The geological subsurface, particularly porous formations, can offer grid-scale energy storage options , , either by storing a chemical energy carrier, such as hydrogen or methane , , or by storing mechanical energy as compressed air , , , or as sensible heat (e.g., , ).
Geological storage of gaseous methane, which is the major constituent of natural gas, has been well investigated and implemented for decades to stabilise seasonal mismatches between production and demand. Storing mechanical energy in the subsurface using pressurised air for strongly fluctuating conditions represents a novel application.
One way to ensure large-scale energy storage is to use the storage capacity in underground reservoirs, since geological formations have the potential to store large volumes of fluids with minimal impact to environment and society.
Potential storage reservoir sites in the geological underground mainly comprise salt caverns, saline aquifers, depleted hydrocarbon reservoirs and rock caverns. Adapted from . Essentially, a geological reservoir is prepared prior to injection, to effectively create an underground, pressurised storage container.
4.1.6. Geotechnical criteria Geotechnical criteria are related to the construction phase of underground energy storage and include thermal and mechanical rock properties, usually requiring in situ tests to assess the cavern stability.
This supports a stable air–water contact level in the geological storage formation, minimising the energy required for moving formation water during the cyclic operation. This allows for high injection and withdrawal rates and thus a higher overall efficiency.