The storage business
Natural gas storage facilities are important pillars of the German and European energy supply. A steady and continuous production of natural gas stands in contrast to a volatile demand and consumption which follows seasonal and business cycle depended pattern. Gas storage facilities ensure together with the European gas pipeline system the supply of the required gas volumes also in cold periods in winter at any time.
Kinds of storage facilities
There are two ways of storing large quantities of natural gas underground: cavern and porous storage facilities. Both are located at depths of between 500 and 2,500 metres.
Porous storage facilities
Porous reservoir Porous storage facilities make use of the natural porosity and permeability of sandstone strata. The natural gas is stored in the pores and fissures of the sandstone, which absorb the gas like a sponge and discharge it again when required.
Both depleted gas reservoirs and aquifers are suitable for this form of storage. Aquifers are water-bearing rock formations where the water is forced out by injecting gas.
These porous storage facilities have proved their impermeability over millions of years. Impermeable surface layers prevent any gas from escaping to the surface and water-bearing layers form a natural barrier below the porous sandstone.
Cavern storage facilities
Caverns In contrast to porous storage facilities, caverns are artificial cavities in salt seams deep underground. These caverns are created through controlled dissolution of the salt in water (leaching out). They can be up several hundred metres high and up to 80 metres in diameter. The physical properties of the salt seams ensure these caverns are naturally impermeable. Besides compensating for seasonal fluctuations, caverns are mainly used to cover short-term peaks in gas consumption.
The demands of reservoir technology require a certain amount of gas to be always present in a storage facility. This so-called cushion gas is needed to guarantee the minimal storage pressure required for gas injection and withdrawal, and the stability of the cavern.
The injection and withdrawal of gas determine the workflows at a storage facility.
- Every gas storage facility is connected to the mains network of a gas distribution company by means of an underground pipeline, through which the gas flows to and from the facility.
- Incoming gas flows first through a filter, which separates solid matter and liquids from the gas. Then the gas flows through a calibrated volume gauge.
- Before the gas can be injected into the storage facility, the pressure has to be increased by means of compressors driven by a gas motor or turbine.
- Gas coolers dissipate the heat generated by the compression process.
- Finally the gas is fed into the wells via high-pressure pipes and then injected into the storage formations.
Gas is withdrawn from the reservoir through the same wells it was injected in through.
Gas absorbs water while in the reservoir. This water has to be removed to avoid any corrosion or blockage of the mains network through the formation of gas hydrates.
- In a first step, free droplets of water are separated from the gas and injected into a water disposal well.
- The gas is then preheated to ensure that no gas hydrates are formed in the processing plant after the pressure of the gas has been reduced to mains pressure.
- The gas is dehydrated in a glycol absorption plant. Thanks to its hydroscopic nature glycol absorbs the water vapour contained in the gas.
- Now the gas is now in the same state it was when delivered to the storage facility. After passing through a calibrated volume gauge, it is returned to the gas distribution company.
The withdrawal process described above is dependent on the reservoir pressure being high enough for the gas to be forced into the mains network. If this is no longer case towards the end of the withdrawal process, compressors are again employed to raise the pressure.
The storage site
Storing gas underground requires relatively little space for the associated surface facilities. Each gas storage facility has one or more multi-well locations, a compressor plant, equipment for measuring the volume of gas and its properties, and a permanently staffed control station with equipment for operating and monitoring the entire facility. There will also be various buildings housing electrical, measuring and regulating systems, an emergency generator, a compressed air supply, a fuel gas treatment plant and, of course, offices.