This video shows and explains the injection and withdrawal process at our natural gas storage facility ESE in Etzel.
Gas Storage Technology
Why do we store gas underground?
Large capacities
Underground storage holds far more gas than surface options and offers high injection and withdrawal rates.
Security of supply
Storage facilities guarantee supply security and compensate for winter consumption peaks maintaining stability in gas grids.
Flexibility for the future
With more renewables, additional fluctuations occur – storage enables quick response, including green hydrogen storage.
Balancing fluctuations
Seasonal variations (summer/winter) and short-term demand peaks are balanced.
Two types
Porous rock storages accommodate very large volumes and cavern storages can inject and withdraw very quickly on short notice.
Injection and Withdrawal
How does underground gas storage work?
Storage types
Caverns are large, natural or artificial underground cavities created by the excavation of rock salt or by mining. The artificially created caverns in rock salt are of particular importance for underground gas storage. The petrophysical properties of salt guarantee natural impermeability, so that additional sealing is not necessary. The prerequisite for the construction of caverns for underground gas storage is large, underground salt formations. This is particularly the case with the salt domes in northern and central Germany. To build a cavern, rock salt is drilled, then the salt domes are flushed with fresh water, creating cavities in which the natural gas can be safely stored.
Cavern storage can be used to compensate for both seasonal and short-term fluctuations in demand, as it can be filled and emptied more quickly than porous rock storage facilities.
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Porous rock storage facilities are underground gas storage facilities in former natural gas or oil deposits and in aquifer structures. A prerequisite for the storage of gas in porous rock storage facilities is the presence of porous or fissured storage rock in which - usually microscopic - cavities the gas can be stored.
To ensure that the gas is stored safely and permanently, a pore storage facility requires an overlying, gas-tight rock layer as a storage cover. In our storage facilities, the storage rock consists of sandstone and the storage cover consists of dense clay layers many meters thick. The gas is injected and withdrawn via deep boreholes. In addition, all wells are equipped with independently closing underground safety valves.
Porous rock storage facilities are mainly used for seasonal base load coverage, as they usually have a large storage volume, but the gas has to be injected and withdrawn more slowly due to the naturally given flow conditions in the storage rock.
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Cushion gas and working gas
The total amount of gas contained in storage can be divided into working gas and cushion gas. The gas itself is the same, the difference is the function.
The working gas - the actual purpose of a gas storage facility - is the volume of gas that can be stored and withdrawn. This portion of the gas is therefore the usable gas volume that can be marketed.
The cushion gas provides the necessary pressure to allow the working gas to be withdrawn from the storage at high rates even at low storage levels. In addition, maintaining a minimum pressure is necessary to ensure long-term integrity in cavern storage facilities. In aquifer-influenced pore storage facilities, the cushion gas prevents excessive water intrusion into the pore space used for gas storage. The cushion gas limit is a lower limit for the withdrawal process of a gas storage facility, which is determined by official requirements or geological/technical conditions.
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Technical storage curves
The technical storage curves show the injection and withdrawal rate of the storage facility depending on the fill level.
The available system capacity is significantly influenced by the pressure and temperature of the stored gas. However, these are not constant, but change depending on the fill level and the storage operation mode.
The system unit operation with the lowest capability at an assumed storage level and certain pressure and temperature conditions determines the available storage capacity. The course of the technical storage curve is influenced - depending on pressure and temperature - by the different components of the storage, such as separators, compressors and the well.
Another factor influencing storage capacity is the pressure of the gas transmission system from which gas is either injected or withdrawn.
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Hydrogen as a future option
We are equipping our storage facilities for a carbon-neutral future. The underground storage of natural gas has been tried and tested for decades. We assume that natural gas will remain with us for a long time as a bridge to a completely decarbonised energy world in the future. At the same time, we are already preparing our storage facilities for the energy sources of the future. Hydrogen is seen as the key to the energy transition and has the advantage that, like natural gas, it can be stored underground. Large-scale underground hydrogen storage is essential for the European hydrogen market and will help improve its economic viability. We have suitable storage facilities for the hydrogen market and have already started to test their feasibility. A clear regulatory environment is now needed to enable the conversion and development of new underground hydrogen storage facilities.
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Process safety
Uniper plays a leading role in promoting Health, Safety, Security, Environment & Sustainability (HSSE-S) best-practice strategies wherever we operate. We firmly believe that this benefits society, our employees and their families. As part of OGMP 2.0, for example, we have also voluntarily committed ourselves to continuously and transparently report our methane emissions according to defined standards and furthermore reduce them. We want to protect people and the environment from harm, minimise risks, increase safety, and thus ensure that everyone at Uniper feels safe in their jobs and returns home safely at the end of each day. This is our top priority. Our commitment to health and safety naturally includes business partners, neighbours and visitors.
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Monitoring from a distance
As a special feature in the operational management of our storage facilities, we use network control technology primarily for remote monitoring, and in some cases also for control. This process control technology includes the measurement, control and regulation technology for the operation of our storage facilities and provides the operating personnel, known to us as dispatchers, with the necessary data to make targeted decisions and facilitate routine work. The storage facilities are monitored remotely in our dispatching center, which is manned 24/7. This involves reacting to malfunction situations and controlling injection and withdrawal processes. For this purpose, process information such as accounting values, measured values and messages are transmitted to our dispatching control center, where they are processed and displayed in a user-friendly manner. From there, control and actuation commands are sent to the process in the opposite direction. General operation of the storage is of course still possible at the storage facility as well. This type of monitoring and control of gas storage processes brings many advantages and great flexibility.
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