Editing Methane clathrate (section)

==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 [[Blowout (well drilling)#Formation kick|Blowout: 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 |url-status=dead |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 buildup of methane clathrate inside the dome; with its low density of approximately 0.9 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>