Underground natural gas storage sites consist of geological structures having characteristics that enable to stockpile, preserve and, when required, withdraw natural gas.

Storage sites are considered conventional when they are obtained using depleted or semi-depleted gas production reservoirs, semi-conventional when they use depleted oil reservoirs or aquifers (i.e. geological structures containing water), special when they use abandoned coal mines and in cavities obtained in underground salt formations.

The Cellino and Collalto reservoirs are of a conventional type.

In the gas of depleted gas reservoirs: the most significant elements are the shape and size of the reservoir, the breadth and characteristics of the aquifer, the gas-water contact, the characteristics of the reservoir rocks and of the caprocks.
The most important physical parameters of the reservoir rock that must be carefully evaluated and monitored are:

• porosity
• permeability
• interstitial water saturation.

In view of their properties and their current level of developments, Edison Stoccaggio’s two reservoirs can be deemed as basic storage sites, i.e. they are normally used throughout the winter season and they are characterised by high working gas and by a slow decline in daily peak withdrawal performance.