Underground natural gas storage fields grew in popularity shortly after World War II. At the time, the natural gas industry noted that seasonal demand increases could not feasibly be met by pipeline delivery alone. To meet seasonal demand increases, the delivery capacity of pipelines (and thus their size), would have to increase dramatically. However, the technology required to construct such large pipelines to consuming regions was, at the time, unattainable and unfeasible. To be able to meet seasonal demand increases, underground storage fields were the only option.
Natural gas is usually stored underground, in large storage reservoirs. There are three main types of underground storage:
Any underground storage facility is reconditioned to create a sort of storage vessel underground before natural gas is injected into it. Natural gas is injected into the formation and, as the pressure builds as more natural gas is added, the underground formation becomes a sort of pressurized natural gas container. The higher the pressure in the storage facility, the more readily gas may be extracted. Conversely, once enough gas has been extracted the pressure drops enough that it is insufficient to push the natural gas out of the storage facility. This means that, in any underground natural gas storage facility, there will be a certain amount of gas that may never be extracted. This is known as physically unrecoverable gas; it is permanently embedded in the formation.
In addition to this physically unrecoverable gas, underground storage facilities contain what is known as 'base gas' or 'cushion gas'. This is the volume of gas that must remain in the storage facility to provide the required pressurization to extract the remaining gas. In the normal operation of the storage facility, this cushion gas remains underground; however a portion of it may be extracted using specialized compression equipment at the wellhead.
'Working gas' is the volume of natural gas in the storage reservoir that can be extracted during the normal operation of the storage facility. This is the natural gas that is being stored and withdrawn; the capacity of storage facilities normally refers to their working gas capacity. At the beginning of a withdrawal cycle, the pressure inside the storage facility is at its highest; meaning working gas can be withdrawn at a high rate. As the volume of gas inside the storage facility drops, pressure in the storage facility decreases and, consequently, so does the rate at which it can be withdrawn.