Making use of natural conditions
Natural gas has been successfully stored for many decades in natural porous rock formations lying deep underground. There are two types of pore storages: depleted oil and gas fields on the one hand, and water-bearing rock layers – otherwise known as aquifers – on the other hand. The most important type is depleted natural gas fields whose intrinsic tightness has already been demonstrated by their existence over geological time scales. Moreover, the geological structure, reservoir properties (e.g. porosity and permeability), capacity, and the flow behaviour of the gases and liquids within the reservoir are already very well known from the results of exploration and the subsequent production activities. Another beneficial aspect is the ability in many cases to make use of the already existing wells, or to convert them into observation wells. The ability to use a reservoir to store natural gas primarily depends on the existence of adequate permeability within the rock. Using depleted oil fields is more complicated than the conversion of former gas fields because of various problems including purifying the gas when it is produced again at the surface. Depleted oil fields therefore only play a relatively minor role compared to depleted gas fields.
In addition to using depleted oil and gas fields, it is also possible to use natural aquifer formations to store natural gas. This is done by injecting the gas into the storage which then displaces the water in the pore spaces. A key prerequisite for successful storage in a natural aquifer is the presence of adequate porosity and permeability in the storage rock, as well as the presence of gas-tight horizons which effectively seal the aquifer above and below (see Fig.). By choosing structurally favourable sections of the aquifer, e.g. within dome-like structures, it is possible to ensure that the gas remains within this dome-like structure after injection instead of dissipating laterally in an uncontrolled manner. The maximum possible volume of gas which can be stored in a structure of this kind depends amongst other things on the so-called spill point, which is the highest point of the structure below which the gas will spill out into adjacent parts of the formation (see Fig.).
Compared to a depleted oil or gas field, the investment required in geological and reservoir-mechanical investigations are much higher in an aquifer storage because no empirical data is available from previous exploration and production phases.
Compared to artificially constructed salt caverns, pore storages are mainly suitable for long-term and less flexible storage of very large volumes of gas. They are ideal for compensating for seasonal fluctuations in demand (winter – summer).
Pore storages enjoy a very long tradition in the storage of natural gas: their use goes back almost 100 years when a depleted gas field in Welland County, Canada, was used to store gas for the first time. The first aquifer storage was created in the USA in 1931. The first aquifer storage to be used in Germany was in Engelbostel near Hannover in the 1950s, which was also the time when the first gas field was used for natural gas storage in Reitbrook near Hamburg. The first depleted oil field in Germany to be used for the storage of gas was also in Reitbrook and began operations in 1973.
There are currently 21 pore storages operating in Germany which hold approx. 45 % of the usable storage gas in the country. The largest storage is a depleted gas field in Rehden, located between Bremen and Osnabrück. The largest aquifer storage in Germany is also located in Niedersachsen near Kalle close to the border with the Netherlands.
Hardly any new aquifer storages have been developed in recent years because of the considerable exploration costs and risks. There has also been a stagnation in activities involving the conversion of depleted gas fields because of the lack of suitable reservoirs on the one hand, and the greater current interest in developing flexible salt cavern storages on the other hand.