Gas storage has been present in nature for millions of years. Strategic storages mimic that process by taking advantage of a natural formation, old depleted reservoirs, or saline aquifers as subway storage for natural gas.
The processing and subway storage of hydrocarbons in salt caverns is a practice that began in 1915. Salt caverns for oil storage are critical infrastructures in energy systems and are currently used by developed countries mainly because they offer safety in their operation and flexibility regarding the type of hydrocarbons, even gaseous, as in this case, that can be stored.
Salt caverns are artificially created cavities with built-in salt deposits. They are suitable for the storage of liquid hydrocarbons and, in particular, high-pressure gases. Large quantities of gas can be safely stored due to the sizeable geometric volume and high storage pressures. In addition, the unique properties of rock salt guarantee long-term stability. Compared to other geological storages, the specific construction costs are meager as the operation and creation of the cavern are done from the surface through a single borehole, which is equipped with special pipes and equipment. Hydrogen has been successfully stored in salt caverns in the U.K. and the USA.
Salt has several properties that make it ideal for gas storage. Rock salt is technically gas-tight when affected by compressive stress. Its viscous-plastic property causes it to redistribute the accumulated stress in response to cavern construction and operation, making it possible to construct and operate caverns with diameters of up to 100m. Another important property is the high solubility of salt in water, which allows them to be built by leaching.
Rock salt deposits are divided into two types:
- Salt domes: thick formations created from natural salt deposits. The cavities in these structures are usually large in diameter and depth.
- Salt layers: Stratified formations often underlie large basins, with relatively thin salt layers (<300m thick).
Two fundamental processes take place in subway storage: injection and extraction of natural gas.
During the injection process, natural gas coming from the primary pipeline network is compressed to be injected into deep formations. The injected gas, which occupies a much smaller volume than at the surface, displaces the water that fills the pores of the reservoir rock, which is sealed by a top layer of impermeable rock.
In the extraction phase, the natural gas is extracted from storage and treated to meet the required specifications for transport through the storage rock.
Specifications required for transport through the primary gas pipeline network (water dew point, hydrocarbon dew point, gross calorific value, etc.). Finally, it is odorized before being sent to the network.
Compared to above-ground storage, they are protected by a rock cover hundreds of meters thick and therefore have a safety advantage. In addition, subway storages guarantee large storage volumes, which leads to higher energy density.
Subway storage facilities are critical infrastructures in the natural gas value chain, as they make it possible to adjust supply to demand and meet consumption peaks. Subway storage facilities also play an essential role as a tool available to marketers to balance their balance in the transmission network.