View source for Methane clathrate

{{short description|Methane-water lattice compound}}
{{redirect|Fire ice|the book by Clive Cussler|Fire Ice}}
[[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: [[United States Geological Survey]].]]

'''Methane clathrate''' (CH<sub>4</sub>·5.75H<sub>2</sub>O) or (4CH<sub>4</sub>·23H<sub>2</sub>O), also called '''methane hydrate''', '''hydromethane''', '''methane ice''', '''fire ice''', '''natural gas hydrate''', or '''gas hydrate''', is a solid [[clathrate compound]] (more specifically, a [[clathrate hydrate]]) in which a large amount of [[methane]] is trapped within a [[crystal]] structure of water, forming a solid similar to [[ice]].<ref>{{citation |publisher=U.S. Geological Survey |url=https://woodshole.er.usgs.gov/project-pages/hydrates/what.html |date=31 August 2009 |access-date=28 December 2014 |title=Gas Hydrate: What is it? |archive-url=https://web.archive.org/web/20120614141539/http://woodshole.er.usgs.gov/project-pages/hydrates/what.html |archive-date=June 14, 2012}}</ref><ref name="Hassan 12305–12314">{{Cite journal |last1=Hassan |first1=Hussein |last2=Romanos |first2=Jimmy |date=2023-08-09 |title=Effects of Sea Salts on the Phase Behavior and Synthesis of Methane Hydrates + THF: An Experimental and Theoretical Study |url=https://pubs.acs.org/doi/10.1021/acs.iecr.3c00351 |journal=Industrial & Engineering Chemistry Research |language=en |volume=62 |issue=31 |pages=12305–12314 |doi=10.1021/acs.iecr.3c00351 |issn=0888-5885|url-access=subscription }}</ref><ref>{{Cite journal |title=Coupled Numerical Modeling of Gas Hydrate-BearingSediments: From Laboratory to Field-Scale Analyses |last1=Sánchez |first1=M. |last2=Santamarina |first2=C. |last3=Teymouri |first3=M. |last4=Gai |first4=X. |journal=Journal of Geophysical Research: Solid Earth |volume=123 |issue=12 |pages=10,326-10,348 |year=2018 |doi=10.1029/2018JB015966 |hdl=10754/630330 |url=https://repository.kaust.edu.sa/bitstream/10754/630330/1/S-nchez_et_al-2018-Journal_of_Geophysical_Research__Solid_Earth.pdf |bibcode=2018JGRB..12310326S|s2cid=134394736 |hdl-access=free }}</ref><ref>{{Cite journal |title=A pseudo-kinetic model to simulate phase changes in gas hydrate bearing sediments |last1=Teymouri |first1=M. |last2=Sánchez |first2=M. |last3=Santamarina |first3=C. |journal=Marine and Petroleum Geology |article-number=104519 |year=2020 |volume=120 |doi=10.1016/j.marpetgeo.2020.104519 |bibcode=2020MarPG.12004519T |doi-access=free |hdl=10754/664452 |hdl-access=free }}</ref><ref>{{cite journal |last1=Chong |first1=Z. R. |last2=Yang |first2=S. H. B. |last3=Babu |first3=P. |last4=Linga |first4=P. |last5=Li |first5=X.-S. |date=2016 |title=Review of natural gas hydrates as an energy resource: Prospects and challenges |journal=Applied Energy |volume=162 |pages=1633–1652 |doi=10.1016/j.apenergy.2014.12.061 |bibcode=2016ApEn..162.1633C }}</ref><ref>{{Cite journal|doi=10.1039/C8CS00989A|title=Gas hydrates in sustainable chemistry|year=2020|last1=Hassanpouryouzband|first1=Aliakbar|last2 = Joonaki|first2 = Edris|last3 = Vasheghani Farahani|first3 = Mehrdad|last4 = Takeya|first4 = Satoshi|last5 = Ruppel|first5 = Carolyn|last6 = Yang|first6 = Jinhai|last7 = J. English|first7 = Niall|last8 = M. Schicks|first8 = Judith|last9 = Edlmann|first9 = Katriona|last10 = Mehrabian|first10 = Hadi|last11 = M. Aman|first11 = Zachary|last12 = Tohidi|first12 = Bahman|journal=Chemical Society Reviews|volume=49|issue=15|pages=5225–5309|pmid=32567615|bibcode=2020CSRev..49.5225H |s2cid=219971360|doi-access = free|hdl = 1912/26136|hdl-access = free}}</ref> 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 [[sediment]]s on the [[ocean]] floors of the [[Earth]] (around 1100{{nbsp}}m below the sea level).<ref>{{Cite journal |title=Old Gas, New Gas |author=Roald Hoffmann |journal=[[American Scientist]] |volume=94 |issue=1 |pages=16–18 |year=2006 |doi=10.1511/2006.57.16 |url=https://www.americanscientist.org/article/old-gas-new-gas|url-access=subscription }}</ref> Methane hydrate is formed when hydrogen-bonded water and methane gas come into contact at high pressures and low temperatures in oceans.

Methane clathrates are common constituents of the shallow marine [[geosphere]] and they occur in deep [[Sedimentary rock|sedimentary]] structures and form [[outcrop]]s on the ocean floor. Methane hydrates are believed to form by the precipitation or crystallisation of methane migrating from deep along [[Fault (geology)|geological faults]]. Precipitation occurs when the methane comes in contact with water within the sea bed subject to temperature and pressure. In 2008, research on Antarctic [[Vostok Station]] and [[European Project for Ice Coring in Antarctica#Concordia Station at Dome C|EPICA Dome C]] ice cores revealed that methane clathrates were also present in deep [[Antarctica|Antarctic]] [[ice core]]s and record a history of [[atmospheric methane]] concentrations, dating to 800,000 years ago.<ref>{{Cite journal |title=High resolution carbon dioxide concentration record 650,000–800,000 years before present |journal=[[Nature (journal)|Nature]] |volume=453 |pages=379–382 |year=2008 |doi=10.1038/nature06949 |pmid=18480821 |last1=Lüthi |first1=D |last2=Le Floch |first2=M |last3=Bereiter |first3=B |last4=Blunier |first4=T |last5=Barnola |first5=JM |last6=Siegenthaler |first6=U |last7=Raynaud |first7=D |last8=Jouzel |first8=J |last9=Fischer |first9=H |display-authors=8 |issue=7193 |bibcode=2008Natur.453..379L |s2cid=1382081 |url=https://epic.awi.de/id/eprint/18281/1/Lth2008a.pdf|doi-access=free }}</ref> The ice-core methane clathrate record is a primary source of data for [[global warming]] research, along with oxygen and carbon dioxide.

Methane clathrates used to be considered as a potential source of [[abrupt climate change]], following the [[clathrate gun hypothesis]]. In this scenario, heating causes catastrophic melting and breakdown of primarily undersea hydrates, leading to a massive release of methane and accelerating warming. Current research shows that hydrates react very slowly to warming, and that it's very difficult for methane to reach the atmosphere after dissociation.<ref name="Wallmann2018">{{Cite journal|journal=Nature Communications|year=2018|author=Wallmann|display-authors=et al |title=Gas hydrate dissociation off Svalbard induced by isostatic rebound rather than global warming |volume=9 |issue=1 |article-number=83 |doi=10.1038/s41467-017-02550-9 |pmid=29311564 |pmc=5758787 |bibcode=2018NatCo...9...83W}}</ref><ref>{{cite journal |last1=Mau |first1=S. |last2=Römer |first2=M. |last3=Torres |first3=M. E. |last4=Bussmann |first4=I. |last5=Pape |first5=T. |last6=Damm |first6=E. |last7=Geprägs |first7=P. |last8=Wintersteller |first8=P. |last9=Hsu |first9=C.-W. |last10=Loher |first10=M. |last11=Bohrmann |first11=G. |date=23 February 2017 |title=Widespread methane seepage along the continental margin off Svalbard - from Bjørnøya to Kongsfjorden |journal=Scientific Reports |volume=7 |article-number=42997 |doi=10.1038/srep42997 |pmid=28230189 |pmc=5322355 |bibcode=2017NatSR...742997M |s2cid=23568012 |doi-access=free }}</ref> Some active seeps instead act as a minor [[carbon sink]], because with the majority of methane dissolved underwater and encouraging [[methanotroph]] communities, the area around the seep also becomes more suitable for [[phytoplankton]].<ref>{{cite journal |last1=Pohlman |first1=John W. |last2=Greinert |first2=Jens |last3=Ruppel |first3=Carolyn |last4=Silyakova |first4=Anna |last5=Vielstädte |first5=Lisa |last6=Casso |first6=Michael |last7=Mienert |first7=Jürgen |last8=Bünz |first8=Stefan |date=1 February 2020 |title=Enhanced CO2 uptake at a shallow Arctic Ocean seep field overwhelms the positive warming potential of emitted methane |journal=Proceedings of the National Academy of Sciences |volume=114 |issue=21 |pages=5355–5360 |doi=10.1073/pnas.1618926114 |pmid=28484018 |pmc=5448205 |doi-access=free }}</ref> As the result, methane hydrates are no longer considered one of the [[tipping points in the climate system]], and according to the [[IPCC Sixth Assessment Report]], no "detectable" impact on the global temperatures will occur in this century through this mechanism.<ref name="IPCC AR6 WG1 Ch.5">{{Cite journal |last1=Fox-Kemper |first1=B. |last2=Hewitt |first2=H.T.|author2-link=Helene Hewitt |last3=Xiao |first3=C. |last4=Aðalgeirsdóttir |first4=G. |last5=Drijfhout |first5=S.S. |last6=Edwards |first6=T.L. |last7=Golledge |first7=N.R. |last8=Hemer |first8=M. |last9=Kopp |first9=R.E. |last10=Krinner |first10=G. |last11=Mix |first11=A. |date=2021 |editor-last=Masson-Delmotte |editor-first=V. |editor2-last=Zhai |editor2-first=P. |editor3-last=Pirani |editor3-first=A. |editor4-last=Connors |editor4-first=S.L. |editor5-last=Péan |editor5-first=C. |editor6-last=Berger |editor6-first=S. |editor7-last=Caud |editor7-first=N. |editor8-last=Chen |editor8-first=Y. |editor9-last=Goldfarb |editor9-first=L. |title=Chapter 5: Global Carbon and other Biogeochemical Cycles and Feedbacks |journal=Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Full_Report.pdf |publisher=Cambridge University Press, Cambridge, UK and New York, NY, USA |page=5 |doi=10.1017/9781009157896.011}}</ref> Over several millennia, a more substantial {{convert|0.4-0.5|C-change|F-change|sigfig=1}} response may still be seen.<ref name="Schellnhuber2018">{{Cite journal |last1=Schellnhuber |first1=Hans Joachim |last2=Winkelmann |first2=Ricarda |last3=Scheffer |first3=Marten |last4=Lade |first4=Steven J. |last5=Fetzer |first5=Ingo |last6=Donges |first6=Jonathan F. |last7=Crucifix |first7=Michel |last8=Cornell |first8=Sarah E. |last9=Barnosky |first9=Anthony D. |author-link9=Anthony David Barnosky |date=2018 |title=Trajectories of the Earth System in the Anthropocene |journal=[[Proceedings of the National Academy of Sciences]] |volume=115 |issue=33 |pages=8252–8259 |bibcode=2018PNAS..115.8252S |doi=10.1073/pnas.1810141115 |issn=0027-8424 |pmc=6099852 |pmid=30082409 |doi-access=free}}</ref>

== General ==
Methane hydrates were discovered in Russia in the 1960s, and studies for extracting gas from it emerged at the beginning of the 21st century.<ref name=BBCMay17>{{cite web |url=https://www.bbc.com/news/world-asia-china-39971667 |title=China claims breakthrough in mining 'flammable ice' |publisher=BBC |date=May 19, 2017}}</ref>

== Structure and composition ==
[[File:Gas_Hydrate_Crystals.jpg|thumb|microscope image]]
The nominal methane clathrate hydrate composition is (CH<sub>4</sub>)<sub>4</sub>(H<sub>2</sub>O)<sub>23</sub>, or 1 [[mole (unit)|mole]] of methane for every 5.75 moles of water, corresponding to 13.4% methane by mass, although the actual composition is dependent on how many methane molecules fit into the various cage structures of the water [[Crystal structure|lattice]]. The observed density is around 0.9 g/cm<sup>3</sup>, which means that methane hydrate will float to the surface of the sea or of a lake unless it is bound in place by being formed in or anchored to sediment.<ref name=Max>{{Cite book |last=Max |first=Michael D. |title=Natural Gas Hydrate in Oceanic and Permafrost Environments |publisher=Kluwer Academic Publishers |year=2003 |page=62 |url=https://books.google.com/books?id=fd8QFKwcSskC |isbn=978-0-7923-6606-5}}</ref> One litre of fully saturated methane clathrate solid would therefore contain about 120 grams of methane (or around 169 litres of methane gas at 0&nbsp;°C and 1 atm),{{#tag:ref|The average methane clathrate hydrate composition is 1 [[mole (unit)|mole]] of methane for every 5.75 moles of water. The observed density is around 0.9 g/cm<sup>3</sup>.<ref name=Max/> For one mole of methane, which has a molar mass of about 16.043&nbsp;g (see [[Methane]]), we have 5.75 moles of water, with a molar mass of about 18.015&nbsp;g (see [[Properties of water]]), so together for each mole of methane the clathrate complex has a mass of {{nowrap|16.043 g + 5.75 × 18.015 g}} ≈ 119.631&nbsp;g. The fractional contribution of methane to the mass is then equal to {{nowrap|16.043 g / 119.631 g}} ≈ 0.1341. The density is around 0.9&nbsp;g/cm<sup>3</sup>, so one litre of methane clathrate has a mass of around 0.9&nbsp;kg, and the mass of the methane contained therein is then about {{nowrap|0.1341 × 0.9 kg}} ≈ 0.1207&nbsp;kg. At a density as a gas of 0.716&nbsp;kg/m<sup>3</sup> (at 0&nbsp;°C; see the info box at ''[[Methane]]''), this comes to a volume of {{nowrap|0.1207 / 0.716 m<sup>3</sup>}} = 0.1686&nbsp;m<sup>3</sup> = 168.6&nbsp;L.|group="nb"}} or one cubic metre of methane clathrate releases about 160 cubic metres of gas.<ref name=BBCMay17 />

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. (Due to sharing of each water molecule between four cages, there are 46 water molecules and eight methane molecules per unit cell.){{citation needed|date=October 2025}} This compares with a [[hydration number]] of 20 for methane in aqueous solution.<ref name="Dec 2005">{{cite journal |title=Direct Measure of the Hydration Number of Aqueous Methane |journal=[[Journal of the American Chemical Society|J. Am. Chem. Soc.]] |year=2006 |volume=128 |issue=2 |pages=414–415 |doi=10.1021/ja055283f |pmid=16402820 |last1=Dec |first1=Steven F. |last2=Bowler |first2=Kristin E. |last3=Stadterman |first3=Laura L. |last4=Koh |first4=Carolyn A. |last5=Sloan |first5=E. Dendy |bibcode=2006JAChS.128..414D }} 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 [[Magic angle spinning|MAS NMR]] spectrum recorded at 275 [[kelvin|K]] and 3.1 [[Pascal (unit)|MPa]] shows a peak for each cage type and a separate peak for [[gas phase]] methane.{{Citation needed|date=September 2010}} In 2003, a clay-methane hydrate intercalate was synthesized in which a methane hydrate complex was introduced at the interlayer of a sodium-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 |author2=Koster van Groos AF |year=2003 |title=New gas-hydrate phase: Synthesis and stability of clay-methane hydrate intercalate |journal=[[Geology (journal)|Geology]] |volume=31 |issue=7 |pages=653–656 |doi=10.1130/0091-7613(2003)031<0653:NGPSAS>2.0.CO;2 |bibcode=2003Geo....31..653G}}</ref>
[[File:Methane Hydrate phase diagram.jpg|thumb|Methane hydrate phase diagram. The horizontal axis shows temperature from {{val|-15|to|33|u=degC}}, the vertical axis shows pressure from 0 to 120,000 kilopascals (0 to 1,184 atmospheres). Hydrate forms above the line. For example, at 4&nbsp;°C hydrate forms above a pressure of about 50 atm/5000 kPa, found at about 500&nbsp;m sea depth.]]

== Natural deposits ==
{{see also|Methane hydrate stability zone}}
[[Image:Gas hydrates 1996.svg|thumb|Worldwide distribution of confirmed or inferred offshore gas hydrate-bearing sediments, 1996.<br />Source: [[USGS]]]]
[[Image:Gashydrat im Sediment.JPG|thumbnail|Gas hydrate-bearing sediment, from the subduction zone off Oregon]]
[[Image:Gashydrat mit Struktur.jpg|thumbnail|Specific structure of a gas hydrate piece, from the subduction zone off Oregon]]

Methane clathrates are restricted to the shallow [[lithosphere]] (i.e. <&nbsp;2,000&nbsp;m depth). Furthermore, necessary conditions are found only in either continental [[sedimentary rock]]s in polar regions where average surface temperatures are less than 0&nbsp;°C; or in oceanic [[sediment]] at water depths greater than 300&nbsp;m where the [[water mass|bottom water]] temperature is around 2&nbsp;°C. In addition, deep fresh water lakes may host gas hydrates as well, e.g. the fresh water [[Lake Baikal]], Siberia.<ref name="Vanneste 2001">{{Cite journal |last1=Vanneste |first1=M. |year=2001 |title=Multi-frequency seismic study of gas hydrate-bearing sediments in Lake Baikal, Siberia |journal=Marine Geology |volume=172 |issue=1–2 |pages=1–21 |doi=10.1016/S0025-3227(00)00117-1 |last2=De Batist |first2=M |last3=Golmshtok |first3=A |last4=Kremlev |first4=A |last5=Versteeg |first5=W |display-authors=etal |bibcode=2001MGeol.172....1V |title-link=Lake Baikal}}</ref> Continental deposits have been located in [[Siberia]] and [[Alaska]] in [[sandstone]] and [[siltstone]] beds at less than 800&nbsp;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 |bibcode=1995OrGeo..23..997K |url=http://www.che.ncsu.edu/ILEET/CHE596web_Fall2011/resources/natural-gas/Methane%20Hydrate-Geochemistry.pdf |access-date=28 December 2014 |archive-url=https://web.archive.org/web/20141228210733/http://www.che.ncsu.edu/ILEET/CHE596web_Fall2011/resources/natural-gas/Methane%20Hydrate-Geochemistry.pdf |archive-date=28 December 2014}}</ref>

=== Oceanic ===
Methane hydrate can occur in various forms like massive, dispersed within pore spaces, nodules, veins/fractures/faults, and layered horizons.<ref>{{cite journal |last1=Mishra |first1=C. K. |last2=Dewangan |first2=P |last3=Mukhopadhyay |first3=R |last4=Banerjee |first4=D |title=Available online 7 May 2021 1875-5100/© 2021 Elsevier B.V. All rights reserved. Velocity modeling and attribute analysis to understand the gas hydrates and free gas system in the Mannar Basin, India |journal=Journal of Natural Gas Science and Engineering |date=August 2021 |volume=92 |article-number=104007 |doi=10.1016/j.jngse.2021.104007 |s2cid=235544441 |url=https://www.sciencedirect.com/science/article/pii/S1875510021002134|url-access=subscription }}</ref> Generally, it is found unstable at standard pressure and temperature conditions, and 1 m<sup>3</sup> of methane hydrate upon dissociation yields about 164 m<sup>3</sup> of methane and 0.87 m<sup>3</sup> of freshwater.<ref>{{Cite book|last=Sloan|first=E. Dendy|title=Clathrate hydrates of natural gases.|date=2008|publisher=CRC Press|others=Carolyn A. Koh|isbn=978-1-4200-0849-4|edition=3rd|location=Boca Raton, FL|oclc=85830708}}</ref><ref>{{cite journal |last1=Mishra |first1=C K |last2=Dewangan |first2=P |last3=Sriram |first3=G |last4=Kumar |first4=A |last5=Dakara |first5=G |title=Spatial distribution of gas hydrate deposits in Krishna-Godavari offshore basin, Bay of Bengal |journal=Marine and Petroleum Geology |year=2020 |volume=112 |article-number=104037 |doi=10.1016/j.marpetgeo.2019.104037 |bibcode=2020MarPG.11204037M |doi-access=free }}</ref><ref>{{cite journal |last1=Kvenvolden |first1=K A |title=Gas hydrates-geological perspective and global change |journal=Reviews of Geophysics |year=1993 |volume=31 |issue=2 |pages=173–187 |doi=10.1029/93RG00268 |bibcode=1993RvGeo..31..173K }}</ref> There are two distinct types of oceanic deposits.  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 ([[δ13C|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 since the δ<sup>13</sup>C values of clathrate and surrounding dissolved methane are similar.<ref name="Kvenvolden 1995" />  However, it is also thought that freshwater used in the pressurization of oil and gas wells in permafrost and along the continental shelves worldwide combines with natural methane to form clathrate at depth and pressure since methane hydrates are more stable in freshwater than in saltwater.<ref name="Hassan 12305–12314"/>  Local variations may be widespread since the act of forming hydrate, which extracts pure water from saline formation waters, can often lead to local and potentially significant increases in formation water salinity.  Hydrates normally exclude the salt in the pore fluid from which it forms. Thus, they exhibit high electric resistivity like ice, and sediments containing hydrates have higher resistivity than sediments without gas hydrates (Judge [67]).<ref name="MH_future_2011">{{citation |title=Methane Hydrates and the Future of Natural Gas |first=Carolyn |last=Ruppel |series=Gas Hydrates Project |publisher=U.S. Geological Survey |location=Woods Hole, MA |url=https://mitei.mit.edu/system/files/Supplementary_Paper_SP_2_4_Hydrates.pdf |access-date=28 December 2014 |archive-url=https://web.archive.org/web/20151106085926/http://mitei.mit.edu/system/files/Supplementary_Paper_SP_2_4_Hydrates.pdf |archive-date=6 November 2015}}</ref>{{rp|9}}

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 fresh, not salt, 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&nbsp;m depth and continued to 450&nbsp;m, where it reached [[phase equilibrium|equilibrium]] with the gaseous phase. Measurements indicated that methane occupied {{val|0|-|9|u=%}} by volume in the GHSZ, and ~12% in the gaseous zone.<ref name="Dickens 1997">{{Cite journal |last=Dickens |first=GR |author2=Paull CK |author3=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 |bibcode=1997Natur.385..426D |url=https://deepblue.lib.umich.edu/bitstream/2027.42/62828/1/385426a0.pdf |hdl=2027.42/62828|s2cid=4237868 |hdl-access=free }}</ref><ref name=sautter2012>
{{cite web |url=http://oceanexplorer.noaa.gov/explorations/04etta/background/profile/profile.html |title=A Profile of the Southeast U.S. Continental Margin |author=Leslie R. Sautter |work=NOAA Ocean Explorer |publisher=National Oceanic and Atmospheric Administration (NOAA) |access-date=3 January 2015}}</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. Carbon from this type of clathrate 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 formed from a mixture of the two.

The methane in gas hydrates is dominantly generated by microbial consortia degrading organic matter in low oxygen environments, with the methane itself produced by [[methanogen]]ic [[archaea]]. Organic matter in the uppermost few centimeters of sediments is first attacked by aerobic bacteria, generating CO<sub>2</sub>, which escapes from the sediments into the [[water column]]. Below this region of aerobic activity, anaerobic processes take over, including, successively with depth, the microbial reduction of nitrite/nitrate, metal oxides, and then [[sulfate]]s are reduced to [[sulfide]]s.  Finally, methanogenesis becomes a dominant pathway for organic carbon [[remineralization]].

If the sedimentation rate is low (about 1&nbsp;cm/yr), the organic carbon content is low (about 1%), and oxygen is abundant, aerobic bacteria can use up all the organic matter in the sediments faster than oxygen is depleted, so lower-energy [[electron acceptors]] are not used. But where sedimentation rates and the organic carbon content are high, which is typically the case on continental shelves and beneath western boundary current upwelling zones, the [[pore water]] in the sediments becomes [[Hypoxia (environmental)|anoxic]] at depths of only a few centimeters or less.  In such organic-rich marine sediments, sulfate becomes the most important terminal electron acceptor due to its high concentration in [[seawater]]. However, it too is depleted by a depth of centimeters to meters.  Below this, methane is produced. This production of methane is a rather complicated process, requiring a highly reducing environment (Eh {{val|-350|to|-450|u=mV}}) and a pH between 6 and 8, as well as a complex [[syntrophic]], consortia of different varieties of archaea and bacteria. However, it is only archaea that actually emit methane.

In some regions (e.g., Gulf of Mexico, Joetsu Basin) methane in clathrates may be at least partially derive from thermal degradation of organic matter (e.g. petroleum generation), with oil even forming an exotic component within the hydrate itself that can be recovered when the hydrate is disassociated.<ref>Kvenvolden, 1998(incomplete ref)</ref><ref>{{Cite journal |last1=Snyder |first1=Glen T. |last2=Matsumoto |first2=Ryo |last3=Suzuki |first3=Yohey |last4=Kouduka |first4=Mariko |last5=Kakizaki |first5=Yoshihiro |last6=Zhang |first6=Naizhong |last7=Tomaru |first7=Hitoshi |last8=Sano |first8=Yuji |last9=Takahata |first9=Naoto |last10=Tanaka |first10=Kentaro |last11=Bowden |first11=Stephen A. |date=2020-02-05 |title=Evidence in the Japan Sea of microdolomite mineralization within gas hydrate microbiomes |journal=Scientific Reports |language=en |volume=10 |issue=1 |page=1876 |doi=10.1038/s41598-020-58723-y |pmid=32024862 |pmc=7002378 |bibcode=2020NatSR..10.1876S |issn=2045-2322}}</ref>{{Citation needed|date=October 2011}} The methane in clathrates typically has a biogenic isotopic signature and highly variable δ<sup>13</sup>C ({{val|-40|to|-100|u=‰}}), with an approximate average of about −65‰.<ref>Kvenvolden, 1993(incomplete ref)</ref>{{Citation needed|date=October 2011}}<ref>Dickens 1995 (incomplete ref)</ref><ref>{{Cite journal |last1=Snyder |first1=Glen T. |last2=Sano |first2=Yuji |last3=Takahata |first3=Naoto |last4=Matsumoto |first4=Ryo |last5=Kakizaki |first5=Yoshihiro |last6=Tomaru |first6=Hitoshi |date=2020-03-05 |title=Magmatic fluids play a role in the development of active gas chimneys and massive gas hydrates in the Japan Sea |journal=Chemical Geology |language=en |volume=535 |article-number=119462 |doi=10.1016/j.chemgeo.2020.119462 |bibcode=2020ChGeo.53519462S |issn=0009-2541 |doi-access=free}}</ref><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=J. Geol. Soc. Japan |volume=101 |pages=902–924 |doi=10.5575/geosoc.101.902 |issue=11 |doi-access=free}}</ref> Below the zone of solid clathrates, large volumes of methane may form bubbles of free gas in the sediments.<ref name="Dickens 1997"/><ref name="Matsumoto 1996">{{Cite journal |last1=Matsumoto |first1=R. |last2=Watanabe |first2=Y. |last3=Satoh |first3=M. |last4=Okada |first4=H. |last5=Hiroki |first5=Y. |last6=Kawasaki |first6=M. |others=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 |doi=10.5575/geosoc.102.932 |issue=11 |doi-access=free}}</ref><ref>{{cite web |url=http://ethomas.web.wesleyan.edu/ees123/clathrate.htm |title=Clathrates - little known components of the global carbon cycle |publisher=Ethomas.web.wesleyan.edu |date=2000-04-13 |access-date=2013-03-14}}</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.

[[Gas hydrate pingo]]s have been discovered in the Arctic oceans Barents sea. Methane is bubbling from these dome-like structures, with some of these gas flares extending close to the sea surface.<ref>{{cite web |url=https://phys.org/news/2017-06-domes-frozen-methane-blow-outs.html |title=Domes of frozen methane may be warning signs for new blow-outs |publisher=Phys.org |year=2017}}</ref>

==== Reservoir size ====
[[File:Gas_hydrate_under_carbonate_rock.jpg|thumb|Gas hydrate under carbonate rock on the seafloor of the northern Gulf of Mexico]]
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 1970s.<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-Science Reviews |volume=66 |issue=3–4 |pages=183–197 |doi=10.1016/j.earscirev.2003.11.002 |bibcode=2004ESRv...66..183M}}</ref>  The highest estimates (e.g. 3{{e|18}} m<sup>3</sup>)<ref name="Trofimuk 1973">{{Cite journal |last=Trofimuk |first=A. A. |author2=N. V. Cherskiy |author3=V. P. Tsarev |year=1973 |title=[Accumulation of natural gases in zones of hydrate—formation in the hydrosphere] |journal=[[Doklady Akademii Nauk SSSR]] |volume=212 |pages=931–934 |language=ru}}</ref> were based on the assumption that fully dense clathrates could litter the entire floor of the deep ocean. Improvements in our understanding of clathrate chemistry and sedimentology have revealed that hydrates form in only a narrow range of depths ([[continental shelves]]), at only some locations in the range of depths where they could occur (10-30% of the [[Gas hydrate stability zone]]), and typically are found at low concentrations ({{val|0.9|–|1.5|u=%}} by volume) at sites where they do occur. Recent estimates constrained by direct sampling suggest the global inventory occupies between {{convert|1e15|and|5e15|m3|e6mi3|sigfig=2|abbr=off}}.<ref name="Milkov 2004"/>  This estimate, corresponding to 500–2500 gigatonnes carbon (Gt&nbsp;C), is smaller than the 5000 Gt&nbsp;C estimated for all other geo-organic fuel reserves but substantially larger than the ~230 Gt&nbsp;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&nbsp;Gt&nbsp;C in the Arctic,<ref>{{cite journal |last1=MacDonald |first1=G. J. |year=1990 |title=Role of methane clathrates in past and future climates |journal=Climatic Change |volume=16 |issue=3 |pages=247–281 |doi=10.1007/bf00144504 |bibcode=1990ClCh...16..247M|s2cid=153361540 }}</ref>{{Citation needed|date=October 2011}} but no estimates have been made of possible Antarctic reservoirs. These are large amounts. In comparison, the total carbon in the atmosphere is around 800&nbsp;gigatons (see {{section link|Carbon|Occurrence}}).

These modern estimates are notably smaller than the {{val|10,000|to|11,000|u=Gt C}} (2{{e|16}} m<sup>3</sup>) proposed<ref>{{Cite journal |last=Buffett |first=Bruce |author2=David Archer |date=15 November 2004 |title=Global inventory of methane clathrate: sensitivity to changes in the deep ocean |journal=Earth and Planetary Science Letters |volume=227 |issue=3–4 |pages=185–199 |url=http://geosci.uchicago.edu/~archer/reprints/buffett.2004.clathrates.pdf |quote=Preferred ... global estimate of 3<sup>18</sup> g ... Estimates of the global inventory of methane clathrate may exceed 10<sup>19</sup> g of carbon |doi=10.1016/j.epsl.2004.09.005 |bibcode=2004E&PSL.227..185B}}</ref> by previous researchers as a reason to consider clathrates to be a geo-organic 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 suggest 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&nbsp;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]], and [[Northern Canada]].

In 2008, Canadian and Japanese researchers extracted a constant stream of natural gas from a test project at the [[Mallik gas hydrate site]] 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=2008-03-31 |access-date=2008-06-16 |archive-url=https://web.archive.org/web/20080608011136/http://nnsl.com/northern-news-services/stories/papers/mar31_08ma.html |archive-date=2008-06-08}}</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 |access-date=2013-03-21 |work=Natural Resources Canada |date=2007-12-20 |archive-url=https://web.archive.org/web/20110629141558/http://gsc.nrcan.gc.ca/gashydrates/mallik2002/index_e.php |archive-date=June 29, 2011}}</ref>

=== Commercial use ===
Economic deposits of hydrate are termed natural gas hydrate (NGH) and store 164 m<sup>3</sup> of methane, 0.8 m<sup>3</sup> water in 1 m<sup>3</sup> hydrate.<ref>{{Cite book |title=Exploration and Production of Oceanic Natural Gas Hydrate |last1=Max |first1=Michael D. |last2=Johnson |first2=Arthur H. |chapter=Economic Characteristics of Deepwater Natural Gas Hydrate |date=2016-01-01 |publisher=Springer International Publishing |isbn=978-3-319-43384-4 |pages=39–73 |language=en |doi=10.1007/978-3-319-43385-1_2|s2cid=133178393 }}</ref>  Most NGH is found beneath the seafloor (95%) where it exists in thermodynamic equilibrium.  The sedimentary methane hydrate reservoir probably contains 2–10 times the currently known reserves of conventional [[natural gas]], {{as of|2013|lc=y}}.<ref name=ta201304>
{{cite magazine |last=Mann |first=Charles C. |title=What If We Never Run Out of Oil? |magazine=The Atlantic Monthly |date=April 2013 |url=https://www.theatlantic.com/magazine/archive/2013/05/what-if-we-never-run-out-of-oil/309294/ |access-date=23 May 2013}}</ref> This represents a potentially important future source of [[hydrocarbon]] [[fuel]]. However, in the majority of sites deposits are thought 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.

In August 2006, China announced plans to spend 800 million yuan (US$100 million) over the next 10 years to study natural gas hydrates.<ref>{{cite web |url=http://www.chinadaily.com.cn/bizchina/2006-08/25/content_674169_2.htm |title=Agreements to boost bilateral ties |publisher=Chinadaily.com.cn |date=2006-08-25 |access-date=2013-03-14}}</ref> A potentially economic reserve in the Gulf of Mexico may contain approximately {{convert|100|e9m3}} of gas.<ref name="Milkov 2004"/> [[Bjørn Kvamme]] and [[Arne Graue]] at the Institute for Physics and technology at the [[University of Bergen]] have developed a method for injecting {{CO2}} into hydrates and reversing the process; thereby extracting CH<sub>4</sub> by direct exchange.<ref>{{cite web |url=http://www.vg.no/pub/vgart.hbs?artid=184534 |title=Norske forskere bak energirevolusjon, VB nett, in Norwegian |publisher=Vg.no |access-date=2013-03-14 |date=May 2007}}</ref> The University of Bergen's method is being field tested by [[ConocoPhillips]] and state-owned [[Japan Oil, Gas and Metals National Corporation]] (JOGMEC), and partially funded by the U.S. Department of Energy. The project has already reached injection phase and was analyzing resulting data by March 12, 2012.<ref>{{cite web |url=http://www.netl.doe.gov/technologies/oil-gas/FutureSupply/MethaneHydrates/projects/DOEProjects/MH_06553HydrateProdTrial.html |title=The National Methane Hydrates R&D Program DOE/NETL Methane Hydrate Projects |publisher=Netl.doe.gov |date=2013-02-19 |access-date=2013-03-14 |archive-url=https://web.archive.org/web/20130817121951/http://www.netl.doe.gov/technologies/oil-gas/FutureSupply/MethaneHydrates/projects/DOEProjects/MH_06553HydrateProdTrial.html |archive-date=2013-08-17 }}</ref>

On March 12, 2013, JOGMEC researchers announced that they had successfully extracted natural gas from frozen methane hydrate.<ref name=extracts>{{cite news |title=Japan extracts gas from methane hydrate in world first |publisher=BBC |date=March 12, 2013 |url=https://www.bbc.co.uk/news/business-21752441 |access-date=March 13, 2013}}</ref>  In order to extract the gas, specialized equipment was used to drill into and depressurize the hydrate deposits, causing the methane to separate from the ice.  The gas was then collected and piped to surface where it was ignited to prove its presence.<ref name=coup /> According to an industry spokesperson, "It [was] the world's first offshore experiment producing gas from methane hydrate".<ref name=extracts />  Previously, gas had been extracted from onshore deposits, but never from offshore deposits which are much more common.<ref name=coup>{{cite news |title=An Energy Coup for Japan: 'Flammable Ice' |author=Hiroko Tabuchi |author-link=Hiroko Tabuchi |date=March 12, 2013 |work=New York Times |url=https://www.nytimes.com/2013/03/13/business/global/japan-says-it-is-first-to-tap-methane-hydrate-deposit.html |access-date=March 14, 2013}}</ref> The hydrate field from which the gas was extracted is located {{convert|50|km|mi}} from central Japan in the [[Nankai Trough]], {{convert|300|m|ft}} under the sea.<ref name=extracts /><ref name=coup />  A spokesperson for JOGMEC remarked "Japan could finally have an energy source to call its own".<ref name=coup /> Marine geologist Mikio Satoh remarked "Now we know that extraction is possible. The next step is to see how far Japan can get costs down to make the technology economically viable."<ref name=coup />  Japan estimates that there are at least 1.1 trillion cubic meters of methane trapped in the Nankai Trough, enough to meet the country's needs for more than ten years.<ref name=coup />

Both Japan and China announced in May 2017 a breakthrough for [[resource extraction|mining]] methane clathrates, when they extracted methane from hydrates in the [[South China Sea]].<ref name=BBCMay17 /> China described the result as a breakthrough; [[Praveen Linga]] from the Department of Chemical and Biomolecular Engineering at the National University of Singapore agreed "Compared with the results we have seen from Japanese research, the Chinese scientists have managed to extract much more gas in their efforts".<ref>{{Cite news |url=https://www.bbc.com/news/world-asia-china-39971667 |title=China claims breakthrough in 'flammable ice' |work=BBC News |date=2017-05-19}}</ref> Industry consensus is that commercial-scale production remains years away.<ref>{{cite web |url=https://nationalpost.com/news/world/china-japan-extracts-combustible-ice-from-seafloor-a-step-towards-harnessing-a-legendary-frozen-fossil-fuel |title=China and Japan find way to extract 'combustible ice' from seafloor, harnessing a legendary frozen fossil fuel |date=19 May 2017}}</ref>

=== Environmental concerns ===
Experts caution that environmental impacts are still being investigated and that methane—a greenhouse gas with around 81.2 times as much [[global warming potential]] over a 20-year period<ref>Intergovernmental Panel on Climate Change</ref> (GWP20) as carbon dioxide—could potentially escape into the atmosphere if something goes wrong.<ref>{{Cite web |url=https://www.dw.com/en/fire-and-ice-the-untapped-fossil-fuel-that-could-save-or-ruin-our-climate/a-43246890 |title=Fire and ice: The untapped fossil fuel that could save or ruin our climate |last=Hausman |first=Sandy |date=2018-05-31 |website=DW.COM |language=en-GB |access-date=2019-09-14}}</ref> As methane has a much shorter atmospheric lifetime than carbon dioxide, its GWP is much less over longer time periods, with a GWP-100 of 27.9 and a GWP-500 of 7.95.<ref name=":0" />{{Rp|page=7SM-24}}

== Hydrates in natural gas processing ==

=== Routine operations ===
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 like ethane and propane can also form hydrates, although longer molecules (butanes, pentanes) 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 potential for the hydrate to undergo a [[phase transition]] from the solid hydrate to release water and gaseous methane at a high rate when the pressure is reduced. The rapid release of methane gas in a closed system can result in a rapid increase in pressure.<ref name=Max/>

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, like Kinetic Hydrate Inhibitors (increasing the required sub-cooling which hydrates require to form, at the expense of increased hydrate formation rate) and anti-agglomerates, which do not prevent hydrates forming, but do prevent them sticking together to block equipment.

=== Effect of hydrate phase transition during deep water drilling ===
When drilling in oil- and gas-bearing formations submerged in deep water, the reservoir gas may flow into the well bore and form gas hydrates owing to the low temperatures and high pressures found during deep water drilling. The gas hydrates may then flow upward with drilling mud or other discharged fluids. When the hydrates rise, the pressure in the [[Annulus (oil well)|annulus]] decreases and the hydrates dissociate into gas and water. The rapid gas expansion ejects fluid from the well, reducing the pressure further, which leads to more hydrate dissociation and further fluid ejection. The resulting violent expulsion of fluid from the annulus is one potential cause or contributor to the "kick".<ref name=Wang>{{Cite journal |last=Wang |first=Zhiyuan |author2=Sun Baojiang |title=Annular multiphase flow behavior during deep water drilling and the effect of hydrate phase transition |journal=Petroleum Science |volume=6 |pages=57–63 |year=2009 |issue=1 |doi=10.1007/s12182-009-0010-3 |bibcode=2009PetSc...6...57W |doi-access=free}}</ref> (Kicks, which can cause blowouts, typically do not involve hydrates: see {{section link|Blowout (well drilling)|Formation kick}}).

Measures which reduce the risk of hydrate formation include:
* High flow-rates, which limit the time for hydrate formation in a volume of fluid, thereby reducing the kick potential.<ref name=Wang/>
* Careful measuring of line flow to detect incipient hydrate plugging.<ref name=Wang/>
* Additional care in measuring when gas production rates are low and the possibility of hydrate formation is higher than at relatively high gas flow rates.<ref name=Wang/>
* Monitoring of [[Casing (borehole)|well casing]] after it is "[[Shut-in (oil drilling)|shut in]]" (isolated) may indicate hydrate formation. Following "shut in", the pressure rises while gas diffuses through the reservoir to the [[bore hole]]; the rate of pressure rise exhibit a reduced rate of increase while hydrates are forming.<ref name=Wang/>
* Additions of energy (e.g., the energy released by [[Casing (borehole)#Cementing|setting cement]] used in well completion) can raise the temperature and convert hydrates to gas, producing a "kick".

=== Blowout recovery ===
[[File:BP oil containment domes.jpg|thumb|right|Concept diagram of oil containment domes, forming upside-down funnels in order to pipe oil to surface ships. The sunken oil rig is nearby.]]

At sufficient depths, methane complexes directly with water to form methane hydrates, as was observed during the [[Deepwater Horizon oil spill]] in 2010. BP engineers developed and deployed a subsea oil recovery system over oil spilling from a deepwater [[oil well]] {{convert|5000|ft|m}} below [[sea level]] to capture escaping oil. This involved placing a {{convert|125|t|lb|adj=on}} dome over the largest of the well leaks and piping it to a storage vessel on the surface.<ref name=WSJ0503>{{Cite news |url=https://online.wsj.com/article/BT-CO-20100503-700843.html?mod=WSJ_latestheadlines |title=US Oil Spill Response Team: Plan To Deploy Dome In 6–8 Days |first=David |last=Winning |work=Wall Street Journal |publisher=Dow Jones & Company |date=2010-05-03 |access-date=2013-03-21 |archive-url=https://web.archive.org/web/20100506024716/http://online.wsj.com/article/BT-CO-20100503-700843.html?mod=WSJ_latestheadlines |archive-date=May 6, 2010}}</ref> This option had the potential to collect some 85% of the leaking oil but was previously untested at such depths.<ref name=WSJ0503/> BP deployed the system on May 7–8, but it failed due to a buildup of methane clathrate inside the dome; with its low density of approximately 0.9&nbsp;g/cm<sup>3</sup> the methane hydrates accumulated in the dome, adding buoyancy and obstructing flow.<ref>{{Cite news |last=Cressey |first=Daniel |title=Giant dome fails to fix Deepwater Horizon oil disaster |date=10 May 2010 |publisher=Nature.com |url=http://blogs.nature.com/news/thegreatbeyond/2010/05/_giant_dome_fails_to_fix_deepw.html |access-date=10 May 2010}}</ref>

== Methane clathrates and climate change ==
{{excerpt|Clathrate gun hypothesis|paragraphs=1}}
{{excerpt|Clathrate gun hypothesis#Modern deposits|hat=no}}
{{excerpt|Clathrate gun hypothesis#East Siberian Arctic Shelf|hat=no}}
{{excerpt|Clathrate gun hypothesis#Svalbard|hat=no}}

{{excerpt|Clathrate gun hypothesis#Current outlook|paragraphs=2-3|hat=no}}

== Natural gas hydrates for gas storage and transportation ==
Since methane clathrates are stable at a higher temperature than [[liquefied natural gas|liquefied natural gas (LNG)]] (−20&nbsp;°C vs −162&nbsp;°C), there is some interest in converting natural gas into clathrates (Solidified Natural Gas or SNG) rather than liquifying it when transporting it by [[LNG carrier|seagoing vessels]]. A significant advantage would be that the production of natural gas hydrate (NGH) from natural gas at the terminal would require a smaller refrigeration plant and less energy than LNG would. Offsetting this, for 100 tonnes of methane transported, 750 tonnes of methane hydrate would have to be transported; since this would require a ship of 7.5 times greater displacement, or require more ships, it is unlikely to prove economically feasible.{{citation needed|date=February 2014}} Recently, methane hydrate has received considerable interest for large scale stationary storage application due to the very mild storage conditions with the inclusion of [[tetrahydrofuran]] (THF) as a co-guest.<ref>{{Cite journal |doi=10.1016/j.cej.2016.01.026 |title=Rapid methane hydrate formation to develop a cost effective large scale energy storage system |journal=Chemical Engineering Journal |volume=290 |pages=161–173 |year=2016 |last1=Veluswamy |first1=Hari Prakash |last2=Wong |first2=Alison Jia Hui |last3=Babu |first3=Ponnivalavan |last4=Kumar |first4=Rajnish |last5=Kulprathipanja |first5=Santi |last6=Rangsunvigit |first6=Pramoch |last7=Linga |first7=Praveen |bibcode=2016ChEnJ.290..161V }}</ref><ref>{{Cite journal |doi=10.1016/j.apenergy.2018.02.059 |title=A review of solidified natural gas (SNG) technology for gas storage via clathrate hydrates |journal=Applied Energy |volume=216 |pages=262–285 |year=2018 |last1=Veluswamy |first1=Hari Prakash |last2=Kumar |first2=Asheesh |last3=Seo |first3=Yutaek |last4=Lee |first4=Ju Dong |last5=Linga |first5=Praveen |bibcode=2018ApEn..216..262V }}</ref> With the inclusion of [[tetrahydrofuran]], though there is a slight reduction in the gas storage capacity, the hydrates have been demonstrated to be stable for several months in a recent study at −2&nbsp;°C and atmospheric pressure.<ref>{{Cite journal |doi=10.1016/j.fuel.2018.09.126 |title=Molecular level investigations and stability analysis of mixed methane-tetrahydrofuran hydrates: Implications to energy storage |journal=Fuel |volume=236 |pages=1505–1511 |year=2019 |last1=Kumar |first1=Asheesh |last2=Veluswamy |first2=Hari Prakash |last3=Linga |first3=Praveen |last4=Kumar |first4=Rajnish|bibcode=2019Fuel..236.1505K |s2cid=104937420 }}</ref> A recent study has demonstrated that SNG can be formed directly with seawater instead of pure water in combination with THF.<ref>{{Cite journal |doi=10.1016/j.apenergy.2018.10.085 |title=Direct use of seawater for rapid methane storage via clathrate (SII) hydrates |journal=Applied Energy |volume=235 |pages=21–30 |year=2019 |last1=Kumar |first1=Asheesh |last2=Veluswamy |first2=Hari Prakash |last3=Kumar |first3=Rajnish |last4=Linga |first4=Praveen|bibcode=2019ApEn..235...21K |s2cid=106395586 }}</ref>

== See also ==
* [[Future energy development]]
* [[Long-term effects of global warming]]
* [[The Swarm (Schätzing novel)|''The Swarm'' (Schätzing novel)]]
* [[Unconventional (oil & gas) reservoir]]

== Notes ==
{{reflist|group="nb"}}

== References ==
{{reflist|30em}}

== External links ==
* [http://www.straightdope.com/columns/070803.html Are there deposits of methane under the sea? Will global warming release the methane to the atmosphere?] {{Webarchive|url=https://web.archive.org/web/20080430085508/http://www.straightdope.com/columns/070803.html |date=2008-04-30 }} (2007)
* [https://news.bbc.co.uk/2/hi/science/nature/8205864.stm Methane seeps from Arctic sea bed] (BBC)
* [http://www.latimes.com/news/science/environment/la-na-global-warming-methane22-2009feb22,0,6678890.story Bubbles of warming, beneath the ice] (LA Times 2009)
* [https://www.prode.com/fluidproperties/hydrate.php online calculator : hydrate formation conditions with different EOSs]

=== Research ===
* [http://cage.uit.no Centre for Arctic Gas Hydrate, Environment and Climate (CAGE)]
* [https://web.archive.org/web/20160304030313/http://hydrates.mines.edu/ Center for Hydrate Research]
* [https://geology.usgs.gov/connections/mms/joint_projects/methane.htm USGS Geological Research Activities with U.S. Minerals Management Service - Methane Gas Hydrates] {{Webarchive|url=https://web.archive.org/web/20070502033645/http://geology.usgs.gov/connections/mms/joint_projects/methane.htm |date=2007-05-02 }}
* [https://web.archive.org/web/20070105164147/http://www.eee.columbia.edu/research-projects/sustainable_energy/Hydrates/index.html Carbon Neutral Methane Energy Production from Hydrate Deposits] (Columbia University)

=== Video ===
* [https://www.youtube.com/watch?v=U46XOoU0DrM USGS Gas Hydrates Lab] (2012)
* [https://www.youtube.com/watch?v=9q3c9CErdmA Ancient Methane Explosions Created Ocean Craters] (2017)
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{{DEFAULTSORT:Methane Clathrate}}
[[Category:Clathrate hydrates]]
[[Category:Hydrocarbons]]
[[Category:Methane]]
[[Category:Unconventional gas]]
[[Category:Natural gas]]