While natural gas may be stored under pressure in a number of ways, four types of facilities are most common: underground storage in (1) depleted reservoirs in oil and/or gas fields - - the majority of gas storage, (2) aquifers, and (3) salt cavern formations; and above ground storage in (4) Liquefied Natural Gas (LNG) facilities. Each of these types of gas storage is extremely safe.
Each underground storage type has its own physical characteristics (porosity, permeability, retention capability) and economics (site preparation and maintenance costs, deliverability rates, and cycling capability) which govern its suitability for particular applications. Two of the most important characteristics of an underground storage reservoir are its capacity to hold natural gas for future use and the rate at which gas inventory can be withdrawn - its deliverability rate.
Depleted Natural Gas or Oil Fields
Most existing gas storage in the United States is in depleted natural gas or oil fields. Conversion of a field from production to storage takes advantage of existing wells, gathering systems, and pipeline connections. Depleted oil and gas reservoirs are the most commonly used underground storage sites because of their wide availability. Depleted production fields located close to natural gas consuming centers are the most valuable, because this proximity reduces the need for long haul pipeline transportation.
In some areas, most notably the Midwest, natural aquifers have been converted to gas storage reservoirs. An aquifer is suitable for gas storage if the water bearing sedimentary rock formation is overlaid with an impermeable cap rock. While the geology is similar to a depleted production field, the use of an aquifer for gas storage usually requires more base (cushion) gas and greater monitoring of withdrawal and injection performance. Deliverability rates may be enhanced by the presence of an active water drive.
The large majority of salt cavern storage facilities have been developed in salt dome formations in the Gulf Coast states. Salt caverns provide very high withdrawal and injection rates relative to their working gas capacity. Base gas requirements are relatively low. Cavern construction is more costly than depleted field conversions when measured on the basis of dollars per thousand cubic feet of working gas capacity. Still, compared to a depleted field or an aquifer, the ability of a salt cavern storage facility to perform several withdrawal and injection cycles each year reduces the per-unit cost of each thousand cubic feet of gas injected and withdrawn.
There have been efforts to use abandoned mines to store natural gas, with at least one such facility having been used in the United States. Further, the potential for hard-rock cavern natural gas storage is being tested, although no such facilities are commercially operational at present.