Storage fields are divided into three categories: (1) depleted oil and/or gas fields, (2) aquifer storage fields, and 3) salt cavern storage.
Depleted Oil and/or Gas Fields: These reservoirs are naturally occurring, and their potential as secure containers has been proven over the millions of years that the reservoirs held its original deposits of oil and gas. They consist of porous and permeable underground formations (average of 1,000 to 5,000 feet deep) which are confined by impermeable rock or water barriers and identified by a single natural pressure. The working gas requirement is generally about 50% of the total reservoir capacity. Gas is withdrawn in winter season and injected in summer. This type of storage facility can be used for long system supply or peak day.
Aquifer Storage Fields: This type reservoir is bounded partly or completely by water bearing rocks called "Aquifers." The nature of the water in the aquifer may vary from fresh water to nearly saturated brines. They have a high cushion gas requirement, between 50 to 80% generally. They also have high deliverability rates and gas is withdrawn in winter season and injected in summer season.
Salt Cavern Storage: This type of facility uses caverns solution mined in underground salt deposits (Salt domes or Salt formations). Salt caverns usually operate with about 20 to 30 % cushion gas and the remaining capacity as working gas. Working gas can be recycled more than once per year. Salt cavern storage has high deliverability and injection capabilities and is used for short peak day deliverability purpose.
Depleted oil and gas reservoirs account for 87% of the total jurisdictional storage capacity, with salt caverns (3%) and aquifers (10%) accounting for the rest. Depleted oil and gas reservoirs are most common because of their availability and the advantage of using existing infrastructure. Salt caverns are more expensive to construct due to the increase in capital cost associated with the leaching and mining of the salt.
For more detailed information about natural gas storage, visit the following websites:
- EIA: Basics of Underground Natural Gas Storage
- NaturalGas.org: Storage of Natural Gas
- The Energy Information Administration (EIA) - lists weekly reports on the state of natural gas storage.