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Underground Gas Storage

Natural gas is usually stored underground, in large storage reservoirs.

Underground Gas Storage

Underground gas storage (UGS) - is a complex of engineering and technical structures in reservoirs of geological structures, mine workings, as well as in workings-tanks created in rock salt deposits.
There are 3 principal types of underground storage methods: depleted oil or gas reservoirs, aquifers, and salt cavern formations.

Natural UGS facilities are porous sandstone strata in the earth's crust, hermetically sealed from above by a clay dome.
UGS facilities are designed for injection, storage and subsequent gas withdrawal.

The UGS facility includes a subsoil area bounded by a mining allotment, a fund of wells for various purposes, gas collection and treatment systems, and compressor shops.

UGS facilities are being constructed near the route of main gas pipelines and large gas-consuming centers to be able to quickly cover peak gas consumption.
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 3 types of underground storage, depleted reservoirs are the cheapest and the quickest to develop, operate, and maintain.

2 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reservoir are the capability to hold natural gas and the rates at which natural gas inventory can be injected and withdrawn.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.

Aquifers are underground porous, permeable rock formations that act as natural water reservoirs.
However, in certain situations, these water containing formations may be reconditioned and used as natural gas storage facilities

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.

In some instances, aquifer development can take 4 years, which is more than twice the time it takes to develop depleted reservoirs as storage facilities.
Salt Caverns
Underground salt formations are well suited to natural gas storage in that salt caverns, once formed, allow little injected natural gas to escape from the formation unless specifically extracted.
The walls of a salt cavern also have the structural strength of steel, which makes it very resilient against reservoir degradation over the life of the storage facility.

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.