Natural Gas Storage - Measures and Definitions

Measures and Definitions

Characteristics of underground storage facilities need to be defined and measured. A number of volumetric measures have been put in place for that purpose:

• Total gas storage capacity: It is the maximum volume of natural gas that can be stored at the storage facility. It is determined by several physical factors such as the reservoir volume, and also on the operating procedures and engineering methods used.
• Total gas in storage: It is the total volume of gas in storage at the facility at a particular time.
• Base gas (also referred to as cushion gas): It is the volume of gas that is intended as permanent inventory in a storage reservoir to maintain adequate pressure and deliverability rates throughout the withdrawal season.
• Working gas capacity: It is the total gas storage capacity minus the base gas.
• Working gas: It is the total gas in storage minus the base gas. Working gas is the volume of gas available to the market place at a particular time.
• Physically unrecoverable gas: The amount of gas that becomes permanently embedded in the formation of the storage facility and that can never be extracted.
• Cycling rate: It is the average number of times a reservoir’s working gas volume can be turned over during a specific period of time. Typically the period of time used is one year.
• Deliverability: It is a measure of the amount of gas that can be delivered (withdrawn) from a storage facility on a daily basis. It is also referred to as the deliverability rate, withdrawal rate, or withdrawal capacity and is usually expressed in terms of millions of cubic feet of gas per day (MMcf/day) that can be delivered.
• Injection capacity (or rate): It is the amount of gas that can be injected into a storage facility on a daily basis. It can be thought of as the complement of the deliverability. Injection rate is also typically measured in millions of cubic feet of gas that can be delivered per day (MMcf/day).

The measurements above are not fixed for a given storage facility. For example, deliverability depends on several factors including the amount of gas in the reservoir and the pressure etc. Generally, a storage facility’s deliverability rate varies directly with the total amount of gas in the reservoir. It is at its highest when the reservoir is full and declines as gas is withdrawn. The injection capacity of a storage facility is also variable and depends on factors similar to those that affect deliverability. The injection rate varies inversely with the total amount of gas in storage. It is at its highest when the reservoir is nearly empty and declines as more gas is injected. The storage facility operator may also change operational parameters. This would allow, for example, the storage capacity maximum to be increased, the withdrawal of base gas during very high demand or reclassifying base gas to working gas if technological advances or engineering procedures allow.