Underground gas storage (UGS) facilities enable to smooth out seasonal fluctuations in gas demand, reduce peak loads and provide for flexibility and reliability of gas supply. Underground storage is the safe, economical and environmentally friendly way to store large volumes of hydrocarbons, energy sources, chemical products etc. - in liquid, liquefied or gaseous form - in porous or fractured rock strata or in man-made cavities, hosted in geological formations, deep below ground level. For natural gas, underground storage facilities serve to balance supply and demand between summertime and wintertime. When the demand for gas is extremely high, underground storage facilities provide a reserve of gas that is immediately and economically available. This service is known as peak shaving. If a pipeline system fails, or the supply runs seriously short of the requirement, underground storage provides a reserve for managing just such emergencies. Underground storage is also increasingly used for spot-trading. Typical products that can be stored are natural gas, crude oil, air, fuels, propane, butane, as well as chemical and petrochemical products. Huge volumes can be stored. In the case of natural gas storage volumes are often from several hundred million m³ to several billion m³.
The withdrawal rate of a storage site is normally in excess of a hundred thousand m³ per hour and can be as much as several million m³ per hour. Gaseous products are stored underground at high pressures which can range from a few dozen bar to more than 200 bar.The capability to store products underground depends on the existence of suitable geological conditions. Underground storages can be created from:
• aquifer structures under gas-tight layers of cap rock;
• solution-mined caverns in rock salt deposits;
• traditionally mined hard rock cavities;
• disused mines.