I think you need to read this:-

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 If the Conditions allow it, a percentage or all of the working gas capacity could be injected and withdrawn more than once during any season.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.