas quicker than a nearly “empty” storage volume (pressure near minimum allowable, not near zero). The complement to delivery capacity is injection capacity, the rate at which the storage volume can be filled. It is also dependent on state of charge with filling slower when storage is nearly full.

Types of underground storage

There are 3 main types of underground storage formations:

• Depleted oil/gas fields
• Aquifers
• Salt caverns

Depleted natural gas or oil fields close to consumption centres are natural choices to store natural gas. Converting the fields from exploitation to storage is relatively cheap because most piping and pumping equipment can be reused after repurposing. However, conversion from natural gas to hydrogen is not straightforward because of hydrogen properties (equipment embrittlement, gas reactivity with the environment, see details below).

Aquifers are porous water-bearing sedimentary rock formation similar to oil fields. An aquifer capped with a rock formation that is impermeable to gas can be converted into a gas storage by draining water. They generally require a larger portion of cushion gas than gas or oil fields and are less flexible for injection and withdrawal. For natural gas storage, these drawbacks can be partly countered by adding an active water drive, which sustains pressure within the reservoir. Such an approach is less favourable for hydrogen as water/hydrogen separation at extraction is more difficult.

Salt caverns are pure storage volume, rather than porous media for gas/oil fields and aquifers. They benefit from a lower portion of cushion gas and higher injection and delivery rates. They are created by injecting water in a salt formation and extracting the brine. This process named leaching is more expensive on a per-volume basis than conversion of existing oil/gas fields, but allows to optimise the shape of the cavern. The higher injection and withdrawal rates imply that multiple cycles can be performed during a year, so that cost per amount of gas stored and withdrawn over a period is not necessarily higher than for the other approaches. The multiple cycle capability is particularly important for hydrogen applications due to the unpredictable nature of renewable energy resources. A key drawback of salt caverns is that salt formations are not available everywhere on the planet, so that it is not a universal solution.

For safety reasons, underground storage should only be implemented in regions with weak to no seismic activity.

Why preferring salt caverns?

Salt caverns are a particularly advantageous solution for hydrogen storage for multiple reasons:

• Very low permeability to hydrogen
• No water contamination
• No bacterial contamination
• Sculpted to desired shape by leaching

Hydrogen being a smaller molecule than most natural gas components, it is more prone to leak from its storage. A very low permeability of the storage walls is therefore required, which a salt formation provides without relying on the nature of the capping formation (unlike the aquifers).

Hydrogen easily bonds with water, requiring a separator to treat withdrawn gas if any contamination occurs during storage. A mechanically-created salt cavern is naturally water-free as opposed to an aquifer. But a salt cavern created by leaching (water injection) will have some residual water within the walls, at least for the first operating cycle, so that water/hydrogen separation will be required.

Depleted oil/gas fields and aquifer contain bacteria that can convert hydrogen in other gases, consuming some hydrogen and requiring a gas treatment after withdrawal to ensure gas purity. Salt does not allow bacterial life, so that bacterial conversion of hydrogen is not an issue.

The ability to sculpt the reservoir during leaching implies that shape can be optimised for injection and withdrawal (shaped to minimise stress during pressure variations). Salt cavern therefore have a large withdrawal or injection rate compared to their working gas capacity and have relatively low cushion gas requirements. These high rates make salt cavern suitable for high dynamics operation (frequent withdrawal/injection), a key point for hydrogen production from excess renewable electricity, but also for compressed air energy storage systems.

Sculpting a salt cavern can be done from surface with a vertical drilling to inject water and extract the resulting brine. Injecting a liquid lighter than water and no reactive with salt allows directing cavern expansion towards the bottom and controlling its shape. It is therefore possible to create vertical capsule-shaped caverns which are the shapes with less mechanical stress and therefore the largest amplitude between minimum and maximum gas pressure.

Salt caverns are relatively small with up to a few millions m3, while porous formations can have up to a few hundred millions m3. Furthermore, porous formations are more widespread across the globe. Yet, salt cavern compensates small volume by less requirement for cushion gas: a depleted oil or gas field requires about 50% of cushion gas, while salt cavern can operate with only 30% of cushion gas. This is related to the overburden (or lithostatic) pressure created by capping rock (density of rocks times depth). For safety reasons, in-cavern pressure must be kept between 24 and 80% of overburden pressure. A deep salt formation capped with heavy rocks therefore provides the largest pressure difference between minimum and maximum values, maximising the amount of working gas (but also requiring more cushion gas).

To ensure the mechanical stability of the salt cavern over time, some minimum thickness for walls is required: the foot wall thickness can be only 20% of cavern diameter, while hanging wall thickness needs to be at least 75% of cavern diameter. Furthermore, factor of 2 between height and diameter is considered optimal in a bedded salt deposit.

Potential in Europe

The economics of underground storage is largely dependent on the ratio between working (useful) and cushion (“lost”) gas. Because hydrogen is some much lighter than natural gas, this ratio is even more critical and hydrogen storage is necessarily more expensive than natural gas storage (more gas mass required as cushion gas). To partly compensate this drawback, hydrogen is generally stored at higher pressure than natural gas, up to 200 bar.

Because of the potential effect of salt cavern on ground stability, identifying relevant caverns exclude proximity to urban areas as well as major transportation infrastructures like motorways and railways, which in densely-populated areas like Europe, excludes a significant portion of the land.

In Europe, the main salt deposition occurred in Northern Europe and spread from eastern United Kingdom, under the North Sea towards the Netherlands, Denmark across Northern Germany and down to the central basins of Poland. Accordingly, Germany is the country with most onshore underground storage capacity and there is also a large offshore potential.

The analysis of European salt formations after exclusion of near-urban and near-infrastructure locations shows a total potential of 84.8 PWhH₂, but with only about 27% that is onshore. If a further restriction to 50 km from the shore is added (proximity of most wind farms), the potential reduces to 7.3 PWhH₂. Yet, as a comparison, the European potential for pumped hydropower is only 0.054 to 0.123 PWhH₂.

On a per-country basis, Germany has the largest potential with slightly over 40% of all potential, either counting all resources or only the onshore resources. The Netherlands and the United Kingdom follow but with only a bit more than 10% of the potential. Norway has the specificity of having no onshore potential but a large offshore potential. France has widespread bedded-salt deposits but they are mainly located near densely-populated areas, so that its exploitable potential accounts for only about 2% of European potential.

The actual benefit of storage in salt caverns is a very complex subject, depending both on potential excess renewable energy as well as potential hydrogen use. Excess electricity from wind farms (the most abundant variable energy within areas with large storage potential) is highly variable because of dependency on wind but also on energy consumption and electricity market conditions. Best estimations define a bracket of 1 to 7% of total electricity production as excess production.

The hydrogen use side is today mainly driven by economics and varies between countries because:

• Cost of hydrogen production and storage depends on cost of electricity (about 50% of cost is electricity for electrolysis only)
• Green hydrogen will compete with existing hydrogen-production methods in the industrial sector (mainly natural gas reforming, so that local natural gas cost has an influence)
• Green hydrogen will compete with crude-based fuels in the transportation sector, so that local fuel prices have an influence

For argument’s sake, analyses within the European project HyUnder conclude that stored green hydrogen will mainly be targeting the transportation sector in Spain, while mainly targeting the balancing of electricity production between renewable-rich and renewable-scarce periods in Romania. These differences arise from distinct renewable energy resources, storage capabilities, electricity prices and market structure as well as crude-based product prices.