methane hydrates
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Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid
clathrate compound A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. The word ''clathrate'' is derived from the Latin (), meaning ‘with bars, latticed’. Most clathrate compounds are polymeric and completely envelop t ...
(more specifically, a clathrate hydrate) in which a large amount of
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
is trapped within a
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...
structure of water, forming a solid similar to
ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaqu ...
. Originally thought to occur only in the outer regions of the
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
, where temperatures are low and water ice is common, significant deposits of methane clathrate have been found under
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
s on the
ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...
floors of the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
. 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 There are several conflicting usages of geosphere, variously defined. It may be taken as the collective name for the lithosphere, the hydrosphere, the cryosphere, and the atmosphere. The different collectives of the geosphere are able to exchange ...
and they occur in deep
sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
structures and form
outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficia ...
s on the ocean floor. Methane hydrates are believed to form by the precipitation or crystallisation of methane migrating from deep along 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 Vostok Station (russian: ста́нция Восто́к, translit=stántsiya Vostók, , meaning "Station East") is a Russian research station in inland Princess Elizabeth Land, Antarctica. Founded by the Soviet Union in 1957, the station ...
and EPICA Dome C ice cores revealed that methane clathrates were also present in deep Antarctic ice cores and record a history of
atmospheric methane Atmospheric methane is the methane present in Earth's atmosphere. Atmospheric methane concentrations are of interest because it is one of the most potent greenhouse gases in Earth's atmosphere. Atmospheric methane is rising. The 20-year globa ...
concentrations, dating to 800,000 years ago. The ice-core methane clathrate record is a primary source of data for
global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
research, along with oxygen and carbon dioxide.


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.


Structure and composition

The nominal methane clathrate hydrate composition is (CH4)4(H2O)23, or 1
mole Mole (or Molé) may refer to: Animals * Mole (animal) or "true mole", mammals in the family Talpidae, found in Eurasia and North America * Golden moles, southern African mammals in the family Chrysochloridae, similar to but unrelated to Talpida ...
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
lattice Lattice may refer to: Arts and design * Latticework, an ornamental criss-crossed framework, an arrangement of crossing laths or other thin strips of material * Lattice (music), an organized grid model of pitch ratios * Lattice (pastry), an orna ...
. The observed density is around 0.9 g/cm3, 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. 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 °C and 1 atm), or one cubic metre of methane clathrate releases about 160 cubic metres of gas. Methane forms a "structure-I" hydrate with two
dodecahedral In geometry, a dodecahedron (Greek , from ''dōdeka'' "twelve" + ''hédra'' "base", "seat" or "face") or duodecahedron is any polyhedron with twelve flat faces. The most familiar dodecahedron is the regular dodecahedron with regular pentagon ...
(12 vertices, thus 12 water molecules) and six tetradecahedral (14 water molecules) water cages per unit cell. (Because of sharing of water molecules between cages, there are only 46 water molecules per unit cell.) This compares with a
hydration number The hydration number, or solvation number of a compound is defined as the average number of molecules bound to the compound more strongly (by 13.3 kcal/mol or more) than they are bound to other water molecules. The hydration number is dependent o ...
of 20 for methane in aqueous solution. Note: the number 20 is called a magic number equal to the number found for the amount of water molecules surrounding a
hydronium ion In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is d ...
.
A methane clathrate MAS NMR spectrum recorded at 275 K and 3.1
MPa MPA or mPa may refer to: Academia Academic degrees * Master of Performing Arts * Master of Professional Accountancy * Master of Public Administration * Master of Public Affairs Schools * Mesa Preparatory Academy * Morgan Park Academy * Mou ...
shows a peak for each cage type and a separate peak for
gas phase In the physical sciences, a phase is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, magnetiza ...
methane. 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 Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystals, known as clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite gro ...
clay. The upper temperature stability of this phase is similar to that of structure-I hydrate.


Natural deposits

Methane clathrates are restricted to the shallow lithosphere (i.e. < 2,000 m depth). Furthermore, necessary conditions are found only in either continental
sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s in polar regions where average surface temperatures are less than 0 °C; or in oceanic
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
at water depths greater than 300 m where the
bottom water Bottom water is the lowermost water mass in a water body, by its bottom, with distinct characteristics, in terms of physics, chemistry, and ecology. Oceanography Bottom water consists of cold, dense water near the ocean floor. This water is char ...
temperature is around 2 °C. In addition, deep fresh water lakes may host gas hydrates as well, e.g. the fresh water Lake Baikal, Siberia. Continental deposits have been located in
Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive region, geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a ...
and
Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S. ...
in
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...
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.


Oceanic

Methane hydrate can occur in various forms like massive, dispersed within pore spaces, nodules, veins/fractures/faults, and layered horizons. Generally, it is found unstable at standard pressure and temperature conditions, and 1 m^3 of methane hydrate upon dissociation yields about 164 m^3 of methane and 0.87 m^3 of freshwater. There are two distinct types of oceanic deposits. The most common is dominated (> 99%) by
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
contained in a structure I
clathrate A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. The word ''clathrate'' is derived from the Latin (), meaning ‘with bars, latticed’. Most clathrate compounds are polymeric and completely envelop t ...
and generally found at depth in the sediment. Here, the methane is isotopically light ( δ13C < −60‰), which indicates that it is derived from the microbial reduction of CO2. The clathrates in these deep deposits are thought to have formed in situ from the microbially produced methane since the δ13C values of clathrate and surrounding dissolved methane are similar. 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. 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 7. 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 continental rise is a low-relief zone of accumulated sediments that lies between the continental slope and the abyssal plain. It is a major part of the continental margin, covering around 10% of the ocean floor. Formation This geologic str ...
, the GHSZ started at 190 m depth and continued to 450 m, where it reached equilibrium with the gaseous phase. Measurements indicated that methane occupied 0-9% by volume in the GHSZ, and ~12% in the gaseous zone. In the less common second type found near the sediment surface, some samples have a higher proportion of longer-chain
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
s (< 99% methane) contained in a structure II clathrate. Carbon from this type of clathrate is isotopically heavier ( δ13C 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 The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United ...
and the
Caspian Sea The Caspian Sea is the world's largest inland body of water, often described as the world's largest lake or a full-fledged sea. An endorheic basin, it lies between Europe and Asia; east of the Caucasus, west of the broad steppe of Central Asia ...
. 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 methanogenic archaea. Organic matter in the uppermost few centimeters of sediments is first attacked by aerobic bacteria, generating CO2, which escapes from the sediments into the
water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
. 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 The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
s are reduced to sulfides. Finally, methanogenesis becomes a dominant pathway for organic carbon
remineralization In biogeochemistry, remineralisation (or remineralization) refers to the breakdown or transformation of organic matter (those molecules derived from a biological source) into its simplest inorganic forms. These transformations form a crucial link ...
. If the sedimentation rate is low (about 1  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 Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...
in the sediments becomes
anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...
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
seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appr ...
. 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 −350 to −450 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. The methane in clathrates typically has a biogenic isotopic signature and highly variable δ13C (−40 to −100‰), with an approximate average of about −65‰ . Below the zone of solid clathrates, large volumes of methane may form bubbles of free gas in the sediments. 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 pingos 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.


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 An order of magnitude is an approximation of the logarithm of a value relative to some contextually understood reference value, usually 10, interpreted as the base of the logarithm and the representative of values of magnitude one. Logarithmic di ...
per decade since it was first recognized that clathrates could exist in the oceans during the 1960s and 1970s. The highest estimates (e.g. 3 m3) 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 (0.9–1.5% by volume) at sites where they do occur. Recent estimates constrained by direct sampling suggest the global inventory occupies between . This estimate, corresponding to 500–2500 gigatonnes carbon (Gt C), is smaller than the 5000 Gt C estimated for all other geo-organic fuel reserves but substantially larger than the ~230 Gt C estimated for other natural gas sources.USGS World Energy Assessment Team, 2000. US Geological Survey world petroleum assessment 2000––description and results. USGS Digital Data Series DDS-60. The permafrost reservoir has been estimated at about 400 Gt C in the Arctic, 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 gigatons (see Carbon: Occurrence). These modern estimates are notably smaller than the 10,000 to 11,000 Gt C (2 m3) proposed 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 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 Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...
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 Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S. ...
,
Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive region, geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a ...
, 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 The Mallik Methane Hydrate Site (or ''Mallik Gas Hydrate Production Research Well'', ''Mallik test well'') is located in the Beaufort Sea, Canada. Site highlights * First dedicated scientific and technical research site investigating permafrost g ...
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. The Mallik gas hydrate field was first discovered by Imperial Oil in 1971–1972.


Commercial use

Economic deposits of hydrate are termed natural gas hydrate (NGH) and store 164 m3 of methane, 0.8 m3 water in 1 m3 hydrate. 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 Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...
, . This represents a potentially important future source of
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
fuel. However, in the majority of sites deposits are thought to be too dispersed for economic extraction. 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. A potentially economic reserve in the Gulf of Mexico may contain approximately of gas. Bjørn Kvamme and Arne Graue at the Institute for Physics and technology at the
University of Bergen The University of Bergen ( no, Universitetet i Bergen, ) is a research-intensive state university located in Bergen, Norway. As of 2019, the university has over 4,000 employees and 18,000 students. It was established by an act of parliament in 194 ...
have developed a method for injecting into hydrates and reversing the process; thereby extracting CH4 by direct exchange. The University of Bergen's method is being field tested by ConocoPhillips and state-owned
Japan Oil, Gas and Metals National Corporation The , is a Japanese government Independent Administrative Institution which was created in 2004 when the former Japan National Oil Corporation merged with the former Metal Mining Agency of Japan. See also * Critical mineral raw materials * Ene ...
(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. On March 12, 2013, JOGMEC researchers announced that they had successfully extracted natural gas from frozen methane hydrate. 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. According to an industry spokesperson, "It asthe world's first offshore experiment producing gas from methane hydrate". Previously, gas had been extracted from onshore deposits, but never from offshore deposits which are much more common. The hydrate field from which the gas was extracted is located from central Japan in the
Nankai Trough The is a submarine trough located south of the Nankaidō region of Japan's island of Honshu, extending approximately offshore. The underlying fault, the ''Nankai megathrust,'' is the source of the devastating Nankai megathrust earthquakes, w ...
, under the sea. A spokesperson for JOGMEC remarked "Japan could finally have an energy source to call its own". 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." 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. Both Japan and China announced in May 2017 a breakthrough for
mining Mining is the extraction of valuable minerals or other geological materials from the Earth, usually from an ore body, lode, vein, seam, reef, or placer deposit. The exploitation of these deposits for raw material is based on the economic ...
methane clathrates, when they extracted methane from hydrates in the
South China Sea The South China Sea is a marginal sea of the Western Pacific Ocean. It is bounded in the north by the shores of South China (hence the name), in the west by the Indochinese Peninsula, in the east by the islands of Taiwan and northwestern Phi ...
. 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". Industry consensus is that commercial-scale production remains years away.


Environmental concerns

Experts caution that environmental impacts are still being investigated and that methane—a greenhouse gas with around 25 times as much
global warming potential Global warming potential (GWP) is the heat absorbed by any greenhouse gas in the atmosphere, as a multiple of the heat that would be absorbed by the same mass of carbon dioxide (). GWP is 1 for . For other gases it depends on the gas and the time ...
over a 100-year period (GWP100) as carbon dioxide—could potentially escape into the atmosphere if something goes wrong. Furthermore, while cleaner than coal, burning natural gas also creates carbon dioxide emissions.


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. 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 Ethylene glycol (IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...
(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 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". (Kicks, which can cause blowouts, typically do not involve hydrates: see 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. * Careful measuring of line flow to detect incipient hydrate plugging. * 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. * Monitoring of well casing after it is " shut in" (isolated) may indicate hydrate formation. Following "shut in", the pressure rises while gas diffuses through the reservoir to the
bore hole A borehole is a narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water ( drilled water well and tube well), other liquids (such as petrole ...
; the rate of pressure rise exhibit a reduced rate of increase while hydrates are forming. * Additions of energy (e.g., the energy released by setting cement used in well completion) can raise the temperature and convert hydrates to gas, producing a "kick".


Blowout recovery

At sufficient depths, methane complexes directly with water to form methane hydrates, as was observed during the
Deepwater Horizon oil spill The ''Deepwater Horizon'' oil spill (also referred to as the "BP oil spill") was an industrial disaster that began on 20 April 2010 off of the coast of the United States in the Gulf of Mexico on the BP-operated Macondo Prospect, considere ...
in 2010. BP engineers developed and deployed a subsea oil recovery system over oil spilling from a deepwater oil well below
sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardise ...
to capture escaping oil. This involved placing a dome over the largest of the well leaks and piping it to a storage vessel on the surface. This option had the potential to collect some 85% of the leaking oil but was previously untested at such depths. 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/cm3 the methane hydrates accumulated in the dome, adding buoyancy and obstructing flow.


Methane clathrates and climate change


Natural gas hydrates for gas storage and transportation

Since methane clathrates are stable at a higher temperature than liquefied natural gas (LNG) (−20 vs −162 °C), there is some interest in converting natural gas into clathrates (Solidified Natural Gas or SNG) rather than liquifying it when transporting it by 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.. 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 Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...
(THF) as a co-guest. With the inclusion of
tetrahydrofuran Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...
, 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 °C and atmospheric pressure. A recent study has demonstrated that SNG can be formed directly with seawater instead of pure water in combination with THF.


See also

*
Future energy development Energy development is the field of activities focused on obtaining sources of energy from natural resources. These activities include production of renewable, nuclear, and fossil fuel derived sources of energy, and for the recovery and reuse ...
* Long-term effects of global warming * ''The Swarm'' (Schätzing novel) * Unconventional (oil & gas) reservoir


Notes


References


External links


Are there deposits of methane under the sea? Will global warming release the methane to the atmosphere?
(2007)
Methane seeps from Arctic sea bed
(BBC)
Bubbles of warming, beneath the ice
(LA Times 2009)
online calculator : hydrate formation conditions with different EOSs
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Research


Centre for Arctic Gas Hydrate, Environment and Climate (CAGE)Center for Hydrate Research
* ttps://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


USGS Gas Hydrates Lab
(2012)
Ancient Methane Explosions Created Ocean Craters
(2017) {{DEFAULTSORT:Methane Clathrate Clathrate hydrates Hydrocarbons Methane Unconventional gas Natural gas