Methane clathrate

Methane hydrate ( Methanklathrat - from Latin clatratus = cage - even methane, combustible ice or gas condensate called ) consists of methane, which is embedded in the solidified water, the water molecules completely cover the methane. Methane hydrate is therefore called intercalation compound ( clathrate ). Russian scientists hypothesized that methane hydrate in many areas of the world occur naturally. For the first time pure methane hydrate was discovered in 1971 in the Black Sea. The first hole for the investigation of gas hydrates took place in 1997 on the Blake Plateau instead. The commercial production is examined among others the SUGAR project under the direction of GEOMAR.

Properties

In one mole of methane are 5.75 moles of water, so the formula CH4 · 5.75 H2O notation is. The density is 0.9 g / cm ³. The methane is present in a highly compressed form, under normal conditions is equivalent to 1 m³ of gas hydrate while 164 cubic meters of gas and 0.8 cubic meters of water. At room temperature, the gas hydrate is unstable and it escapes methane, which can be ignited - the look is reminiscent of " burning ice " or Esbit.

Crystal structure

Methane hydrate is formed from water and methane gas at a pressure from about 20 bar; this pressure is from about 190 meters water depth reached ( 19 bar water pressure plus 1 bar air pressure) and at temperatures of 2 to 4 ° C. Gas hydrates crystallize in the cubic crystal system, the cage shape is determined by the stored gas molecule. The most common form for storage of methane is the dodecahedron. To date, three crystal structures of methane hydrates: Type I with storage of methane and Ethanhydraten as well as carbon dioxide and Dihydrogensulfid, type II in propane and isobutane and H-type case of longer hydrocarbons such as methylcyclohexane.

Occurrence

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 is low enough. Other deposits are found in the permafrost of the polar regions. The minimum formation depth is about 300 m in the Arctic and at about 600 m in the tropics. Salt in the sea water results in a reduction of the stability of the field, since with increasing salt content are required lower temperatures and higher pressures. Long-chain hydrocarbons in the hydrates have an opposite effect. The natural occurrence is estimated at twelve billion tons of methane, which is more than twice as much carbon is there possibly bound as in all oil, gas and coal reserves in the world. Methane hydrate is commonly found at depths of 500 to 1000 meters.

Gas hydrates form yellow form to gray, transparent to translucent masses that fill the pore spaces of the sediment and related deposits. The sediment is cemented with this potentially stabilizing effect. Methane hydrate is under standard conditions with density 900 kg / m³ lighter than water and rises. Due to the over water is about 4.5 times higher bulk modulus of methane hydrate this buoyancy is maintained in every ocean depth, so increases with depth even slightly. This methane hydrate remains lying on the seabed against his buoyancy in water, it requires the adhesive mixing with heavier material, such as sand or rock to a sufficient ratio. Methane hydrate therefore is typically found in the depths of the sea bottom, where it fills pores and tends to rise to the top.

The known methane hydrate, the United States Geological Survey (USGS ) listed. Only guesses of occurrence at great ocean depths are based on the detection of a so-called bodensimulierenden reflector ( BSR) of seismic waves at the lower limit of the gas hydrate layer - including free gas and liquid water is suspected, while the upper limit is not set by a defined limit. The BSR is a necessary but not a sufficient condition for gas hydrates. Seismic reflectors may arise, inter alia, the diagenesis.

In test drilling large fields in the Tarn and Eileen were found at the Prudhoe Bay with at least eight major coal seams in 300 to about 800 meters deep and about 40-60 billion cubic meters of gas hydrates in Alaska.

On the basis of the latest scientific calculations assume researchers that there are about 4 billion tons of methane hydrate under the ice of Antarctica.

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

Since 1976 Messojacha field is reduced by injection process methane from methane hydrates in Siberia.

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

In 1997, the first detection of methane hydrate in the sediments of Lake Baikal, Russia has been rendered. As part of the " Baikal Drilling " project, the hydrate was detected in the sediment core BDP -97. In 1980 appeared a publication in which it was suspected that there might be gas hydrate deposits in the deep sediment layers of Lake Baikal.

Japan examines the sands of the Nankai trench, which contain about 20 percent of gas hydrate. For the first time Japan holds methane hydrate from 2013 to 1000 m depth, 330 m below the seabed 80 km off the coast from the deep sea.

Obviously there are far more methane than previously anticipated. Also in the Mediterranean rising methane was detected.

Formation

In oceans, methane is formed by a certain group of archaea, the methanogens. They reduce, for the purpose of energy production, the dissolved carbon dioxide or other C1 compounds to methane. This biochemical process known as methanogenesis.

During the formation of methane hydrate, the water must be supersaturated with methane, also have certain pressure and temperature conditions prevail. Only at high pressures and low temperatures, methane hydrates are stable. In the presence of hydrogen sulfide or carbon dioxide to methane hydrate can form at lower pressures and slightly higher temperatures. Large deposits in addition to those in oceans are in the ice sheet of Greenland and in the Antarctic and in the permafrost.

At the subduction zone off Oregon, the oceanic Juan de Fuca plate is subducted beneath the continental North American plate by plate tectonics. Here, the subducted sediment is squeezed at greater depths and pore water with high methane content is transported upwards. Near the sediment surface of this dissolved methane result of cooling in the stability field of methane hydrate and methane hydrate is formed in the sediment or at the sediment surface. Through this process, most of the ascending methane is bound in the sediment and deposited near the sediment surface. At the sediment surface can be detected, the points at which this pore water, the sediment leaves (Cold Seep ), on the occurrence of bacterial mats and great clam and worm colonies. This fauna uses the remaining present in the rising water, methane and hydrogen sulfide, in order to extract using special methane or sulfidoxidierender bacteria energy livelihoods, regardless of light in exposed habitats by means of photosynthesis is the primary energy source of communities in contrast.

The low proportion of 13C suggests a microbial origin. Organic matter in marine sediments can be implemented by microorganisms under anaerobic conditions, among others, to methane, the methane hydrate forms with the surrounding water. Gerald Dickens, Paläogeologe at Rice University in Houston, assumes that in the Paleocene to around 55 million years ago when the temperatures were four to five degrees higher than today, in the sea large amounts of organic substances were formed. These materials were probably precursors to the formation of methane hydrate.

With regard to the assumption that in the methane hydrate twice as much carbon could be included as in all known deposits of fossil fuels ( natural gas, petroleum, coal, oil sands) together, critics fear that the exploitation of these deposits - originated about 60 million years Paleocene - could bring back its climate through the greenhouse effect again. At the end of the Paleocene, there was a global, sudden temperature rise of about 5-6 ° C. The Paleocene / Eocene Thermal Maximum was triggered by a sudden release of methane. As a source become unstable methane hydrate are discussed on the seabed.

Importance

The pure extraction of methane from its hydrate in the sea floor is not yet economically, and the large amounts of methane bound can at most an energy carrier of the future hope. Because methane hydrate decomposed in the upper water column at lower pressure and higher temperature and thereby escape large amounts of gaseous methane, the reduction of Methanhydratfelder is difficult. The first and only industrial- commercial plant on earth is mined in the methane hydrate is located in the Siberian city of Krasnoyarsk. In Japan, the Norwegian Spitsbergen and other countries is already being intensively researched.

Methane hydrate seems great influence on the climate to have, because methane is a greenhouse gas with - viewed 23 times stronger effect than carbon dioxide over 100 years ( see Global warming and methane hydrate ) - According to William Dillon of the U.S. Geological Survey.

Methanhydratfelder in the Gulf of Mexico are the habitat of polychaetes Hesiocaeca methanicola and appearing with him methanotrophs.

The presence of large quantities of methane hydrate in the Bermuda Triangle is among other things as explanation for the phenomena occurring allegedly used there (see the Bermuda Triangle, Section methane hydrate deposits and blowout ). Scientists like William Dillon deem it unlikely, but concede that large releases of methane would reduce the density of the affected water masses so that ships could no longer swim it.

Geologically are ahead of Norway and been proven in the Caribbean methane gas releases which by sea-level fall ( pressure reduction ), slipping exposed Hydratmassen and tsunamis could have been triggered.

Another reason for the intense research in the field of gas hydrates is the possible use of Kohlenstoffdioxidhydrat as Kohlenstoffdioxidspeicher. In this case, CO2 is to be stored as the hydrate at the seabed. At the same time methane would be released from the seabed by its introduction.

Global warming and methane hydrate

The stability of methane hydrate can be affected due to the induced global warming pressure and temperature changes in the ocean. With a warming of the ground water, the thickness of the stability zone of methane hydrates is reduced. So far, stable methane hydrate become unstable - decomposes the methane hydrate, methane is released and goes partially in the gaseous state. Above a temperature increase of 3 degrees Kelvin of the surrounding water, the thickness of the methane hydrate stability zone decreases significantly, with a temperature increase of 8 K it disappears completely. Even with small changes in temperature can cause a gas escape into the open water. With the collapse of the methane hydrates continue to the stability of the soil and landslides reduced ( Storegga effect) as well as tsunamis can be the result.

Once the methane hydrate is outside the stability zone, free methane forms in sediments and below the hydrate layer. The free methane can escape by diffusion or gas rising from the sea floor in the ocean water, under certain circumstances, larger quantities can accumulate, which are then released abruptly ( blowout ). Cracked out single methane hydrate without Sedimentanhaftungen are lighter than water, rise and decay partly on their way up and transport the methane quickly to higher water layers.

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

Escape of methane in large scale in the atmosphere is a danger especially on the long term. Due to the stable temperature stratification and the slow mixing of the oceans, higher water temperatures will only become clear in the course of centuries on the seabed and in deeper sediment layers. Only in well-mixed relatively shallow ocean areas with hydrate in surface sediment is a short term release to be expected.

In the course of global warming may lead to high methane concentrations in the atmosphere to an intensifying feedback by chronic release of methane over thousands of years, since the methane increases the greenhouse effect. Abschmelzendes continental ice, the water of the sea level rises, thus increasing the pressure on the seafloor, the hydrate can stabilize only to a small extent. Strong release of methane seems to be done maximum 2 55 million years ago during the Paleocene / Eocene temperature maximum and the Eocene Thermal. It was then that a global warming of the atmosphere; geologists find in the rocks of that time today evidence of a rapid increase in the methane content of the air. The resulting consequences could affect the Earth's climate over tens of thousands of years.

In addition to the methane hydrates in the ocean, the methane contained in the permafrost may be released by the heating as soon as the ground thaws.

Possible consequences of the release

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

With the dissolution of the hydrate methane is released. It rises from the sediment slowly in the form of small bubbles in the clear water. On his way to the surface it can be broken down in two ways. On the one hand, the methane can be oxidized anaerobically SO42 - still in the seabed caused by bacteria and archaea with the aid of. On the other hand, it may be oxidized aerobically by bacteria in the water column, where CO2 is released. By this oxidation, there is a decrease in the oxygen concentration and the CO 2 formed is increased, the concentration of carbonic acid ( H2CO3 ) in the ocean thus contributing to a further acidification ( dissociation of carbonic acid H2CO3 → HCO 3 H ). In the long term, it is expected that a new carbon dioxide balance between ocean and atmosphere forms. About one-fifth of the carbon dioxide formed in the ocean is discharged into the atmosphere. The atmospheric CO2 concentration would increase accordingly and continue to contribute to the greenhouse effect.

If methane is abruptly released - it may be caused by a destabilization of methane by a landslide, as happened, for example ahead of Norway in the so-called Storegga landslide about 8000 years ago - can rise to the sea surface, a large portion of the gaseous methane and the increase methane concentration in the atmosphere greatly. The atmospheric methane is oxidised to CO2 by an average life of approximately 8 years.

Also for the navigation, it is rising methane pose a threat. Scottish scientists to the fact, for example, returns the sinking of a discovered in the North Sea in Hexenloch fishing boat. The rising gas bubbles can therefore reduce the density of sea water so much that ships lose their buoyancy.

Mining methods

Even without global warming can be released during the degradation of methane hydrate as the degradation of crude oil and natural gas methane. There is a risk that there will be mass harmful releases.

According to statements by Timothy Collet from the U.S. Geological Survey, the expectations of the existing and recoverable amount of methane hydrate in the area of Alaska and northern Canada were far exceeded. It is estimated that the quantities stored excel at gas hydrate resources of recoverable gas from conventional far.