The two types of underground gas storage
Basically there are two ways of storing gas underground: the storage cavern and the storage reservoir. The two differ with regard to the reservoir rock and the storage mechanism.
Caverns are large natural or manmade underground cavities. The man-made cavities are created by leaching them in rock salt or by using mine workings. The artificially leached salt caverns are particularly important for storing gas underground.
Salt domes provide underground cavities
The petro-physical properties of salt guarantee the natural tightness of the rock salt caverns and make an additional lining, such as required for storage caverns in disused mine workings, unnecessary.
The use of salt caverns as underground gas storage facilities depends on the occurrence of huge underground salt deposits at accessible depths. The salt domes in northern and central Germany are particularly suitable for this. Large cavities for storing gas can be created in the rock salt, which is mainly located in socalled salt stocks only a few hundred metres under the earth's surface, and can easily be tapped through borings.
Cushion gas provides the necessary pressure
The gas held in a gas storage facility is always divided into cushion and working gas. The cushion gas is the volume of gas that is necessary to ensure the minimum storage pressure necessary for optimal gas injection and withdrawal. In caverns, the cushion gas is also necessary to ensure stability. The proportion of cushion gas is roughly one third to a half of the maximum storage volume and remains permanently in the storage facility.
Working gas is the gas volume which can be stored or withdrawn at any time in addition to the cushion gas.
Storage reservoirs are mainly used for covering the seasonal base load as they often have a large storage volume and, due to the natural flows in the reservoir rock, mostly sandstone, react more slowly to changes in withdrawal rates in the storage well holes.
Porous rock offers excellent storage characteristics
The porosity and fissurization of the rock are ideal prerequisites for storing gas.
Storage reservoirs are underground gas storage facilities in depleted gas or oil deposits as well as in aquifer horizons. Since gas and oil have previously been extracted from these deposits, they have already been well examined before they are used for gas storage and their storage behaviour is known. The cap rock layers which mostly consist of mudstone or rock salt have been impervious to gas for millions of years and therefore ensure safe storage operations.
Aquifers are porous, water-filled rock strata which are covered by impermeable cap rock. The injection of storage gas displaces the resident water from the porous spaces and creates an artificial gas deposit. When the gas is withdrawn, the displaced water pushes the stored gas back into the well hole.