Methane hydrate

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Burning methane hydrate (small picture: model of the molecular structure)

Methane hydrate (also called methane clathrate - from Latin clatratus = cage -, methane ice or flammable ice ) consists of methane that is stored in solidified water , with the water molecules completely enclosing the methane. Methane hydrate is therefore called an intercalation compound ( clathrate ). Russian scientists hypothesized that methane hydrate occurs naturally in many areas of the world. Pure methane hydrate was first discovered in the Black Sea in 1971 . The first drilling for the investigation of gas hydrates took place in 1997 on the Blake Plateau in the North Atlantic. Funding techniques are also being investigated in the SUGAR project funded by the Federal Ministry for Economic Affairs and Energy and the Federal Ministry of Education and Research under the direction of GEOMAR .

properties

There are 5.75 moles of water for one mole of methane, so the formula is written as CH 4  · 5.75 H 2 O. The density is 0.9 g / cm³. The methane is in a highly compressed form, under normal conditions 1 m³ gas hydrate corresponds to 164 m³ gas and 0.8 m³ water. At room temperature the gas hydrate is unstable and methane escapes, which can be ignited - the appearance is reminiscent of "burning ice" or Esbit .

Crystal structure

Methane hydrate is formed from water and methane gas at a pressure of around 2 MPa - this pressure is reached from around 200 m water depth - and temperatures of 2 to 4 ° C. Gas hydrates crystallize in a cubic crystal system , the shape of the cage being determined by the embedded gas molecule. The most common form of methane storage is the dodecahedron .

So far, three crystal structures of methane hydrates have been found: Type I with the incorporation of methane and ethane hydrates as well as with carbon dioxide and dihydrosulfide, Type II with propane and isobutane and Type H with longer-chain hydrocarbons such as methylcyclohexane.

Occurrence

Known and suspected deposits of methane hydrate on the continental margins and in permafrost

Methane hydrate is thermodynamically stable only under certain pressure and temperature conditions and therefore forms in large quantities on the continental slopes where the pressure is high and the temperature low enough. Further occurrences can be found in the permafrost soils of the polar regions. The minimum formation depth is around 300 m in the Arctic and around 600 m in the tropics. Salt in sea water leads to a reduction in the stability field, since lower temperatures and higher pressures are necessary with increasing salt content. Long chain hydrocarbons in the hydrates have the opposite effect. The natural occurrence is estimated at twelve trillion tons of methane hydrate, which means that there is possibly more than twice as much carbon bound as in all oil , natural gas and coal reserves in the world. Methane hydrate is usually found at depths of 500 to 1000 m.

Gas hydrates form yellow to gray, transparent to translucent masses that fill the pore spaces of the sediment and form coherent deposits. The sediment is cemented with a possibly stabilizing effect. Methane hydrate is lighter than water under standard conditions with a density of 900 kg / m³ and rises. Due to the compression modulus of methane hydrate, which is around 4.5 times higher than that of water , this buoyancy is retained at every depth of the sea, and even increases slightly with depth. In order for methane hydrate to remain on the sea floor against its buoyancy in the water, it needs to be mixed with heavier material, such as sand or rock, in sufficient proportions. Methane hydrate is therefore typically found in the depths of the seabed, where it fills pores and tends to rise to the top.

The United States Geological Survey (USGS) has listed the known methane hydrate deposits . Only assumptions about the occurrence in great depths of the sea are based on the evidence of a so-called soil-simulating reflector (BSR) of seismic waves at the lower limit of the gas hydrate layer - below this, free gas and liquid water are assumed, while the upper limit cannot be determined by a defined limit. The BSR is a necessary but not a sufficient condition for gas hydrates. Seismic reflectors can arise from diagenesis , among other things .

Test drilling in Alaska revealed large fields in Tarn and Eileen on Prudhoe Bay with at least eight large seams at a depth of 300 to 800 meters and about 40 to 60 billion cubic meters of gas hydrate.

Based on the latest scientific calculations, researchers assume that there are around 4 billion tons of methane hydrate under the Antarctic ice sheet.

In Canada , a large field was found in Mallik in the Northwest Territories in the Mackenzie Delta , where scientists from the USA, Europe, including Germany, Japan, India and China have worked out mining methods.

Since 1976, methane has been extracted from methane hydrates in the Siberian Messojacha field using an injection process .

In 1996, the German research ship " Sonne " discovered large methane hydrate deposits for the first time during a research voyage led by the IFM-GEOMAR Institute in Kiel, around 100 km west of Oregon .

In 1997 the first evidence of methane hydrate in the sediments of the Russian Lake Baikal was made . In the course of the “Baikal Drilling” project, the hydrate was detected in the sediment core BDP-97. As early as 1980, a publication appeared in which it was assumed that there could be gas hydrate deposits in the deep sedimentary layers of Lake Baikal.

Japan is studying the sands of the Nankai Trench , which contain around 20 percent gas hydrate. For the first time in 2013, Japan recovered methane hydrate from a depth of 1000 meters, 330 meters below the sea floor, 80 kilometers off the coast from the deep sea.

Apparently there is far more methane hydrate than previously thought. Rising methane has also been discovered in the Mediterranean.

Emergence

In the oceans methane is produced by a specific group of archaea , the methanogens . For the purpose of generating energy, they reduce C 1 compounds to methane . This biochemical process is called methanogenesis .

When methane hydrate is formed, the water must be supersaturated with methane, and certain pressure and temperature conditions must also prevail. Methane hydrates are only stable at high pressures and low temperatures. In the presence of hydrogen sulfide or carbon dioxide, methane hydrate can form even at lower pressures and slightly higher temperatures. Large deposits in addition to those in the oceans are found in the ice sheet of Greenland and Antarctica as well as in the permafrost soils.

Structure of a methane hydrate chunk from the Oregon subduction zone

At the subduction zone off Oregon , plate tectonics subducts the oceanic Juan de Fuca plate below the continental North American plate . The subducted sediment is pressed out at greater depths and pore water with a high methane content is transported upwards. In the vicinity of the sediment surface, this dissolved methane cools down into the stability field of methane hydrate, and methane hydrate is formed in the sediment or on the sediment surface. Through this process, most of the rising methane is bound in the sediment and deposited near the sediment surface. On the sediment surface, the places where this pore water leaves the sediment ( cold sea ) can be identified by the appearance of bacterial mats and large colonies of mussels and worms. This fauna uses the remaining methane and hydrogen sulphide in the rising water in order to use special methane or sulphide-oxidizing bacteria to generate energy as a basis for life, regardless of light, which, in contrast, is the primary source of energy for communities through photosynthesis in exposed habitats .

The low proportion of 13 C suggests microbial formation. Organic substances in marine sediments can be converted by microorganisms under anaerobic conditions into methane, among other things, which forms methane hydrate with the surrounding water. Gerald Dickens , paleogeologist at Rice University in Houston , believes that in the Paleocene , around 55 million years ago, when temperatures were four to five degrees higher than today, large amounts of organic matter were formed in the ocean. These substances were probably the starting materials for the formation of methane hydrate.

With regard to the assumption that the methane hydrate deposits could contain twice as much carbon as in all known deposits of fossil fuels ( natural gas , crude oil , coal , tar sands ) combined, critics fear that the exploitation of these deposits - originated around 60 million years ago in the Paleocene - whose climate could be brought back by the greenhouse effect . At the end of the Paleocene there was a sudden global temperature increase of around 5 to 6 ° C. The Paleocene / Eocene temperature maximum was triggered by a sudden release of methane. Methane hydrate deposits on the ocean floor that have become unstable are discussed as a source.

meaning

raw material

The extraction of methane from its hydrate in the sea floor is not economical. The large amounts of bound methane are ecologically questionable as an energy source of the future. Because methane hydrate decomposes in the higher water layers at lower pressure and higher temperature and large amounts of gaseous methane escape as a result, the breakdown of the methane hydrate fields is difficult. The first and only industrial and commercial plant on earth in which methane hydrate is broken down is located in Krasnoyarsk, Siberia . Intensive research is already being carried out in Japan, the Norwegian Spitzbergen and other countries.

Experts estimate that just as much carbon is stored in gas hydrates as in the known coal deposits: it is estimated that there are around twelve trillion tons of methane hydrate in the sea floor. In the SUGAR project , German researchers are developing technologies for how this gas - mostly methane - can be extracted. There are many deposits in the territories of Asian countries and the USA and Canada, which is why Dr. Mathias Haeckel from GEOMAR believes it is possible that methane from gas hydrates could be an important energy source in the future. Japan, China, Taiwan and South Korea have large reserves of gas hydrate, but have very few conventional oil and gas reserves. "This could perhaps be seen as a kind of energy revolution, insofar as the gas market is shifting from Russia and the Arab countries to Asia," said Haeckel. The marine researcher Haeckel points out, however, that gas hydrates are also a fossil fuel, so gas production is only a bridging technology until we can switch to alternative energy sources.

Global warming potential

Methane hydrate seems to have a major influence on the climate, because methane is a greenhouse gas with a global warming potential of 28, that is, an effect 28 times stronger than carbon dioxide seen over 100 years. (See also section Global Warming and Methane Hydrate )

Influence on the sea

Methane hydrate fields in the Gulf of Mexico are the habitat of the polychaete Hesiocaeca methanicola and the methanotrophic bacteria that occur with it .

The presence of large amounts of methane hydrate in the Bermuda Triangle is used, among other things, to explain the phenomena supposedly occurring there (see Bermuda Triangle , section Methane hydrate occurrence and blowout ). Scientists like William Dillon think this is unlikely, but concede that large releases of methane would reduce the density of the affected water masses so that ships could no longer swim on it.

The formation of methane hydrate under the glacier tongues solidifies the sediment there, so that in places glaciers flow more slowly into the sea than without this effect.

In geological terms, methane gas releases off Norway and in the Caribbean have been proven, which could have been triggered by falling sea levels or dwindling glaciation (pressure reduction), sliding of exposed hydrate masses and tsunamis.

Use as storage

Work is underway to use methane hydrate as an energy store on land. In the process, methane should initially be stored in large quantities as methane hydrate and then used as fuel if required.

Another reason for the intensive research in the field of gas hydrates is the possible use of carbon dioxide hydrate as carbon dioxide storage. The aim is to store CO 2 as a hydrate on the sea floor. At the same time, however, its discharge would release methane from the sea floor.

Global warming and methane hydrate

The stability of methane hydrate deposits can be influenced as a result of the pressure and temperature changes in the ocean caused by global warming . With a warming of the ground water the thickness of the stability zone of the methane hydrates is reduced. Methane hydrate deposits that were previously stable become unstable - the methane hydrate breaks down, methane is released and some of it changes into a gaseous state. From a temperature increase of 3 K in the surrounding water of the methane hydrates, the thickness of the stability zone decreases significantly, at a temperature increase of 8 K it disappears completely. Even small changes in temperature can cause the gas to escape into the free water. With the disintegration of methane hydrates, the stability of the sea ​​floor continues to decrease , and landslides ( Storegga effect) and tsunamis can result.

As soon as the methane hydrate is outside the stability zone, free methane forms in the sediment and below the hydrate layer. The free methane can escape diffusively or as a gas ascending from the sea floor into the ocean water, under certain circumstances larger quantities can accumulate, which are then suddenly released (blowout). Broken out individual methane hydrate chunks without sediment adhesion are lighter than water, rise and partially disintegrate on their way up and transport the methane quickly to higher water layers.

According to the IPCC, scenarios are possible in which the global surface temperature of the seas could rise by 5 K above the pre-industrial value by the year 2100. Due to the polar amplification , temperatures in the Arctic could rise by up to 10 K.

Large amounts of methane released into the atmosphere are particularly dangerous in the long term. Due to the stable temperature stratification and the slow mixing of the oceans, higher water temperatures will only appear on the sea floor and in deeper sediment layers over the course of centuries. A short-term release is only to be expected in well-mixed, relatively shallow sea areas with hydrate deposits in the surface sediment.

In the course of global warming, a chronic methane release over millennia can lead to high methane concentrations in the atmosphere, to an intensifying feedback , since the methane intensifies the greenhouse effect. Melting continental ice, the water of which causes the sea level to rise and thus increases the pressure on the sea floor, can only stabilize the hydrate deposits to a small extent. A strong release of methane appears to have occurred 55 million years ago during the Paleocene / Eocene temperature maximum and the Eocene Thermal Maximum 2 . At that time there was a global warming of the atmosphere; In the rocks of that time, geologists are now finding evidence of a rapid increase in the methane content of the air. The consequences of this could affect the earth's climate for tens of thousands of years.

In 2006, in its special report "The future of the seas - too warm, too high, too acidic", the WBGU estimated the danger that a destabilization of methane hydrate deposits on the sea floor could lead to a rapid release of larger climate-relevant quantities of methane hydrate within the 21st century would be classified as "very low". The WBGU rated the probability of a “chronic methane release over many centuries to millennia due to the slow penetration of global warming into the deeper ocean layers and sediments” as “significantly more important”. At the same time, he emphasized that “climate change could be intensified in the long term by the release of methane from hydrates” and said: “This feedback effect harbors the risk that mankind will lose control of the greenhouse gas concentration in the atmosphere, since methane is not released from the sea floor taxable or limitable ".

Methane hydrates can also occur in the permafrost soils of the continental Arctic regions at a depth of 100 to approx. 2000 m below the land surface. As a result of the warming, the methane in it can be released as soon as the ground thaws. A study from 2016 estimated the contribution of the methane released from permafrost to global warming as “still relatively low”.

Possible consequences of a release

The consequences of a methane release depend on the mechanisms of propagation: (i) diffusion or release of fine bubbles, (ii) blowout , and also on (iii) the rate of release.

When the hydrate structure dissolves, methane is released. It rises slowly from the sediment in the form of small bubbles into the open water. It can be broken down in two ways on its way to the surface. On one hand, the methane can still seabed anaerobically by bacteria and archaea with the aid of SO 4 2- is oxidized to be. On the other hand, it can be aerobically oxidized by bacteria in the water column , releasing CO 2 . This oxidation leads to a decrease in the oxygen concentration and the CO 2 formed increases the concentration of carbonic acid (H 2 CO 3 ) in the ocean and thus contributes to further acidification ( dissociation of the carbonic acid H 2 CO 3 → HCO 3 - + H + ). In the long term, it can be expected that a new carbon dioxide balance will form between the ocean and the atmosphere. About a fifth of the carbon dioxide formed in the ocean is released into the atmosphere. The atmospheric CO 2 concentration would increase accordingly and further intensify the greenhouse effect.

If methane is released suddenly - this can be triggered by a destabilization of the methane by a landslide, as happened for example in the so-called Storegga slide off Norway about 8000 years ago - a large part of the gaseous methane can rise to the sea surface and the Significantly increase the methane concentration in the atmosphere. The atmospheric methane is oxidized to carbon dioxide (CO 2 ) and water (H 2 O) after an average life of about eight years .

The rising methane can also pose a threat to shipping . Scottish scientists attribute this to, for example, the sinking of a fishing trawler discovered in the Witches' Hole in the North Sea . The rising gas bubbles can therefore reduce the density of the sea ​​water so much that ships lose their ability to swim.

Mining methods

  1. Conversion into a gaseous state by increasing the temperature.
  2. Pressure relief, which can usually already result when the hydrate is exposed to atmospheric pressure during drilling or by artificially creating crevices and cracks.
  3. Pumping in carbon dioxide gas or heated water.
  4. Injecting the antifreeze methanol into the reservoir.

Even without global warming, methane can be released when methane hydrate is broken down, as when oil and gas are broken down. There is a risk of climate-damaging mass releases.

According to Timothy Collet of the US Geological Survey , expectations of the amount of methane hydrate present and recoverable in the Alaska and Northern Canada area have been exceeded by far. It is estimated that the stored quantities of gas hydrate far exceed the resources of conventionally extractable gas.

See also

literature

  • G. Bohrmann : Gas hydrates of the oceans. In: PdN-ChiS. 6/54, year 2005, pp. 2–7.
  • A. Boetius : Microbial methane turnover in the sea. In: PdN-ChiS. 6/54, year 2005, pp. 8–12.

Individual evidence

  1. ↑ Alaska Oil and Gas Conservation Commission: Kuparuk River Unit, Tarn Oil Pool ( Memento of the original dated December 30, 2013 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. , July 11, 2005. @1@ 2Template: Webachiv / IABot / doa.alaska.gov
  2. ↑ JL Wadham, S. Arndt, S. Tulaczyk, M. Stibal, M. Tranter, J. Telling, GP Lis, E. Lawson, A. Ridgwell, A. Dubnick, MJ Sharp, AM Anesio, and CEH Butler: Potential methane reservoirs beneath Antarctica . In: Nature . tape 488 , 2012, p. 633-637 , doi : 10.1038 / nature11374 .
  3. ^ US Geological Survey : The Mallik Project
    GeoForschungsZentrum Potsdam : International Continental Scientific Drilling Program ( Memento of July 29, 2007 in the Internet Archive ), October 18, 2006.
  4. ^ Japan Gas, Oil and Metals National Corporation, Methane Hydrate Technology Research Center: Research Consortium for Methane Hydrate Resources in Japan .
  5. ↑ Map ( memento from July 11, 2007 in the web archive archive.today ) and picture ( memento from January 30, 2006 in the internet archive ) of the drilling.
  6. ^ Research History of Lake Gas Hydrates according to Kuzmin MI, Kalmychkov GV, Geletiy VA, et al .: First discovery of gas hydrates in sedimentation mass of the Baikal lake.  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. Report RAN, 1998, t. 362, no. 4, pp. 541-543.@1@ 2Template: Dead Link / www.oilandgaseurasia.com  
  7. ^ Research History of Lake Gas Hydrates according to Evremova AG, Andreeva MV, Levshenko TV, et al .: On gases in Baikal lake bed sediments.  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. Gas industry. Series: Geology and exploration of gas and gas condensate fields, 1980, No. 2, pp. 15-27.@1@ 2Template: Dead Link / www.oilandgaseurasia.com  
  8. ↑ orf.at: Japan harbors methane hydrate from deep sea for the first time , from March 12, 2013.
  9. ↑ Japan extracts gas from methane hydrate in world first. In: BBC News. March 12, 2013, accessed March 23, 2013 .
  10. ^ J. Zachos, M. Pagani, L. Sloan, E. Thomas: Trends, Rhythms and Aberrations in Global Climate 65 Ma to Present. In: Science Vol. 292, 2001, pp. 686-693 (PDF file; 1.89 MB).
  11. ↑ Source of energy in the sea floor .
  12. ↑ Marie Heidenreich: Gas hydrate could lead to a worldwide energy revolution. In: Research for Sustainable Development (FONA). August 14, 2017. Retrieved August 17, 2017 .
  13. ↑ G. Myhre, D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch, J.-F. Lamarque, D. Lee, B. Mendoza, T. Nalajima, A. Robock, G. Stephens, T. Takemura, H. Zhang et al. : Climate Change 2013: The Physical Science Basis . In: Intergovernmental Panel on Climate Change (Ed.): Working Group I contribution to the IPCC Fifth Assessment Report . September 30, 2013, Chapter 8: Anthropogenic and Natural Radiative Forcing, pp. Table 8.1.A, pages 8–88 to 8–99 ( climatechange2013.org [PDF; 2.9 MB ; accessed on October 13, 2013] Final Draft Underlying Scientific-Technical Assessment).
  14. ^ US Geological Survey , Woods Hole Science Center: Bermuda Triangle ( Memento of March 1, 2000 in the Internet Archive ), September 25, 2006.
  15. ↑ Monica Winsborrow et al .: Regulation of ice stream flow through subglacial formation of gas hydrates. Nature Geoscience, 2016, doi: 10.1038 / ngeo2696 (discussed in scinexx ).
  16. ↑ Axel Mörer-Funk: Dresden researchers build energy storage systems based on the model of the deep sea. In: www.ingenieur.de. March 15, 2017. Retrieved June 10, 2019 .
  17. ↑ WBGU: Special Report 2006 - Chapter 6: Methane hydrates in the sea floor. ( Memento from June 11, 2008 in the Internet Archive ).
  18. ↑ Helmholtz Center for Ocean Research Kiel: Dynamics of the Ocean Floor. ( Memento of October 2, 2006 in the Internet Archive ).
  19. ↑ a b The future of the seas - too warm, too high, too acidic. In: Special report. German Advisory Council on Global Change (WBGU), 2006, accessed on June 10, 2019 . P. 98.
  20. ↑ Risk of climate change from thawing permafrost? (PDF) In: UBA background paper. Federal Environment Agency, August 2006, accessed on June 10, 2019 . P. 16.
  21. ↑ Risk of climate change from thawing permafrost? (PDF) In: UBA background paper. Federal Environment Agency, August 2006, accessed on June 10, 2019 . P. 13.
  22. ↑ Permafrost researchers determine for the first time the amount of methane released through the thawing process. In: www.awi.de. Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, August 25, 2016, accessed on June 10, 2019 .
  23. ↑ KW Anthony, R. Daaen, P. Anthony, T. Schneider von Deimling, C.-L. Ping, JP Chanton, G. Grosse: Methane emissions proportional to permafrost carbon thawed in Arctic lakes since the 1950s . In: Nature Geoscience . tape 9 , 22 August 2016, p. 679-682 .
  24. ↑ BBC News: North Sea wreck in methane mystery. November 29, 2000 (accessed July 23, 2013).
  25. ↑ USGS Canadian north slope gas hydrate energy ressources ferc.gov (PDF).

Web links

Commons : Methane Hydrate  - Collection of pictures, videos and audio files
Wiktionary: Methane hydrate  - explanations of meanings, word origins, synonyms, translations