Subsurface solutions: geological reservoirs for hydrogen storage

IDRIC researchers Dr Omid Shahrokhi and Prof John Andresen share an overview of the potential solutions and challenges of large-scale underground hydrogen storage. Their projects funded by IDRIC brings together Heriot-Watt University, the University of Manchester, and the British Geological Survey (BGS) to address some of the questions involved in the need for large-scale energy storage to reduce emissions from national electricity and gas grids.

Types of Geological Reservoirs as Storage Candidates

Several types of subsurface structures show promise for hydrogen storage:

  • Salt Caverns – Salt has desirable properties: it’s impermeable to gas and can self-heal minor fractures. Salt caverns are created by dissolving salt deposits with water, and they’re already used for natural gas storage. These salt deposits are not widely distributed. Typical hydrogen storage capacities of salt caverns are tens of Gigawatt hours.
  • Depleted Gas Fields – Once hydrocarbon gases are extracted, these reservoirs offer pre-existing infrastructure and understood geology, potentially speeding the adaptation for hydrogen storage. The geological formations that have stored hydrocarbons for millions of years, can provide safe options for hydrogen storage too. Typical storage capacities of these fields are thousands of Gigawatt hours (TWh).
  • Aquifers – Deep saline aquifers are water-filled underground porous rock formations. These have huge potential storage capacity as they are widely distributed in the world but require careful assessment to avoid compromising groundwater.

Key Considerations

Geological hydrogen storage is not without challenges and concerns:

  • Purity and Cycling – Repeated injection and withdrawal of hydrogen risks introducing impurities to an expensive commodity (hydrogen streams produced from electrolysers or steam reformation). Most geological reservoirs also hold microbes that can consume hydrogen and change them to other undesirable chemicals. Monitoring and management measures are needed to maintain hydrogen quality.
  • Geological Suitability – Not all underground formations are equal. Careful analysis of a site’s structure, porosity (empty spaces within rocks), permeability (the ability of the rocks to allow fluid flow within it), and caprock (impermeable rock layer preventing gas escape) is vital. For example, in certain formations, repeated gas injection and withdrawal can cause seismic activity.
  • Storage Efficiency – Some hydrogen loss in the subsurface is likely. This can be due to trapping, microbial activity, or reaction with certain minerals. Minimising hydrogen loss while optimising the amount of hydrogen retrieved per cycle is an ongoing research focus. Unlike carbon dioxide where permanent trapping is the main objective, temporary storage with minimal trapping is the main objective here.

Benefits of geological hydrogen storage

Geological hydrogen storage allows us to tap into vast, cheap, and safe storage capacities. It could play a pivotal role in:

  • Safety – Underground storage of very large volumes of hydrogen as a flammable gas over the surface is not safe. Underground storage formations are mainly void of oxygen in substantial amounts. This prevents any fire or explosion within the reservoirs. At the wellbore several shutting valves control (and can fully stop) the flow of hydrogen in and out of the reservoir in any hazardous situation.
  • Cost of storage – Compared to storage in high-pressure and cryogenic vessels, the cost of hydrogen storage per kilogram H­2 is lower in underground storage sites. See this report.
  • Seasonal Balancing – Storing excess hydrogen generated from renewable energy (solar, wind) during periods of abundance to use when these sources are less powerful.
  • Energy Security – Creating strategic hydrogen reserves that buffer against disruptions and enhance energy resilience.
  • Grid Balancing – Integrating geological storage into electricity grids could smooth out supply-demand mismatches and help maintain stability.

The Road Ahead

The science of geological hydrogen storage is advancing quickly. Research focuses on better understanding reservoir behaviour, knowledge of hydrogen interactions with underground minerals and microbes, and improving storage efficiency. While few pilot projects are already demonstrating the technical feasibility (Underground Sun Storage in Austria, HyChico in Argentina), widespread commercialisation will require addressing safety, regulation, and public acceptance.

The Role of Research

Researchers across three UK institutions are collaborating to provide a more holistic study of subsurface hydrogen storage.

Researchers at Heriot-Watt University focus on how multiphase flow phenomena affect hydrogen injectivity (the rate at which hydrogen can be stored through a wellbore into a reservoir) and hydrogen lost to trapping in the subsurface at actual reservoir conditions (elevated temperature and pressure). Both parameters directly affect the economic feasibility of underground hydrogen storage in porous media.

Researchers at BGS study geochemical and bacterial interactions of hydrogen in the subsurface environment and researchers at the University of Manchester focus on fundamental pore-scale mechanisms governing hydrogen storage and modelling scaleup from pore to reservoir scale.

Storing vast amounts of hydrogen is an essential element to realising decarbonised industrial clusters as well as hard-to-abate carbon emissions like transportation, and domestic heating. UK has the potential to supply over 8% of total EU hydrogen need (See “Commercial Models for Future Hydrogen Production” from Crown Estate Scotland) and replace a large portion of 17.8% of EU natural gas imported from Great Britain (See this Eurostat article). Geological reservoirs present an immense opportunity to do just that – storing clean energy below the ground to create a greener energy future above.