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.

Ikoku defines three types of gas involved in a gas storage reservoir.¹² The first is the base gas, or cushion gas, that remains when the base pressure is reached. The base pressure is the pressure at which production is stopped and injection begins. The second type of gas is the working gas, or working storage, that is produced and injected during the cycle process. The third type is the unused gas that essentially is the unused capacity of the reservoir. Figure 4.5 defines these three types of gas on a p/z plot.

Figure 4.5 p/z plot showing different types of gas in a gas storage reservoir.

The base pressure, and therefore the amount of base gas, is defined by deliverability needs. Sufficient pressure must be maintained in the reservoir for reservoir gas to be delivered to transporting pipelines. Economics dictates the pressure at which injection of gas during the summer months ends. Compression costs must be balanced with the projected supply and demand of the winter months. In theory, for a volumetric reservoir, the cycles of injection and production simply run up and down the p/z versus G_p curve between the pressure limits just discussed.

In certain applications, the use of the delta pressure concept may be advantageous.¹¹ The delta pressure is defined as the pressure at maximum storage minus the initial reservoir pressure. Under the right conditions, an amount of gas larger than the initial gas in place can be achieved. This again is dictated by the economics of the given situation.

Hollis presented an interesting case history of the considerations involved in changing the Rough Gas Field in the North Sea over to a storage reservoir.¹³ Considerations in the design of storage and deliverability rates included the probability of a severe winter occurring in the demand area. A severe winter was given a probability of 1 in 50. Hollis concluded that the differences between offshore and onshore storage facilities are due mainly to economic factors and the integrated planning that must take place in offshore development.

Storage is a useful application of gas reservoirs. We encourage the reader to pursue the references for more detailed information, if it becomes necessary.