The PDMS and the two IGS publications on underground storage (see references below) use the term "Working Gas" to mean "Working Gas Capacity" (U.S. Energy Information Administration, 2004).
Stated differently, as used by the IGS, Total Base Gas plus Working Gas equals Total Capacity.
Technically defined (U.S. Energy Information Administration, 2004), the volume of "Working Gas" should change as gas is injected and withdrawn from the reservoir. It is only when the entire Working Gas Capacity is filled to its maximum that Working Gas and Working Gas Capacity are equal.
The value is reported in MCF (thousand cubic feet of gas).
Follow this link to a more thorough discussion of < Terminology used to describe gas storage volumes >
Dawson, T. A. and G. S. Carpenter,1963, Underground storage of natural gas in Indiana: Indiana Geological Survey, Special Report 01, 29 p.
U.S. Energy Information Administration, 2004, The basics of underground natural gas storage: U.S. Energy Information Administration Web site, http://www.eia.gov/pub/oil_gas/natural_gas/analysis_publications/storagebasics/storagebasics.html., date accessed, July 21, 2011.
Keller, S. J., 1998, Underground storage of natural gas in Indiana: Indiana Geological Survey, Special Report 59, 77 p.