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Underground Storage


When Nicor first started storing gas, it was in low-pressure, above-ground tanks and interconnected high-pressure cylinders buried under the surface of the soil. Then, in 1954, a pipeline supplier developed the state's first underground facility. Called an aquifer, it was located near Herscher, southwest of Kankakee.

An underground aquifer reservoir is a natural underground formation made of water-filled, porous sandstone layers covered by a solid dome-shaped caprock. Through wells, natural gas was injected into the reservoir, displacing the water and making the gas available for withdrawal later.

Nicor shared some of Herscher's storage capacity, but as customers grew, we decided we needed our own storage fields.


In 1957, we found the potential for storage capabilities in Illinois. After conducting tests and determining that the location was prime for a storage facility, the first cubic foot of gas was injected in 1958. Our goal was to provide reserves for the 1960-61 heating season. As our customer base and service territory grew, we continued to develop new storage fields.


With our underground storage aquifers underneath the Illinois farmland, we are able to purchase gas when it is likely to be less expensive. We can store it throughout the year, and then retrieve it for customers during peak winter months. Storage fields also give us flexibility to deliver gas to customers who purchase their own supplies. This allows those customers to buy gas at the best price and store it for future use.

Facts and Figures

  • We have about 150 billion cubic feet annual storage capacity.
  • We can meet about 50 percent of peak-day needs.
  • Storage supplies one-third of our normal winter deliveries.
  • Injection and Withdrawal

Injection and Withdrawal

When natural gas enters the storage station, it is filtered and compressed to increase its pressure to levels higher than that of the reservoir's pressure. When gas enters the aquifer reservoir, water is displaced from the sandstone. The displaced water provides us with the pressure needed to withdraw the gas.

There are two kinds of gas in the aquifer:

  • Base gas – a cushion for working storage gas that supports reservoir pressure
  • Working gas – gas available for withdrawal

Gas must be at higher pressures than the transmission pipeline for it to flow into the system. At some fields, the gas is already at that higher pressure. At others, gas must be pumped into the transmission line. The gas must be dehydrated before it starts being distributed.