L51381 Non-Recoverable Gas in Underground Storage

It is well known that when gas accumulates in a partially water saturated porous reservoirs, either by natural entrapment or by artificial injection, a significant fraction of that gas cannot subsequently be recovered. The actual quantity of non-recoverable gas depends on many factors, some relating to the reservoir itself and others relating to operational conditions or necessities. The gas storage industry has long recognized the distinction between base gas and working gas, the former being the volume of gas intentionally left in the reservoir to provide energy and space to accommodate the latter which is the gas normally available for distribution. The distinction between base gas and working gas is always somewhat arbitrary. By definition, base gas is non-recoverable gas under "normal" operating conditions, but in practice base gas is occasionally withdrawn during periods of peak demand. Clearly, base gas is non-recoverable only within certain, sometimes arbitrary, operational limits. This research program has included an exhaustive investigation of the theoretical considerations relating to the entrapment of non-recoverable gas, the practice of storage operators and various laboratories in determining non-recoverable gas saturation and in making various measurements upon which these determinations may be based, and extensive experimentation on both core samples obtained from a variety of storage reservoirs and upon sand pack models simulating some of the features observed in the core studies.