Page 64 - Handbook of Natural Gas Transmission and Processing Principles and Practices

1.13. Underground Gas Storage
Strategically, underground gas storage provides security of supply in case there are disruptions to
production and transmission. This could be due to commercial reasons, such as sales gas price
negotiation, political reasons, or an outage. It is a means to balance seasonal variations in
consumption; the winter demand is typically greater than the summer usage. It can also enhance
the effectiveness of gas transport and production, where it can be stored locally to where it is being
used. Commercially, natural gas is a commodity for trading.
There are three principal types of underground storage methods: depleted oil or gas reservoirs,
aquifers, and salt cavern formations.

1.13.1. Depleted Reservoirs

The most common form of underground storage is depleted gas reservoirs. They are formations
that have already been exhausted of most of their recoverable oil and natural gas. The empty
reservoirs can be used to hold natural gas. Typically, an extensive pipeline network is located close
to the reservoir sites for the convenience of the injection and withdrawal operation. Of the three
types of underground storage, depleted reservoirs are the cheapest and the quickest to develop,
operate, and maintain.
Two of the most important characteristics of an underground storage reservoir are the capability
to hold natural gas and the rates at which natural gas inventory can be injected and withdrawn.
Note that most of these depleted oil fields had been injected with nitrogen for enhanced oil
recovery, and consequently, the nitrogen content in these fields would vary during withdrawal. In
some reservoirs the nitrogen content in the reservoir during the withdrawal mode can vary from 3
mol% to 30%. In these facilities, a nitrogen rejection unit must be installed to remove the nitrogen
content to meet pipeline specification.

1.13.2. Aquifers
In the United States, 16% of storage capacity is in aquifers. Aquifers are underground porous,
permeable rock formations that act as natural water reservoirs. An aquifer is suitable for natural gas
storage if the water-bearing sedimentary rock formation is overlaid with an impermeable cap rock.
While the geology of aquifers is similar to the depleted production fields, they are more expensive
to develop for natural gas storage, and consequently, there are limited numbers of aquifer natural
gas storage, found only in areas where there are no other alternatives.

1.13.3. Salt Caverns
Salt caverns are formed out of existing salt bed deposits. Most of the large salt caverns are located in
the salt domes along the Gulf Coast in the US. Salt caverns in Northeastern, Midwestern, and
Western States are also available but the applications are limited by the lack of suitable geology.
The cavern is man-made by drilling a well down into the formation, and pumping water through
the completed well to dissolve the salt which returns to the surface as brine. The walls of the cavern
are very resilient against reservoir degradation.
As the salt cavern is an open vessel, it offers very high deliverability. Flow rates can be high and
they can be brought on stream and ramped to full flow quickly. They are best for peak loads and
short term trading rather than long term seasonal storage. Peak load can be provided by salt
caverns, where the deliverability is higher, turnovers will be higher and facilities are smaller. Salt
caverns turnover can be daily or weekly, entirely dictated by commercial trading.

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