The United States is home to three types of natural gas storage facilities: depleted fields, aquifers and salt caverns. Depleted fields still hold more natural gas than any facility type, but since 2007, salt caverns have become the most common new sites for storage. Salt caverns offer several advantages. They are highly flexible in that they operate under extreme pressure making injection or delivery of natural gas a much quicker process. Salt caverns can also cycle gas more frequently; in other scenarios, there are just two seasons for injection (summer) and withdrawal (winter). Finally, salt caverns require a lower pressure for delivery of gas to customers. This helps to offset their slightly higher capital costs.
The first salt cavern created for storage was in Melville, Saskatchewan, Canada. Built in 1961, the 300,000 barrel cavern is 3,700 feet deep, and eventually led to the technology being used in the United States. In the 1970s, salt caverns were built in Louisiana, Mississippi and Alabama. These salt domes in the southern states were naturally constructed by the deposition of salt in an ancient marine basin. The salt caverns are also used for the disposal of non-hazardous oilfield wastes (NOW), and fall under the oversight of the Safe Drinking Water Act’s Underground Injection Control (UIC) program. There are five levels of hazard classification for injection wells, with the most serious being Class I – i.e. the injection of hazardous and radioactive wastes under a drinking water well. Class II wells, including waste from injection for bringing natural gas to the surface, have been an issue lately as well as they are linked to the debate about ‘fracking’ and its environmental costs.
As stated earlier, salt caverns are created in existing salt domes, many of which were formed some 30 million years ago. When they are created by humans, their locations have been given much thought. Depth to the salt layers, proximity to population centers and future land-use in the area are all considered when designating a salt cavern storage site. In Texas alone, there are 11 caverns at 7 different sites throughout the state. All were selected to minimize the potential for groundwater contamination and/or leakage.
Once a salt cavern is build or a naturally occurring dome is selected, they are filled with brine and then wastes are injected as a slurry with water and brine. The caverns act as separators, sending hydrocarbons to the top and wastes settle on the bottom. Once a cavern is filled, it is closed in a process where plugs surround water-bearing areas. Over time, pressure builds as gas is formed through bacterial degradation and corrosion. Even though salt is highly impermeable, some of the gas can seep into the formation over time due to the underground pressurization.
While there are few situations where leakage might occur, here are several likely scenarios: inadvertent access; failure of a cavern seal; leaky soluble areas; or cavern roof collapse. Naturally occurring radioactive materials (NORM) can be highly hazardous; however the potential of salt caverns for long-term storage of both fuel sources and waste may be our best, and safest, bet for future energy needs.