Lesum salt dome

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The Lesum salt dome is a near-surface salt structure in the Burglesum district of Bremen , the Blockland district of Bremen and the neighboring municipality of Ritterhude in Lower Saxony . It contains cavern storage facilities in which diesel oil as part of the strategic oil reserve for times of crisis and natural gas are stored. The construction of the caverns began in 1968.

Geography and geology

The Bremen district of Burgdamm lies completely above the salt dome, the districts of Lesum, Werderland, Burg-Grambke and Blockland as well as the municipality of Ritterhude partially. The from the zechstein - Salinar salt dome formed has a areal extent of 5.5 × 3.5 km and a secondary thickness of about 4600 m; the highest point is only 135 m below sea ​​level or 157 m below ground level.

In the transition period between the Permian and Triassic geological ages up to approx. 250 million years ago, thick deposits of various salts formed from evaporation from the Zechstein Sea . The load on younger rock layers triggered salt tectonics , especially in weak areas of this cover rock . Correspondingly, the Lesum salt dome was created by mainly lateral displacement of the younger rocks; the salt rock rose more than four kilometers to near the surface.

Later the salt dome was narrowed horizontally ( compressively overprinted ) and a salt intrusion developed at a depth of approx. 2.5 km , which penetrated far between the rock layers.

Due to its proximity to the surface, the salt dome extends into the aquifer. The water primarily dissolves the easily soluble parts of the salt rock. This created a gypsum hat that continuously loses its load-bearing capacity due to the loss of material. It can sink holes occur. In the Geest area of ​​Lesum-Ritterhude, around 20 circular, partially filled with soft layers, subsidence with a diameter of up to 200 m were identified as sinkholes above the Lesum salt dome. The pattern of their distribution suggests that sinkholes are also present in the Burg-Grambke marshland.

Ruschdahlmoor, 2011

The Ruschdahlmoor is the visible result of two sinkholes . The fen was later overgrown by raised bog peat. With a peat thickness of 32 meters, it is considered the deepest bog in Germany and one of the deepest in Europe. C14 studies date the peat to a period between 9120 and 8612 BC. Chr.

At least the gypsum hat is still exposed to the loosening effect of the groundwater. In the map of the Geogefahren in Lower Saxony, Erdfall- and reduce areas of an area is shown above the salt dome Lesum.

Cavern storage

Georeferencing Map with all coordinates of the cavern storage section : OSM
Lesum cavern field; Cavern head
Lesum cavern field; movable housing for two cavern heads

The cavern field is located in the Burg-Grambke district near the Lesum (Lage) . The owner is the Petroleum Storage Association (EBV). Nine caverns were created here. Five caverns are operated by Nord-West Kavernengesellschaft mbH (NWKG) on behalf of EBV for the storage of diesel oil. Two caverns each are leased to Storengy and swb / wesernetz for the storage of natural gas.

The salt dome extends here to less than 200 m below the surface level. The high points of the caverns are a few hundred meters deeper in the salt rock. The cavities have heights of about 200-300 m and diameters of about 30-40 m. They are irregularly cylindrical or egg to pear-shaped.

Extraction structure in the Lesum
End point of the brine pipeline in Werderland, valve group and pig station, on the left pipelines for introducing the brine into the Weser

To create the caverns, a borehole is sunk to the planned bottom of the cavern and two pipe strings are installed. To dissolve the cavity, fresh water is injected and the resulting brine is displaced from the cavity and drained away . The fresh water is taken directly from the neighboring Lesum (Lage) via an extraction structure . The brine is an approximately eight-kilometer pipeline in the district Werderland amounting Lemwerder in the Weser initiated (position) .

Fresh water and brine pipes remain connected to the finished mineral oil caverns because they are required for removal and storage processes. There is an approx. 5.5 kilometer long pipeline to a tank farm with a sea-going terminal at the oil port (Lage) for the transport of oil .

The mineral oil floats on the aqueous phase (brine). The two pipelines end near the high point (oil) or low point. During storage, mineral oil is pressed into the cavern and the displaced brine is drained away. When removing from storage, fresh water is pressed in and the oil is displaced into the product line. Because fresh water dissolves the surrounding salt, caverns become larger and their shape changes each time they are stored. They are therefore long-term storage facilities with as few changes as possible. At the Lesum location, there has not yet been any outsourcing for consumption purposes.

Diesel oil is stored in the five caverns of the NWKG with a depth of 470–1010 m. The maximum individual volume is 390,000 m³. The first day of drilling was September 7, 1968. Deviating information: 600–900 m depth, light heating oil. The total volume is 1,289,800 m³.

Caverns for gas are separated from the pipes for fresh water and brine after the first filling. They are filled by injecting compressed gas under high pressure. This pressure is used for the removal. Gas caverns are not depressurized during normal operation. There always remains cushion gas that counteracts the rock pressure . Gas caverns are also suitable for frequent changes in storage and retrieval, for example to adapt to seasonal fluctuations in consumption. L-gas will be stored until the market area conversion , which will take effect here on April 1, 2021 .

As a service provider, Storengy operates two caverns at a depth of 1251–1664 m with a total usable working gas volume of 147 million m³ ( standard volume ). The storage facility is directly connected to the pipeline network ( Gasunie Deutschland ). Deviating information: 1312–1765 m depth.

The two Wesernetz caverns were designed to compensate for seasonal fluctuations in consumption in their own supply network. Their depth is 1050-1350 m, the available working gas 68 million m³. The storage facility is connected to the long-distance network and the local supply network via its own high-pressure line to the multifunctional network interconnection point Riedemannstrasse (Lage) in the Oslebshausen district.

history

Due to the obligation for manufacturers of certain petroleum products to have minimum stocks in force since 1966, Mobil Oil AG had two caverns built here for its refinery at the Bremen oil port from 1968. The refinery was shut down just six years later. The EBV, founded on the basis of the Petroleum Stocking Act, took over the caverns. In the period that followed, the NWKG created further caverns here for the EBV.

At the end of the 1980s and the beginning of the 1990s, two caverns were created for swb / wesernetz. Two more caverns were commissioned by Exxon Mobile in 2000 and sold to Storengy in 2011.

As a result of the market area adjustment, the storage facilities must also be converted from L-gas to H-gas. In view of the greater storage capacity across the network, wesernetz no longer considers operating its own storage facilities to be sensible. The gas from the two caverns is therefore to be completely withdrawn by the end of 2021 and fed into the Bremen supply network. The caverns should then be returned to the EBV; the NWKG will use it as a mineral oil storage facility.

In November 2019, with great technical effort (device tower, pressure lock, high-pressure pumps), an additional pipe string was brought down to the ground in the first cavern and connected to the fresh water extraction. Then the gas extraction began, interrupted by phases in which water was injected into the cavern under high pressure in order to maintain the cavern pressure and finally to completely displace the gas. The corresponding measures are to be taken on the second cavern from autumn 2020.

The two Storengy caverns are to be operated as storage for H gas from April 2021.

Criticism and environmental influences

Around 70 objections were raised during the approval process in 1968. In this - as in the public hearings - the introduction of the brine into surface waters was criticized as an additional "unreasonable salinization". Marsh farmers from the Lemwerder / Berne area in particular worried about drinking water for their cattle because they used Weser water. In addition, dangers from oil storage, contamination of the groundwater, concerns about the stability of the caverns and the unclear question of liability in the event of damage caused by the brining out were mentioned.

After the announcement of the work to decommission the Wesernetz gas caverns, reservations were expressed by the population: The necessary heavy goods traffic through narrow streets will lead to road damage and shocks to the adjacent buildings; When the water is drawn off, the water level of the lesum is reduced and fish are sucked in; Subsidence occurs when the gas extraction reduces the volume of the caverns as a result of rock pressure; Reduction of the stability of the caverns as a result of the increase in volume due to the loosening effect of the fresh water introduced; the brine displaced during the subsequent storage of mineral oil is discharged into the Weser.

The effects of the actual water use are u. a. depending on the ratio of the volume flows of the withdrawal or discharge in relation to that of the waters. The maximum volume flows of fresh water extraction and brine introduction are already limited by the available cross-section of the pipes introduced into the cavern. The water abstraction therefore has no noticeable influence on the water level of the Lesum, which is also tidal water with a mean tidal range of more than three meters.

The brine is discharged into the Weser, the mean discharge of which is 318 m³ / s (corresponding to 1,144,800 m³ / h). The chloride content of the Weser (mean value 236 mg / l) will therefore remain within the normal range of fluctuation despite the introduction of brine. Apart from that, the discharge of concentrated brine, due to its high salt concentration near the discharge point (i.e. before sufficient dilution), undoubtedly has effects on living things.

The storage of mineral oil and gas in salt caverns is carried out on a large scale and is considered safe if the regulations are observed. Nevertheless, accidents occur with the release of large quantities of oil or gas (examples) . The main reasons for this are the technical equipment installed above and below ground.

Concerns about the stability of the caverns when the gas is completely withdrawn are unfounded because the cavity is filled with water during this process. As a result of the dissolving effect, the cavern volume is increased - as is the case with brine extraction or the removal of mineral oil. The salt removal increases over time until an equilibrium is reached through saturation of the brine.

When the salt dome was formed, differences in pressure and the plastic reaction of the salt rock were the cause. The constant loss of volume in caverns is based on these principles. The higher lithostatic pressure compared to the internal pressure of the cavern ( hydrostatic pressure , gas pressure) causes the salt rock to creep into the cavities (convergence). This process begins with the brine and lasts at least for the duration of use. Convergence not only reduces the usable volume of the cavern, it also deforms it. Gradually, the volume of the mountain is shrinking, resulting in a depression on the surface of the day. Cavern operators are obliged to forecast the extent of the subsidence and to document it through annual measurements.

Whether caverns can be permanently secured after the end of their use by filling them with brine and a pressure-tight seal is a matter of technical dispute. On the other hand, the assumption is that the cavern will be completely filled with salt through convergence if it has not been filled with solid material. The prognosis of the subsidence is based on the feasibility of the permanently pressure-tight Kevernen lock. In the event of failure, this means that the cavern will leak in the upper area, the brine can be pressed out and the subsidence will be correspondingly larger.

Web links

Commons : Lesum salt dome  - collection of images, videos and audio files

Individual evidence

  1. ↑ a b Salt structure in the NIBIS map server. nibis.lbeg.de, accessed on June 30, 2020 . Blue: salt dome, ring around 500m deep, red: deeper salt intrusion
  2. ↑ a b Structure data sheet Lesum. bgr.de, accessed on June 30, 2020 (PDF, 307KB).
  3. ↑ Geological section. nibis.lbeg.de, accessed on July 1, 2020 .
  4. ↑ a b Dieter Ortlam: The geological structure in North Bremen . In: Heimatverein Lesum (ed.): Burglesumer Heimatbuch . Bremen 1985, p. 11-30 .
  5. ^ Senator for Environment, Building and Transport of the Free Hanseatic City of Bremen: Landscape Program Bremen 2015. Senator for Environment, Building and Transport of the Free Hanseatic City of Bremen, accessed on April 29, 2020 .
  6. ↑ sinkholes and subsidence areas. nibis.lbeg.de, accessed on July 2, 2020 .
  7. ↑ Hypothetical drilling. nibis.lbeg.de, accessed on July 9, 2020 .
  8. ↑ Cavern construction. nwkg.de, accessed on July 8, 2020 . Explanation of the principle, specific details related to the Rüstringen location. Explanatory video available via link graphic cavern construction (right).
  9. ↑ Cavern operation. nwkg.de, accessed on July 8, 2020 . Explanatory video available via the link graphic cavern operation (right).
  10. ↑ Julia Ladebeck: Germany's diesel stock is in Grambke. www.weser-kurier.de, October 27, 2012, accessed on April 26, 2020 .
  11. ^ Location Lesum - NWKG. nwkg.de, accessed on July 6, 2020 .
  12. ↑ a b c Annual Report 2019 "Oil and Natural Gas in the Federal Republic of Germany". lbeg.niedersachsen.de, June 22, 2020, accessed on July 7, 2020 (PDF, 4.98 MB). Data as of December 31, 2019, operator information; Natural gas: Table 24a, PDF-S. 51; Mineral oil: Table 25, PDF-S. 54
  13. ^ German Bundestag (ed.): Printed matter 17/5705 . May 4, 2011, p. 4 ( online [PDF; 344 kB ; accessed on July 13, 2020]).
  14. ↑ a b c Lesum natural gas storage facility. storengy.de, accessed on July 14, 2020 .
  15. ↑ Saved data. gasunie.de, accessed on July 14, 2020 .
  16. ↑ gas network. wesernetz.de, accessed on July 14, 2020 .
  17. ↑ Conditions for storage access. wesernetz, accessed on July 14, 2020 (PDF, 80 KB).
  18. ↑ Law of September 9, 1965 ( Federal Law Gazette 1965 I p. 1217 )
  19. ↑ newspaper clippings 1968. bremen-grambke.de, accessed on July 9, 2020 .
  20. ↑ Law of July 25, 1978 ( Federal Law Gazette 1978 I p. 1073 )
  21. ↑ a b Klaus Grunewald: 500 million liters from the Lesum. www.weser-kurier.de, accessed on April 26, 2020 .
  22. ↑ Gas storage: GdF Suez subsidiary Storengy purchases with Linklaters on a large scale. juve.de, accessed on July 14, 2020 .
  23. ↑ a b Julia Ladebeck: In the future oil will be stored in the caverns. weser-kurier.de, December 19, 2019, accessed on July 15, 2020 .
  24. ↑ a b Project handover of the Lesum storage facility. Information from the “Committee for Environment and Climate Protection” of the Burglesum Advisory Board on December 5, 2019. wesernetz, accessed on July 15, 2020 (PDF, 1603 KB). P. 6: Illustration of cavern L201, P. 11: Scheme of emptying, P. 13: Timetable, P. 18: Scheme of fresh water supply, P. 19: Relation of hourly water withdrawal / outflow of the Lesum
  25. ↑ To compare the picture of the pressure lock : Reinforcement measures on gas caverns. storag-etzel.de, accessed on July 18, 2020 .
  26. ↑ a b c Volker Kölling: Gas storage facilities are flooded with 500 million liters of Lesum water. buten un within, accessed on June 30, 2020 . (Animation of the process in the video from second 54)
  27. ↑ unknown: grief with salt from Grambker Moor . In: Bremer Nachrichten . November 23, 1968, p. 6 . available from Rainer Meyer: Grambke today, yesterday & the day before yesterday. Grambker district portal, accessed on June 30, 2020 .
  28. ↑ unknown: No concerns about oil storage in the salt dome . In: Burg-Lesumer Vereinsblatt . November 30, 1967, p. 29 . available at Grambker district portal: Grambke today, yesterday & the day before yesterday. Rainer Meyer, accessed June 30, 2020 .
  29. ↑ Master data of the Wasserhorst gauge. pegelonline.wsv.de, accessed on July 20, 2020 (MThw - MTnw).
  30. ↑ hydrology. fgg-weser.de, accessed on July 20, 2020 .
  31. ↑ Bremen-Hemelingen measuring station. bauumwelt.bremen.de, accessed on July 20, 2020 . Average of the 14-day composite samples from 2019
  32. ↑ gas network. wesernetz.de, accessed on July 24, 2020 : "The volume of gas caverns is continuously shrinking ..."
  33. ↑ a b Ralf E. Krupp: Brief report on the long-term safety of solution caverns in the Etzel salt dome . April 17, 2012 ( online [PDF; 1.5 MB ; accessed on July 25, 2020]). P. 6 Pressure conditions, P. 7: Convergence, P. 8: Subsidence, P. 9: Post-operational phase, controversy
  34. ↑ Environmental impact. storag-etzel.de, accessed on July 24, 2020 .
  35. ↑ NWKG informs. nwkg.de, accessed on July 22, 2020 (Numbers for the Lesum location are currently not provided.).

Coordinates: 53 ° 10 ′ 0 ″  N , 8 ° 42 ′ 48 ″  E