Methane clathrate
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[[Image:Burning hydrate inlay US Office Naval Research.jpg|right|frame|"Burning ice". Methane, released by heating, burns; water drips.<br>
Inset: clathrate structure (University of Göttingen, GZG. Abt. Kristallographie).<br>
Source: [[USGS]]]]
'''Methane clathrate''', also called '''methane hydrate''' or '''methane ice''', is a solid form of water that contains a large amount of [[methane]] within its [[crystal]] structure (a [[clathrate hydrate]]). Originally thought to occur only in the outer regions of the [[Solar System]] where temperatures are low and water ice is common, significant deposits of methane clathrate have been found under sediments on the [[ocean]] floors of [[Earth]]. <ref> {{Citation | title=Old Gas, New Gas | author=Roald Hoffmann | publisher=American Scientist | volume=94 | issue=1 | pages=pp. 16–18 | year=2006}} </ref>
Methane clathrates are common constituents of the shallow marine geosphere, and they occur both in deep sedimentary structures, and as outcrops on the ocean floor. Methane hydrates are believed to form by migration of gas from depth along [[geological fault]]s, followed by precipitation, or crystallization, on contact of the rising gas stream with cold sea water. Methane clathrates are also present in deep Antarctic [[ice cores]], and store a record of atmospheric methane concentrations, dating to 800,000 years ago.<ref> {{Citation | title=High resolution carbon dioxide concentration record 650,000-800,000 years before present | author=Dieter Luthi, et.al. | publisher=Nature | volume=453 | pages=pp. 379–382 | year=2008}}</ref> The ice-core methane clathrate record is a primary source of data for [[global warming]] research, along with oxygen and carbon dioxide.
At higher pressures, methane clathrates remain stable at temperatures up to 18 °C. The average methane clathrate hydrate composition is 1 [[mole (unit)|mole]] of methane for every 5.75 moles of water, though this is dependent on how many methane molecules "fit" into the various cage structures of the water lattice. The observed density is around 0.9 g/cm³. One liter of methane clathrate solid would therefore contain, on average, 168 liters of methane gas (at [[Standard temperature and pressure|STP]]).
Methane forms a '''structure I hydrate''' with two [[dodecahedron|dodecahedral]] (20 vertices thus 20 water molecules) and six [[tetradecahedron|tetradecahedral]] (24 water molecules) water cages per unit cell. The hydration value of 20 can be determined experimentally by [[Magic angle spinning|MAS NMR]].<ref name="Dec 2005">{{cite journal| title=Direct Measure of the Hydration Number of Aqueous Methane| first=Steven F.| last= Dec| coauthors= Kristin E. Bowler, Laura L. Stadterman, Carolyn A. Koh, and E. Dendy Sloan, Jr.| journal= [[J. Am. Chem. Soc.]]| year=2006| volume=128| issue=2| pages=414–415| doi=10.1021/ja055283f}} Note: the number 20 is called a [[magic number (chemistry)|magic number]] equal to the number found for the amount of water molecules surrounding a [[hydronium ion]].</ref> A methane clathrate spectrum recorded at 275 [[kelvin|K]] and 3.1 [[MPa]] shows a peak for each cage type and a separate peak for [[gas phase]] methane. Recently, a '''clay-methane hydrate intercalate''' was synthesized in which a methane hydrate complex was introduced at the interlayer of a Na-rich montmorillonite clay. The upper temperature stability of this phase is similar to that of structure I hydrate.<ref name="Guggenheim 2003">{{cite journal| last=Guggenheim| first= S| coauthors= Koster van Groos AF| year=2003| title=New gas-hydrate phase: Synthesis and stability of clay-methane hydrate intercalate| journal=Geology| volume= 31| issue=7| pages= 653–656| doi=10.1130/0091-7613(2003)031<0653:NGPSAS>2.0.CO;2}}</ref>
== Natural deposits ==
[[Image:Gas hydrates 1996.svg|right|thumbnail|350px|Worldwide distribution of confirmed or inferred offshore gas hydrate-bearing sediments, 1996.<br>Source: [[USGS]]]]
Methane clathrates are restricted to the shallow [[lithosphere]] (i.e. < 2000 m depth). Furthermore, necessary conditions are found only either in polar continental [[sedimentary rock]]s where surface temperatures are less than 0 °C; or in oceanic [[sediment]] at water depths greater than 300 m where the [[water mass|bottom water]] temperature is around 2 °C. In addition, deep lakes may host gas hydrates as well, e.g. the freshwater Lake Baikal, Siberia<ref name="Vanneste 2001">{{cite journal| last=Vanneste| first= M. et al.| year=2001| title= Multi-frequency seismic study of gas hydrate-bearing sediments in Lake Baikal, Siberia| journal=Marine Geology| volume=172| pages=1–21| doi= 10.1016/S0025-3227(00)00117-1}}</ref>. Continental deposits have been located in [[Siberia]] and [[Alaska]] in [[sandstone]] and [[siltstone]] beds at less than 800 m depth. Oceanic deposits seem to be widespread in the [[continental shelf]] (see Fig.) and can occur within the sediments at depth or close to the sediment-water interface. They may cap even larger deposits of gaseous methane.<ref name="Kvenvolden 1995">{{cite journal| last=Kvenvolden| first= K.| year=1995| title= A review of the geochemistry of methane in natural gas hydrate| journal= Organic Geochemistry| volume=23| issue=11-12| pages=997–1008| doi= 10.1016/0146-6380(96)00002-2}}</ref>
===Oceanic===
There are two distinct types of oceanic deposit. The most common is dominated (> 99%) by [[methane]] contained in a structure I [[clathrate]] and generally found at depth in the sediment. Here, the methane is isotopically light ([[Carbon-13|δ<sup>13</sup>C]] < -60‰) which indicates that it is derived from the microbial [[redox|reduction]] of [[carbon dioxide|CO]]<sub>2</sub>. The clathrates in these deep deposits are thought to have formed in-situ from the microbially-produced methane, as the δ<sup>13</sup>C values of clathrate and surrounding dissolved methane are similar.<ref name="Kvenvolden 1995"/>
These deposits are located within a mid-depth zone around 300-500 m thick in the sediments (the [[Gas Hydrate Stability Zone]], or GHSZ) where they coexist with methane dissolved in the pore-waters. Above this zone methane is only present in its dissolved form at concentrations that decrease towards the sediment surface. Below it, methane is gaseous. At [[Blake Ridge]] on the Atlantic [[continental rise]], the GHSZ started at 190 m depth and continued to 450 m, where it reached [[phase equilibrium|equilibrium]] with the gaseous phase. Measurements indicated that methane occupied 0-9% by volume in the GHSZ, and ~12% in the gaseous zone.<ref name="Dickens 1997">{{cite journal| last=Dickens| first= GR| coauthors= Paull CK, Wallace P| year=1997| title=Direct measurement of in situ methane quantities in a large gas-hydrate reservoir| journal=Nature| volume= 385| issue=6615| pages= 426–428| doi=10.1038/385426a0}}</ref>
In the less common second type found near the sediment surface some samples have a higher proportion of longer-chain [[hydrocarbon]]s (<99% methane) contained in a structure II clathrate. Methane is isotopically heavier ([[Carbon-13|δ<sup>13</sup>C]] is -29 to -57 ‰) and is thought to have migrated upwards from deep sediments where methane was formed by thermal decomposition of [[organic matter]]. Examples of this type of deposit have been found in the [[Gulf of Mexico]] and the [[Caspian Sea]].<ref name="Kvenvolden 1995"/>
Some deposits have characteristics intermediate between the microbially- and thermally-sourced types and are considered to be formed from a mixture of the two.
The methane in gas hydrates is dominantly generated by bacterial degradation of organic matter in low oxygen environments. Organic matter in the uppermost few centimetres of sediments is first attacked by aerobic bacteria, generating CO<sub>2</sub>, which escapes from the sediments into the water column. In this region of aerobic bacterial activity sulfates are reduced to sulfides. If the sedimentation rate is low (<1 cm/kyr), the organic carbon content is low (<1% ), and oxygen is abundant, aerobic bacteria use up all the organic matter in the sediments. But where sedimentation rates and the organic carbon content are high, the pore waters in the sediments are anoxic at depths of only a few cm, and methane is produced by anaerobic bacteria. This production of methane is a rather complicated process, requires the activity of several varieties of bacteria, a reducing environment (Eh -350 to -450 mV), and a pH between 6 and 8. In some regions (e.g., Gulf of Mexico) methane in clathrates may be at least partially derived from thermal degradation of organic matter, dominantly in petroleum.<ref>Kvenvolden, 1998</ref> The methane in clathrates typically has a bacterial isotopic signature and highly variable δ<sup>13</sup>C (-40 to -100‰), with an approximate average of about -65 ‰ .Kvenvolden, 1993; Dickens et al., 1995; <ref name="Matsumoto 1995">{{cite journal| last=Matsumoto| first= R.| year=1995| title=Causes of the δ<sup>13</sup>C anomalies of carbonates and a new paradigm 'Gas Hydrate Hypothesis'| journal= Jour. Geol. Soc. Japan| volume= 101| pages=902–924}}</ref> Below the zone of solid clathrates, large volumes of methane may occur as bubbles of free gas in the sediments.<ref>Dickens et al., 1997</ref><ref name="Matsumoto 1996">{{cite journal| last=Matsumoto| first= R.| coauthors=Watanabe, Y., Satoh, M., Okada, H., Hiroki, Y., Kawasaki, M., and ODP Leg 164 Shipboard Scientific Party| year= 1996| title= Distribution and occurrence of marine gas hydrates - preliminary results of ODP Leg 164: Blake Ridge Drilling| journal= J. Geol. Soc. Japan| volume= 102| pages=932–944}}</ref><ref>[http://ethomas.web.wesleyan.edu/ees123/clathrate.htm Clathrates - little known components of the global carbon cycle<!-- Bot generated title -->]</ref>
The presence of clathrates at a given site can often be determined by observation of a "Bottom Simulating Reflector" (BSR), which is a seismic reflection at the sediment to clathrate stability zone interface caused by the unequal densities of normal sediments and those laced with clathrates.
====Reservoir size====
The size of the oceanic methane clathrate reservoir is poorly known, and estimates of its size decreased by roughly an [[order of magnitude]] per decade since it was first recognized that clathrates could exist in the oceans during the 1960s and 70s.<ref name="Milkov 2004">{{cite journal| last=Milkov| first= AV| year=2004| title= Global estimates of hydrate-bound gas in marine sediments: how much is really out there?| journal=Earth-Sci Rev| volume= 66| issue=3-4| pages= 183–197| doi= 10.1016/j.earscirev.2003.11.002}}</ref> The highest estimates (e.g. 3{{e|18}} m³<ref name="Trofimuk 1973">{{cite book| last=Trofimuk| first= A.A.| coauthors= Cherskiy, N.V. and Tsarev, V.P.| year= 1973| title= Accumulation of natural gases in zones of hydrate—formation in the hydrosphere| publisher= Doklady Akademii Nauk SSSR 212| pages=931–934| language=Russian}}</ref>) were based on the assumption that fully dense clathrates could litter the entire floor of the deep ocean. However, improvements in our understanding of clathrate chemistry and sedimentology have revealed that hydrates only form in a narrow range of depths ([[continental shelves]]), only at some locations in the range of depths where they could occur (10-30% of the GHSZ), and typically are found at low concentrations (0.9-1.5% by volume) at sites where they do occur. Recent estimates constrained by direct sampling suggest the global inventory lies between 1{{e|15}} and 5{{e|15}} m³ (1 quadrillion to 5 quadrillion).<ref name="Milkov 2004"/> This estimate, corresponding to 500-2500 gigatonnes carbon (Gt C), is smaller than the 5000 Gt C estimated for all other fossil fuel reserves but substantially larger than the ~230 Gt C estimated for other natural gas sources.<ref name="Milkov 2004"/><ref name="USGS 2000">USGS World Energy Assessment Team, 2000. US Geological Survey world petroleum assessment 2000––description and results. USGS Digital Data Series DDS-60.</ref> The permafrost reservoir has been estimated at about 400 Gt C in the Arctic,<ref>MacDonald, 1990</ref> but no estimates have been made of possible Antarctic reservoirs.
These are large amounts. For comparison the total carbon in the atmosphere is around 700 gigatons<ref>[http://www.agiweb.org/geotimes/nov04/feature_climate.html Geotimes — November 2004 — Methane Hydrate and Abrupt Climate Change<!-- Bot generated title -->]</ref>.
These modern estimates are notably smaller than the 10,000 to 11,000 Gt C (2{{e|16}} m³) proposed by previous workers as a motivation considering clathrates as a fossil fuel resource (MacDonald 1990, Kvenvolden 1998). Lower abundances of clathrates do not rule out their economic potential, but a lower total volume and apparently low concentration at most sites<ref name="Milkov 2004"/> does suggests that only a limited percentage of clathrates deposits may provide an economically viable resource.
=== Continental ===
Methane clathrates in continental rocks are trapped in beds of [[sandstone]] or [[siltstone]] at depths of less than 800 m. Sampling indicates they are formed from a mix of thermally and microbially derived gas from which the heavier hydrocarbons were later selectively removed. These occur in [[Alaska]], [[Siberia]] as well as [[Northern Canada]].
In 2008, Canadian and Japanese researchers extracted a constant stream of natural gas from a test project at the Mallik gas hydrate field in the [[Mackenzie River]] delta. This was the second such drilling at Mallik: the first took place in 2002 and used heat to release methane. In the 2008 experiment, researchers were able to extract gas by lowering the pressure, without heating, requiring significantly less energy.<ref>{{cite news |first=Brodie |last=Thomas |title=Researchers extract methane gas from under permafrost |url=http://www.nnsl.com/northern-news-services/stories/papers/mar31_08ma.html |work=Northern News Services |date=2008-04-31 |accessdate=2008-06-16 }}</ref> The Mallik gas hydrate field was first discovered by [[Imperial Oil]] in 1971-1972.<ref>{{cite web |url=http://gsc.nrcan.gc.ca/gashydrates/mallik2002/index_e.php |title=Geological Survey of Canada, Mallik 2002 |accessdate=2008-06-16 |work=Natural Resources Canada |date=2007-12-20 }}</ref>
=== Commercial use ===
The sedimentary methane hydrate reservoir probably contains 2-10x the currently known reserves of conventional [[natural gas]]. This represents a potentially important future source of [[hydrocarbon]] [[fuel]]. However, in the majority of sites deposits are likely to be too dispersed for economic extraction.<ref name="Milkov 2004"/> Other problems facing commercial exploitation are detection of viable reserves; and development of the technology for extracting methane gas from the hydrate deposits. To date, there has only been one field commercialy produced where some of the gas is thought to have been from Methane clathrates, [[Messoyakha Gas Field]].
A research and development project in [[Japan]] is aiming for commercial-scale extraction by [[2016]].<ref>[http://www.mh21japan.gr.jp/english/mh21/02keii.html Background and organization<!-- Bot generated title -->]</ref> In August of 2006, China announced plans to spend 800 million yuan (US$100 million) over the next 10 years to study natural gas hydrates.<ref>[http://www.chinadaily.com.cn/bizchina/2006-08/25/content_674169_2.htm Agreements to boost bilateral ties<!-- Bot generated title -->]</ref> A potentially economic reserve in the Gulf of Mexico may contain ~10<sup>10</sup> m<sup>3</sup> of gas.<ref name="Milkov 2004"/> Bjørn Kvamme and Arne Graue at the Institute for Physics and technology at the [[University of Bergen]] has developed a method for injecting CO<sub>2</sub> into hydrates and reversing the process; thereby extracting CH<sub>4</sub> by direct exchange <ref>http://www.vg.no/pub/vgart.hbs?artid=184534 Norske forskere bak energirevolusjon, VB nett, in Norwegian </ref>.
== Hydrates in natural gas processing ==
Methane clathrates (hydrates) are also commonly formed during natural gas production operations, when liquid water is condensed in the presence of methane at high pressure. It is known that larger hydrocarbon molecules such as ethane and propane can also form hydrates, although as the molecule length increases (butanes, pentanes), they cannot fit into the water cage structure and tend to destabilise the formation of hydrates.
Once formed, hydrates can block pipeline and processing equipment. They are generally then removed by reducing the pressure, heating them, or dissolving them by chemical means (methanol is commonly used). Care must be taken to ensure that the removal of the hydrates is carefully controlled, because of the risk of massive increases in pressure as the methane is released, and the potential for the hydrate to let go with high velocity is exposed to a high pressure differential.
It is generally preferable to prevent hydrates from forming or blocking equipment. This is commonly achieved by removing water, or by the addition of ethylene glycol (MEG) or methanol, which act to depress the temperature at which hydrates will form. In recent years, development of other forms of hydrate inhibitors have been developed, being Kinetic Hydrate Inhibitors (which dramatically slow the rate of hydrate formation) and anti-agglomerates, which do not prevent hydrates forming, but do prevent them sticking together to block equipment.
== Methane clathrates and climate change ==
{{main|Clathrate Gun Hypothesis}}
Methane is a powerful [[greenhouse gas]]. Despite its short atmospheric [[half life]] of 7 years, methane has a [[global warming potential]] of 62 over 20 years and 21 over 100 years (IPCC, 1996; Berner and Berner, 1996; vanLoon and Duffy, 2000). The sudden release of large amounts of natural gas from methane clathrate deposits has been hypothesized as a cause of past and possibly future [[climate]] changes. Events possibly linked in this way are the [[Permian-Triassic extinction event]], the [[Paleocene-Eocene Thermal Maximum]].
== Natural gas hydrates (NGH) vs. liquified natural gas (LNG) in transportation==
Since methane clathrates are stable at a higher temperature (−20 vs −162 °C) than [[LNG]], there is some interest in converting natural gas into clathrates rather than liquifying it when transporting it by [[LNG carrier|seagoing vessels]]. Accordingly, the production of NGH from NG at the terminal would require a smaller plant than LNG would.
== Methane clathrates in popular fiction ==
{{trivia|date=January 2008}}
The book ''Mother of Storms'' by [[John Barnes (author)|John Barnes]] offers a fictional example of catastrophic climate change caused by methane clathrate release.
Another book is ''[[The Life Lottery]]'' by [[Ian Irvine]], in which unprecedented seismic activity triggers a release of methane hydrate, reversing global cooling.
[[Clive Cussler]]'s ''[[Fire Ice]]'' also mentions methane hydrate. It tells of how a Russian mining industrialist wants to detonate a bomb into three pockets off of the American coast creating large tsunamis. The intent was to swamp Boston, Charleston, and Miami.
In the anime ''[[Ergo Proxy]]'', "the cascading release of methane hydrate" wipes out 85% of life on Earth. This suggests either an explosive event or the release of large quantities of methane from clathrates due to global warming.
In the German bestseller [[The Swarm (novel)|''The Swarm'' (''Der Schwarm'')]], an undersea intelligence known as the Yrr heats methane hydrate deposits to cause tsunamis in the [[North Sea]].
In the movie ''[[Stealth (film)|Stealth]]'', high-end fighter planes use [[Pulse detonation engine]]s which use methane hydrate as the fuel.
In ''The Great Sea Battle'', an episode of [[Zoids: Guardian Force]], the [[Ultrasaurus (Zoids)|Ultrasaurus]] was able to fend off an attack from the [[Death Stinger]] by using depth charges to ignite an undersea pocket of methane hydrate.
In [[Detective Conan: Jolly Roger in the Deep Azure]], an island by the name of Yorioyajima was said to have sunk into the bottom of the sea 300 years ago following an earthquake, which had separated the methane and water molecules of the methane hydrate holding the island up, forming the sea ruin which the story revolves around.
In the anime ''[[Code Geass R2]]'', the protagonist uses a methane hydrate deposit to fend off a naval attack by the Empire of Brittania, capsizing the entire fleet.
The novel ''[[The Far Shore of Time]]'' by [[Frederik Pohl]] features an alien race attempting to destroy humanity by bombing the methane clathrate reserves, thus releasing the gas into the atmosphere.
The [[Namco]] [[PS2]] video game [[Death By Degrees]] revolves around an underground organization called KOMETA. Their intended super weapon was a satellite that could heat up the Methane Hydrate in the ocean (particularly in the [[Bermuda Triangle]]). The result is bubbling which causes ships to lose their [[buoyancy]]. The weapon thus has the power to destroy naval vehicles from afar, sinking them to the bottom of the sea.
== See also ==
* [[Future energy development]]
* [[Gas hydrate]], a more general category, and associated problems and remedies in pipeline transport.
* [[Bermuda Triangle]]: deposits of Methane hydrate have been hypothesised as the reason for some alleged disappearances.
* [[Clathrate compound]]
== References ==
{{reflist}}
== External links ==
*[http://www.spiegel.de/international/world/0,1518,523178,00.html China and India Exploit Icy Energy Reserves]
*[http://geology.usgs.gov/connections/mms/joint_projects/methane.htm USGS Geological Research Activities with U.S. Minerals Management Service - Methane Gas Hydrates]
*[http://kristall.uni-mki.gwdg.de/index.html Uni. Göttingen, GZG Abt. Kristallographie ] -> Research ->Gashydrate (Clathrat-Hydrate) ENG
*[http://www.ifm-geomar.de/ IFM-GEOMAR, Kiel, DE] [http://www.ifm-geomar.de/index.php?id=gh-allgemein&L=0 Burning ice picture]
* [http://www.ornl.gov/info/reporter/no16/methane.htm Methane Hydrates] - discusses U.S. government funding of methane hydrates research
* [http://www.mh21japan.gr.jp/english/mh21/02keii.html A research and development project] in [[Japan]]
*[http://www.pet.hw.ac.uk/research/hydrate/index.htm Centre for Gas Hydrate Research]
*[http://www.netl.doe.gov/scngo/NaturalGas/hydrates/about-hydrates/about_hydrates.htm All about Hydrates]
*[http://www.eee.columbia.edu/research-projects/sustainable_energy/Hydrates/index.html Sustainable energy ]- Carbon Neutral Methane Energy Production from Hydrate Deposits
*[http://www.gsc.nrcan.gc.ca/gashydrates/canada/pdf/natural_gas_studies_in_canada.pdf Natural gas studies in Canada]
*[http://www.straightdope.com/columns/070803.html Are there deposits of methane under the sea? Will global warming release the methane to the atmosphere?]
*[http://www.marcsteinmetz.com/pages/tecflux/etecflux_minis.html Picture story by German photographer Marc Steinmetz]
[[Category:Clathrate hydrates]]
[[Category:Hydrocarbons]]
[[Category:Natural gas]]
[[Category:Alternative energy]]
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