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Frequently asked questions


On this page you will find a selection of the most frequently asked questions about the Etzel cavern facility. These questions and answers are constantly being expanded.

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Infobox in Etzel

Etzel Cavern Info Center
Beim Postweg 2
26446 Friedeburg
Tel. +49 (0)4465 977 93 39

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Etzel cavern facilities

A cavern (modified from the Latin "cavum") is an artificially created underground cavity.

75 caverns in operation: 51 for natural gas, 24 for crude oil.

The cavern volume is about 46 million cubic meters.

Average about 60 meters in diameter; 300 to 500 meters high, usually in a range of about 1,000 to 1,500 meters below ground level.

About 12 kilometers long, 5 kilometers wide. It rises from a depth of about 4,000 meters to about 750 meters below the surface.

The area is about 1,500 ha (15 km²).

Cavern construction

Individual work steps in cavern construction are:

→ Construction of a cavern site

→ Drilling operation

→ Drilling completion

→ flushing out the cavity (brine operation) in the salt dome

→ pipeline connection of the cavern site

→ installation of the underground equipment (oil or gas side)

→ filling of the cavern with natural gas or crude oil 

The caverns in Etzel have a volume of between 300,000 m³ and 700,000 m³.

The plant capacity of STORAG ETZEL enables the simultaneous production of up to 25 caverns.

The necessary brine operation is carried out with the help of 16 powerful pumps in the so-called main pump station.

The construction of a cavern takes a total of three to five years.

The longest time is needed to bore out the underground caverns (approx. 30 months of brine operation).

Use of caverns

Use of the caverns for energy storage (oil and gas) for an operating period of up to 100 years.

The thick layers of salt in Etzel that surround the caverns are stable and dense.

The chemical properties and crystal structures of the salt alone prevent salt and the hydrocarbon compounds oil or gas from forming a bond. This is because salts are only soluble in water and do not dissolve in other liquids, which include petroleum, for example.  

Natural gas production and pipeline transport from the natural gas fields in the North Sea or from Siberia, which are often thousands of kilometers away, is continuous in large quantities and not dependent on consumption.

The cavern storage facilities buffer the energy source natural gas in order to be able to compensate for seasonal and daily demand peaks, as well as any supply interruptions, and thus ensure the energy supply for our population and industry every day.

After initial filling, the completed cavern storage facility is operated with the help of the surface facilities at the Etzel site.

In the case of natural gas, this is a gas operating facility of the gas cavern tenants; in the case of oil caverns, it is the main pumping station of STORAG ETZEL at the central operating site. If the storage medium is withdrawn in order to feed it into the existing long-distance pipelines for oil and gas, this is called "withdrawal".

The reverse process of filling is called "injection". In each case, the cavern is always kept under pressure.  

Natural gas is stored in the caverns at the Etzel site at a pressure of up to 200 bar with the help of our tenants' above-ground gas compressor stations.

This allows a large quantity of natural gas to be stored in a space-saving manner.

The content of a gas cavern is equivalent to the annual consumption of 20,000 households or a city of 80,000 inhabitants.

The crude oil stored in the caverns is removed by displacement using seawater (or brine).

For this purpose, the powerful pumps in the main pump station of STORAG ETZEL are used. 

The caverns in Etzel are used exclusively for the storage of liquid and gaseous hydrocarbons.

In addition to the storage of crude oil and natural gas, energy can also be stored in caverns in the form of compressed air or also hydrogen (H2). Compressed air and hydrogen can be supplied, for example, with the help of regenerative energy sources (wind power, photovoltaics / electrolysis).

At the end of their useful life, the caverns are decommissioned in such a way that they pose no danger to people, nature or the environment.

In accordance with a recognized procedure and under the supervision of the mining authority, the caverns will be filled with brine and stored in a long-term safe manner, the well will be sealed, the equipment will be dismantled and the site will be recultivated.

Documents from public meetings held in 2012 are available below (in German):

Stand der Kavernenverwahrung_2012

Gebirgsmechanischer Standsicherheitsnachweis für die Verwahrung der Kavernen in Stade

Druckaufbautest in der Kaverne K102_09_2012

Krupp-Gutachten 04.10.2012

Environmental impact

Construction activities at the Etzel site are limited in terms of location and time.

Possible disturbance may be caused by construction operations as well as truck traffic.

Organizational measures have been taken to reduce these disturbances (noise, light, dust), such as noise abatement measures, regulation of construction traffic via designated roads outside local areas, establishment of speed limits, and cleaning and control measures. 

Around the existing main operating site of STORAG ETZEL and the Etzel Gas Warehouse, 3 new gas stations of our partner companies, i.e. operating areas with buildings and facilities for the storage and retrieval of natural gas, have been built and put into operation in recent years.  

Here, solutions were developed together with the partners, the responsible authorities and landscape planners as to how a visual impairment through architecture and plantings can be reduced. 

Furthermore, in connection with the expansion that has taken place since 2006, there have been extensive official obligations to carry out compensatory and replacement measures at the Etzel site. For this purpose, STORAG ETZEL has acquired around 80 hectares of compensation areas and implemented the landscape management measures. 

Following the general experience that mining causes ground subsidence, monitoring of the surface for changes in elevation was started even before the first caverns of the Etzel cavern facility were excavated in 1974 in order to record the effects of cavern construction and operation on the surface.  

In addition to the magnitude of the elevation changes, the extent of the area of influence, which is limited by the "zero subsidence line" above which no elevation change can be detected, is also of interest. For this purpose, a so-called precision leveling is carried out annually by an independent expert in a closely meshed fixed point network, which is coupled to the official measurement network of the national survey.   

The fixed point network of height monitoring at the Etzel site in brief:

→ Number of measuring points (2019): approx. 800.

→ Monitored area: approx. 148 km².

→ Total distance: > 300 km

→ Accuracy: 0.5 mm/km

→ Monitoring since: 1973

→ Measurement cycle: 1 x per year

→ Duration and period of measurement: approx. 4 weeks each August / September.

The last annual measurement was made in August / September 2022. The evaluation of the measurements of the annual height monitoring in the vicinity of the Etzel cavern facility showed for the year 2022 that the total subsidence (1974 - 2022) in the center of the trough (diameter about 8 km) is 67 cm. This value corresponds to the expectations from the subsidence forecast.

The deepest point of the total subsidence is located north of the federal road B436. Since the changes in elevation at approx. 900 measuring points have been determined continuously every year since 1974 by fine leveling, statements can be made about the mining-induced annual subsidence contribution. From 2021 to 2022, about 5.3 cm of subsidence have been added. The very shallow subsidence trough is not visible to the human eye in the landscape.

According to experience and assessments by recognized experts, subsidence from cavern construction and operation in Etzel will not lead to break-off edges or numerous local depressions in the subsidence depression. Rather, a shallow, uniform subsidence depression will form over the Etzel cavern field. In the process, the center of the depression, and thus the deepest point, gradually shifts from the south field to the north field lying north of the B436.

The subsidence is made uniform, among other things, by the salt and the predominantly plastic earth layers above it, as well as by the mutual superposition of the subsidence effect of each individual cavern [technical term: "superposition"]. The shallow subsidence depression is, even with increasing subsidence amounts, not perceptible to the naked eye.   

It is also noteworthy in this context that due to the soil conditions prevailing in Etzel, especially the marsh with its intercalated peat layers, it naturally occurs that the upper soil layers move, e.g. depending on the water content, and are by no means absolutely static.

The natural variations in elevation in some areas are not taken into account in the annual elevation monitoring (leveling) because the measuring points are "anchored" to the solid ground on long steel rods.

Of course, residents living near the cavern facility can obtain information from STORAG ETZEL about details of height monitoring in the vicinity of Etzel, Horsten and Marx. Experts will be happy to provide information on the methods, procedure and altitude values of nearby measuring points. Please use the STORAG ETZEL citizen phone number 04465/ 977 93 39 for your inquiry or send us an e-mail:

Information center

Subsidence is unavoidable in mining. Based on current investigations, subsidence from cavern construction and operation at the Etzel site is not expected to cause damage, e.g. to buildings or public infrastructure. In the event of damage from cavern construction and operation, the law stipulates that the polluter must compensate for such damage.

In 2015, STORAG ETZEL and the local citizens' initiative jointly agreed to monitor a total of 36 buildings in the vicinity of the cavern facility as a precautionary confidence-building measure and to regularly check them for possible subsidence damage and inclinations.

Until 2018, 18 representative buildings were included in the monitoring by the citizens' initiative and STORAG ETZEL after consultation with the respective house owners. At the beginning, the houses are initially measured on the basis of height bolts and assessed by a recognized building expert in the ACTUAL condition. Overall, it is a mixture of different building types/classes (BI: residential buildings / STORAG ETZEL: historic buildings, infrastructure objects such as waterworks Kleinhorsten, agricultural properties, sensitive residential buildings).

In the following years, the buildings in question are surveyed every 2 years in connection with the height observation of the cavern field (annual levelling) with regard to inclination. Every 5 years, the buildings are also surveyed by an expert in order to be able to detect changes and possible subsidence effects at an early stage. The monitoring procedure is in accordance with the standards and practices of surveying and mining damage assessment recognized in Germany. You can view the survey overview of the building monitoring program here.

→ Survey overview

Economic factor cavern facility

STORAG ETZEL invested over 800 million euros in cavern development in the last expansion phase up to 2017. Since 2015, the company has been spending a total of around 100 million euros on plant and operational safety until probably 2020.  

The investments of the tenant consortia from the gas sector add up to this. Cavern construction and operation make a substantial contribution to business tax revenue in the municipality of Friedeburg.

A significant proportion of the companies working on the cavern facility are based in Lower Saxony. The cavern facility thus contributes to tax revenue and employment in Lower Saxony beyond the region.

The company's investments mainly concern technically demanding projects. Overall, only a few other investors in the wider catchment area have a similarly positive economic effect.

Up to 200 permanent employees of the Etzel cavern facility live in the immediate vicinity of Etzel and contribute with their income to the economic stability in northwestern Germany in recent years.

The cavern business has employed many local and regional companies since the early 1970s. It creates and secures jobs not only directly at the Etzel site, but also in the surrounding area. In the current expansion phase, around 800 million has already been invested by STORAG ETZEL at the site between 2006 and today.  

The list of neighboring communities in which suppliers and service providers of STORAG ETZEL are based - and pay their taxes - is long: In addition to larger cities such as Oldenburg and Wilhelmshaven, communities such as Bad Zwischenahn also benefit from the Etzel site. In zip code area "26" (Ems-Jade area / Wesermarsch) itself, for example, one fifth of the orders worth around 9 million euros remained in 2015. STORAG ETZEL appreciates not only the short distances but also the competence and flexibility of the local companies.

In the municipality of Friedeburg (Etzel is part of the municipality's territory), millions of euros in trade tax have been accruing for years.

According to an estimate by the financial administration of the municipality of Friedeburg, about two-thirds of the respective trade tax revenue in recent years was related to cavern construction and operation. In 2018, €4.6 million in business tax was generated from cavern construction and operation, while in 2017 the figure was around €7.3 million (2016: €6.0 million, 2015: €4.3 million, 2014: €9.7 million).

In recent years, Friedeburg has thus ranked several times in the top 10 of the approximately 400 municipalities in Lower Saxony; the municipality is thus able to provide its citizens with numerous voluntary benefits. A large part of the trade tax also benefits the district of Wittmund and the state of Lower Saxony through the pay-as-you-go system. STORAG ETZEL itself relocated its corporate headquarters to the municipality of Friedeburg at the turn of 2016.