Inventory Verification of Gas Storage Fields
Authors: Mayfield J.F. 
Published: December 2007

Summary
In gas storage field operations, some fields experience an ongoing unaccounted-for loss of significant volumes of gas. The procedure used by some operators to determine such losses and thereby verify gas inventory is discussed here. Two gas storage fields in which the recovery mechanism is gas expansion are described; in one, no gas loss has been experienced, and in the other there is an ongoing unaccounted-for gas loss. Also described is an aquifer gas-storage field with an active water drive that experiences an ongoing unaccounted-for gas loss.

Introduction
During the primary production period of a gas field, espedally older gas fields, the operator usually is not aware of any gas loss from the reservoir, such as loss to shallower or deeper formations by means of wellbore communication, gas vented from surface production equipment, or gas lost by leaks in such equipment. Even if the operator is aware, a reasonably accurate accounting is seldom attainable or required. However, in gas storage operations, losses can be experienced from the usual causes encountered during primary production as well as from other causes. Because of the necessity of maintaining storage-field performance and because of cost considerations, the operator of a gas storage field must give close attention to verification of gas inventories. Consequently, the operators of storage fields, including those fields that originally contained only water (aquifer gas storage fields), routinely gather reservoir performance data to verify that gas injected into the field is indeed within the reservoir and, in the event gas is being lost, to determine the magnitude of such loss.
Most gas storage fields were originally gas fields that were converted to gas storage after depletion of the native gas reserves. Many of these fields were bounded downstructure by water; the production of native gas allowed water to expand into the gas reservoir, reducing the pressure in the aquifer in the vicinity of the field. To inject into the gas reservoir a volume of storage gas equal to that produced from the field during primary production, reservoir space voided by primary production and now occupied by encroached water must be regained during gas injection by the movement of water back into the aquifer. Bypassing of encroached water and movement of gas beyond the original gas/water contact has been observed during gas injection in numerous gas storage fields. This is because the aquifer pressure has been reduced by primary production and gas will flow through the paths of least resistance. The result is gas loss that is now unrecoverably tied up in the aquifer as residual gas saturation. Further, to return the average reservoir pressure to its original value obviously requires exceeding the original pressure by some increment at the injection wells. Although usually quite small, this increase over original pressure may result in gas flow and gas losses through the caprock overlying the gas storage formation.
If gas is being lost, the storage-field deliverability (the rate at which gas can be withdrawn from a storage field at given book inventory) declines from year to year because the actual or real gas inventory is decreasing each year. The deliverability of the field must be maintained by the operator for meeting its peak day delivery requirements during the winter heating season. Also, the seasonal volume of gas available for withdrawal will be reduced each year by unrecognized losses and such volume, therefore, will be unavailable for withdrawal.