Editing Clathrate hydrate (section)

== Hydrates on Earth ==

=== Natural gas hydrates ===
{{main|Methane clathrate}}
Naturally on [[Earth]] gas hydrates can be found on the [[seabed]], in ocean sediments,<ref>{{cite report |last1=Kvenvolden |first1=Keith A. |last2=McMenamin |first2=Mark A. |title=Circular |year=1980 |chapter=Hydrates of natural gas; a review of their geologic occurrence |doi=10.3133/cir825 |doi-access=free }}</ref> in deep lake sediments (e.g. [[Lake Baikal]]), as well as in the [[permafrost]] regions. The amount of [[methane]] potentially trapped in natural [[Methane clathrate|methane hydrate]] deposits may be significant (10<sup>15</sup> to 10<sup>17</sup> cubic metres),<ref>{{cite news |last1=Marshall |first1=Michael |date=26 March 2009 |title=Ice that burns could be a green fossil fuel |url=https://www.newscientist.com/article/dn16848-ice-that-burns-could-be-a-green-fossil-fuel/ |work=New Scientist }}</ref> which makes them of major interest as a potential energy resource. Catastrophic release of methane from the decomposition of such deposits may lead to a global climate change, referred to as the "[[clathrate gun hypothesis]]", because [[methane|CH<sub>4</sub>]] is a more potent greenhouse gas than [[carbon dioxide|CO]]<sub>2</sub> (see [[Atmospheric methane]]). The fast decomposition of such deposits is considered a [[geohazard]], due to its potential to trigger [[landslide]]s, [[earthquake]]s  and [[tsunami]]s. However, natural gas hydrates do not contain only methane but also other [[hydrocarbon]] gases, as well as [[Hydrogen sulfide|H<sub>2</sub>S]] and [[Carbon dioxide|CO<sub>2</sub>]]. [[Air hydrates]] are frequently observed in polar ice samples.

[[Pingo]]s are common structures in permafrost regions.<ref>{{cite journal |last1=Ussler |first1=W. |last2=Paull |first2=C. K. |last3=Lorenson |first3=T. |last4=Dallimore |first4=S. |last5=Medioli |first5=B. |last6=Blasco |first6=S. |last7=McLaughlin |first7=F. |last8=Nixon |first8=F. M. |title=Methane Leakage from Pingo-like Features on the Arctic Shelf, Beaufort Sea, NWT, Canada |journal=AGU Fall Meeting Abstracts |year=2005 |volume=2005 |pages=C11A–1069 |bibcode=2005AGUFM.C11A1069U }}</ref> Similar structures are found in deep water related to methane vents. Significantly, gas hydrates can even be formed in the absence of a liquid phase. Under that situation, water is dissolved in gas or in liquid hydrocarbon phase.<ref>{{cite journal |last1=Youssef |first1=Z. |last2=Barreau |first2=A. |last3=Mougin |first3=P. |last4=Jose |first4=J. |last5=Mokbel |first5=I. |title=Measurements of Hydrate Dissociation Temperature of Methane, Ethane, and CO<sub>2</sub> in the Absence of Any Aqueous Phase and Prediction with the Cubic Plus Association Equation of State |journal=Industrial & Engineering Chemistry Research |date=15 April 2009 |volume=48 |issue=8 |pages=4045–4050 |doi=10.1021/ie801351e }}</ref>

In 2017, both Japan and China announced that attempts at large-scale [[resource extraction]] of methane hydrates from under the seafloor were successful.  However, commercial-scale production remains years away.<ref>{{cite news |title=China claims breakthrough in 'flammable ice' |url=https://www.bbc.com/news/world-asia-china-39971667 |work=BBC News |date=19 May 2017 }}</ref><ref>{{cite journal |title=China and Japan find way to extract 'combustible ice' from seafloor, harnessing a legendary frozen fossil fuel |journal=National Post |date=19 May 2017 |url=https://nationalpost.com/news/world/china-japan-extracts-combustible-ice-from-seafloor-a-step-towards-harnessing-a-legendary-frozen-fossil-fuel }}</ref>

The 2020 Research Fronts report identified gas hydrate accumulation and mining technology as one of the top 10 research fronts in the geosciences.<ref>{{Cite web|url=https://discover.clarivate.com/ResearchFronts2020_EN|title=Web of Science}}</ref>

=== Gas hydrates in pipelines ===
Thermodynamic conditions favouring hydrate formation are often found in [[Pipeline transport|pipelines]]. This is highly undesirable, because the clathrate crystals might agglomerate and plug the line<ref>{{cite journal |doi=10.1021/ef800189k |title=Investigation of Interactions between Gas Hydrates and Several Other Flow Assurance Elements |year=2008 |last1=Gao |first1=Shuqiang |journal=Energy & Fuels |volume=22 |issue=5 |pages=3150–3153 }}</ref> and cause [[flow assurance]] failure and damage valves and instrumentation. The results can range from flow reduction to equipment damage.

==== Hydrate formation, prevention and mitigation philosophy ====
Hydrates have a strong tendency to [[agglomerate]] and to adhere to the pipe wall and thereby plug the pipeline. Once formed, they can be decomposed by increasing the temperature and/or decreasing the pressure. Even under these conditions, the clathrate dissociation is a slow process.

Therefore, preventing hydrate formation appears to be the key to the problem. A hydrate prevention philosophy could typically be based on three levels of security, listed in order of priority:
# Avoid operational conditions that might cause formation of hydrates by depressing the hydrate formation temperature using [[glycol dehydration]];
# Temporarily change [[operating conditions]] in order to avoid hydrate formation;
# Prevent formation of hydrates by addition of chemicals that (a) shift the hydrate equilibrium conditions towards lower temperatures and higher pressures or (b) increase hydrate formation time ([[Reaction inhibitor|inhibitor]]s)

The actual philosophy would depend on operational circumstances such as pressure, temperature, type of flow (gas, liquid, presences of water etc.).

==== Hydrate inhibitors ====
When operating within a set of parameters where hydrates could be formed, there are still ways to avoid their formation. Altering the gas composition by adding chemicals can lower the hydrate formation temperature and/or delay their formation. Two options generally exist:
* Thermodynamic inhibitors
* [[Kinetic Inhibitor|Kinetic inhibitors and anti-agglomerants]]

The most common thermodynamic inhibitors are [[methanol]], [[ethylene glycol|monoethylene glycol]] (MEG), and [[diethylene glycol]] (DEG), commonly referred to as [[glycol]]. All may be recovered and recirculated, but the economics of methanol recovery is not favourable in most cases. MEG is preferred over DEG for applications where the temperature is expected to be −10&nbsp;°C or lower due to high viscosity at low temperatures. [[Triethylene glycol]] (TEG) has too low vapour pressure to be suited as an inhibitor injected into a gas stream. More methanol is lost in the gas phase when compared to MEG or DEG.

The use of [[Kinetic Inhibitor|kinetic inhibitors]] and anti-agglomerants in actual field operations is a new and evolving technology. It requires extensive tests and optimisation to the actual system. While kinetic inhibitors work by slowing down the kinetics of the nucleation, anti-agglomerants do not stop the nucleation, but stop the agglomeration (sticking together) of gas hydrate crystals.  These two kinds of inhibitors are also known as [[LDHI|low dosage hydrate inhibitors]], because they require much smaller concentrations than the conventional thermodynamic inhibitors.  Kinetic inhibitors, which do not require water and hydrocarbon mixture to be effective, are usually polymers or copolymers and anti-agglomerants (requires water and hydrocarbon mixture) are polymers or [[zwitterion]]ic&nbsp;– usually ammonium and COOH&nbsp;– surfactants being both attracted to hydrates and hydrocarbons.