Gas in an underground storage facility is divided into two categories, working gas (top gas) and cushion gas (base gas).
Working gas (top gas) - the volume of gas in the reservoir above the designed level of cushion gas. If the Conditions allow it, a percentage or all of the working gas capacity could be injected and withdrawn more than once during any season.
Cushion gas (base gas) - the volume of gas, including native gas, needed as a permanent inventory in a storage reservoir to maintain adequate reservoir pressure and deliverability rates throughout the withdrawal season.
For more detailed information about natural gas storage, visit the following websites:
- EIA: Basics of Underground Natural Gas Storage
- The Energy Information Administration (EIA) - lists weekly reports on the state of natural gas storage