Gas Reservoirs as Storage Reservoirs

The demand for natural gas is seasonal. During winter months, there is a much greater demand for natural gas than during the warmer summer months. To meet this variable demand, several means of storing natural gas are used in the industry. One of the best methods of storing natural gas is with the use of depleted gas reservoirs. Gas is injected during the warm summer months when there is an overabundance and produced during the winter months when there is a shortage of supply. Katz and Tek have presented a good overview of this subject.¹¹

Katz and Tek listed three primary objectives in the design and operation of a gas storage reservoir: (1) verification of inventory, (2) retention against migration, and (3) assurance of deliverability. Verification of inventory simply means knowing the storage capacity of the reservoir as a function of pressure. This suggests that a p/z plot or some other measure of material balance be known for the reservoir of interest. Retention against migration refers to a monitoring system capable of ascertaining if the injected gas remains in the storage reservoir. Obviously, leaks in casing and so on would be detrimental to the storage process. The operator needs to be assured that the reservoir can be produced during peak demand times in order to provide the proper deliverability. A major concern with the deliverability is that water encroachment not interfere with the gas production. With these design considerations in mind, it is apparent that a good candidate for a storage reservoir would be a depleted volumetric gas reservoir. With a depleted volumetric reservoir, the p/z versus G_p curve is usually known and water influx is not a problem.