Methane clathrate

Methane clathrate

Overview

Methane clathrate, also called methane hydrate, hydromethane, methane ice, "fire ice", natural gas hydrate or just gas hydrate, is a solid clathrate compound
Clathrate compound
A clathrate, clathrate compound or cage compound is a chemical substance consisting of a lattice of one type of molecule trapping and containing a second type of molecule...

 (more specifically, a clathrate hydrate
Clathrate hydrate
Clathrate hydrates are crystalline water-based solids physically resembling ice, in which small non-polar molecules or polar molecules with large hydrophobic moieties are trapped inside "cages" of hydrogen bonded water molecules...

) in which a large amount of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 is trapped within a crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

 structure of water, forming a solid similar to ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

. Originally thought to occur only in the outer regions of the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 where temperatures are low and water ice is common, significant deposits of methane clathrate have been found under sediments on the ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

 floors of Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

.
Discussion
Ask a question about 'Methane clathrate'
Start a new discussion about 'Methane clathrate'
Answer questions from other users
Full Discussion Forum
 
Unanswered Questions
Encyclopedia

Methane clathrate, also called methane hydrate, hydromethane, methane ice, "fire ice", natural gas hydrate or just gas hydrate, is a solid clathrate compound
Clathrate compound
A clathrate, clathrate compound or cage compound is a chemical substance consisting of a lattice of one type of molecule trapping and containing a second type of molecule...

 (more specifically, a clathrate hydrate
Clathrate hydrate
Clathrate hydrates are crystalline water-based solids physically resembling ice, in which small non-polar molecules or polar molecules with large hydrophobic moieties are trapped inside "cages" of hydrogen bonded water molecules...

) in which a large amount of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 is trapped within a crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

 structure of water, forming a solid similar to ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

. Originally thought to occur only in the outer regions of the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 where temperatures are low and water ice is common, significant deposits of methane clathrate have been found under sediments on the ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

 floors of Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

. The worldwide amounts of methane bound in gas hydrates is conservatively estimated to total twice the amount of carbon to be found in all known fossil fuels on Earth.

Methane clathrates are common constituents of the shallow marine geosphere
Geosphere
The term geosphere is often used to refer to the densest parts of Earth, which consist mostly of rock and regolith. The geosphere consists of the inside of the Earth or other planets or bodies....

, and they occur both in deep sedimentary
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

 structures, and as outcrops on the ocean floor. Methane hydrates are believed to form by migration of gas from depth along geological faults, followed by precipitation, or crystallization, on contact of the rising gas stream with cold sea water. Methane clathrates are also present in deep Antarctic ice core
Ice core
An ice core is a core sample that is typically removed from an ice sheet, most commonly from the polar ice caps of Antarctica, Greenland or from high mountain glaciers elsewhere. As the ice forms from the incremental build up of annual layers of snow, lower layers are older than upper, and an ice...

s, and record a history of atmospheric methane
Atmospheric methane
Atmospheric methane levels are of interest due to its impact on climate change. Atmospheric methane is one of the most potent and influential greenhouse gases on Earth. The 100-year global warming potential of methane is 25, i.e...

 concentrations, dating to 800,000 years ago. The ice-core methane clathrate record is a primary source of data for global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

 research, along with oxygen and carbon dioxide.

Structure and composition


The average methane clathrate hydrate composition is 1 mole
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...

 of methane for every 5.75 moles of water, though this is dependent on how many methane molecules "fit" into the various cage structures of the water lattice. The observed density is around 0.9 g/cm3. One litre of methane clathrate solid would therefore contain, on average, 168 litres of methane gas (at STP).

Methane forms a structure I hydrate with two dodecahedral (12 vertices, thus 12 water molecules) and six tetradecahedral
Tetradecahedron
A tetradecahedron is a polyhedron with 14 faces. There are numerous topologically distinct forms of a tetradecahedron, with many constructible entirely with regular polygon faces....

 (14 water molecules) water cages per unit cell. This compares with a hydration number of 20 for methane in aqueous solution. A methane clathrate MAS NMR
Magic angle spinning
In nuclear magnetic resonance, magic angle spinning is a technique often used to perform experiments in solid-state NMR spectroscopy.By spinning the sample at the magic angle θm In nuclear magnetic resonance, magic angle spinning (MAS) is a technique often used to perform experiments in...

 spectrum recorded at 275 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

 and 3.1 MPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

 shows a peak for each cage type and a separate peak for gas phase methane. Recently, a clay-methane hydrate intercalate was synthesized in which a methane hydrate complex was introduced at the interlayer of a sodium-rich montmorillonite
Montmorillonite
Montmorillonite is a very soft phyllosilicate group of minerals that typically form in microscopic crystals, forming a clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite family, is a 2:1 clay, meaning that it has 2 tetrahedral sheets sandwiching a central...

 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
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

 (i.e. < 2,000 m depth). Furthermore, necessary conditions are found only either in polar continental sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

s where surface temperatures are less than 0 °C; or in oceanic sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

 at water depths greater than 300 m where the bottom water
Water mass
An oceanographic water mass is an identifiable body of water with a common formation history which has physical properties distinct from surrounding water...

 temperature is around 2 °C. In addition, deep fresh water lakes may host gas hydrates as well, since methane hydrates, in particular, cannot be formed in salt water (e.g. the fresh water Lake Baikal, Siberia). Continental deposits have been located in Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...

 and Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...

 in sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

 and siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...

 beds at less than 800 m depth. Oceanic deposits seem to be widespread in the continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...

 (see Fig.) and can occur within the sediments at depth or close to the sediment-water interface
Sediment-water interface
In oceanography and limnology, the sediment-water interface is the boundary between bed sediment and the overlying water column. The topography of this interface is often dynamic, as it is affected by physical processes and biological processes ....

. They may cap even larger deposits of gaseous methane.

Oceanic


There are two distinct types of oceanic deposit. The most common is dominated (> 99%) by methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 contained in a structure I clathrate and generally found at depth in the sediment. Here, the methane is isotopically light (δ13C
Carbon-13
Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...

 < -60‰) which indicates that it is derived from the microbial reduction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 of CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

. The clathrates in these deep deposits are thought to have formed in situ from the microbially produced methane, as the δ13C values of clathrate and surrounding dissolved methane are similar. However, it is also thought that fresh water used in the pressurization of oil and gas wells in permafrost and along the continental shelves world wide, combine with natural methane to form clatherate at depth and pressure, since methane hydrates cannot form, initially, in salt water. Local variations may be very common, as 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, and thus they have high electric resistivity just as ice and sediments containing hydrates have a higher resistivity compared to sediments without gas hydrates (Judge [67]).

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
Continental rise
The continental rise is an underwater feature found between the continental slope and the abyssal plain. This feature can be found all around the world, and it represents the final stage in the boundary between continents and the deepest part of the ocean...

, 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
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

s (< 99% methane) contained in a structure II clathrate. Carbon from this type of clathrate is isotopically heavier (δ13C
Carbon-13
Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...

 is -29 to -57 ‰) and is thought to have migrated upwards from deep sediments, where methane was formed by thermal decomposition of organic matter
Organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...

. Examples of this type of deposit have been found in the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...

 and the Caspian Sea
Caspian Sea
The Caspian Sea is the largest enclosed body of water on Earth by area, variously classed as the world's largest lake or a full-fledged sea. The sea has a surface area of and a volume of...

.

Some deposits have characteristics intermediate between the microbially and thermally sourced types and are considered to be formed from a mixture of the two.

The methane in gas hydrates is dominantly generated by microbial consortia degrading organic matter in low oxygen environments, with the methane itself produced by methanogen
Methanogen
Methanogens are microorganisms that produce methane as a metabolic byproduct in anoxic conditions. They are classified as archaea, a group quite distinct from bacteria...

ic archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

. Organic matter in the uppermost few centimetres of sediments is first attacked by aerobic bacteria, generating CO2, 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 sulfates are reduced to sulfides. Finally, once sulfate is used up, methanogenesis becomes a dominant pathway for organic carbon remineralization.

If the sedimentation rate is low (about 1 cm/kyr), 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 anoxic
Hypoxia (environmental)
Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...

 at depths of only a few centimeters or less. In such organic-rich marine sediments, sulfate then becomes the most important terminal electron acceptor due to its high concentration in seawater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...

, although 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, although it is only archaea that actually emit methane.

In some regions (e.g., Gulf of Mexico) methane in clathrates may be at least partially derived from thermal degradation of organic matter, dominantly in petroleum. 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 occur as 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.

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
Order of magnitude
An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...

 per decade since it was first recognized that clathrates could exist in the oceans during the 1960s and '70s. 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 only form in a narrow range of depths (continental shelves), only at some locations in the range of depths where they could occur (10-30% of the GHSZ), and typically are found at low concentrations (0.9-1.5% by volume) at sites where they do occur. Recent estimates constrained by direct sampling suggest the global inventory occupies between one and five million cubic kilometres (0.24 to 1.2 million cubic miles). This estimate, corresponding to 500-2500 gigatonnes carbon (Gt C), is smaller than the 5000 Gt C estimated for all other fossil fuel reserves but substantially larger than the ~230 Gt C estimated for other natural gas sources. 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, for comparison the total carbon in the atmosphere is around 700 gigatons.

These modern estimates are notably smaller than the 10,000 to 11,000 Gt C (2 m³) proposed by previous workers as a motivation considering clathrates as a fossil fuel resource (MacDonald 1990, Kvenvolden 1998). Lower abundances of clathrates do not rule out their economic potential, but a lower total volume and apparently low concentration at most sites 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
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

 or siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...

 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 a Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...

, Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...

 as well as Northern Canada
Northern Canada
Northern Canada, colloquially the North, is the vast northernmost region of Canada variously defined by geography and politics. Politically, the term refers to the three territories of Canada: Yukon, Northwest Territories, and Nunavut...

.

In 2008, Canadian and Japanese researchers extracted a constant stream of natural gas from a test project at the Mallik gas hydrate field in the Mackenzie River
Mackenzie River
The Mackenzie River is the largest river system in Canada. It flows through a vast, isolated region of forest and tundra entirely within the country's Northwest Territories, although its many tributaries reach into four other Canadian provinces and territories...

 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
Imperial Oil
Imperial Oil Limited is Canada's largest petroleum company. The company is engaged in the exploration, production and sale of crude oil and natural gas. It is controlled by US based ExxonMobil, which owns 69.6% of its stock...

 in 1971-1972.

Commercial use


The sedimentary methane hydrate reservoir probably contains 2–10 times the currently known reserves of conventional natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

. This represents a potentially important future source of hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

 fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

. However, in the majority of sites deposits are likely 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. To date, there has only been one field commercially produced where some of the gas is thought to have been from methane clathrates, Messoyakha Gas Field
Messoyakha Gas Field
The Messoyakha gas field is a natural gas field located in the north of the West Siberian Basin, where there are many large fields, where the presence of gas hydrates has been documented. The Messoyakha gas field is often used as an example of production of in-situ gas hydrates...

, supplying the nearby Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

n city of Norilsk
Norilsk
Norilsk is an industrial city in Krasnoyarsk Krai, Russia, located between the Yenisei River and the Taymyr Peninsula. Population: It was granted city status in 1953. It is the northernmost city in Siberia and the world's second largest city north of the Arctic Circle...

.

A research and development project in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

 is aiming for commercial-scale extraction by 2016. 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 ~1010 m3 of gas. Bjørn Kvamme and Arne Graue at the Institute for Physics and technology at the University of Bergen
University of Bergen
The University of Bergen is located in Bergen, Norway. Although founded as late as 1946, academic activity had taken place at Bergen Museum as far back as 1825. The university today serves more than 14,500 students...

 have developed a method for injecting CO2 into hydrates and reversing the process; thereby extracting CH4 by direct exchange.

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 such as ethane and propane can also form hydrates, although as the molecule length increases (butanes, pentanes), they cannot fit into the water cage structure and tend to destabilise the formation of hydrates.

Once formed, hydrates can block pipeline and processing equipment. They are generally then removed by reducing the pressure, heating them, or dissolving them by chemical means (methanol is commonly used). Care must be taken to ensure that the removal of the hydrates is carefully controlled, because of the potential for the hydrate to undergo a phase transition from the solid hydrate to release water and gaseous methane at a high rate as the pressure is reduced. The rapid release of methane gas in a closed system can result in a rapid increases 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
Ethylene glycol is an organic compound widely used as an automotive antifreeze and a precursor to polymers. In its pure form, it is an odorless, colorless, syrupy, sweet-tasting liquid...

 (MEG) or methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

, which act to depress the temperature at which hydrates will form (i.e. common antifreeze
Antifreeze
Antifreeze is a freeze preventive used in internal combustion engines and other heat transfer applications, such as HVAC chillers and solar water heaters....

). In recent years, development of other forms of hydrate inhibitors have been developed, such as Kinetic Hydrate Inhibitors (which dramatically slow the rate of hydrate formation) and anti-agglomerates, which do not prevent hydrates forming, but do prevent them sticking together to block equipment.

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. As they rise, the pressure in the annulus
Annulus (oil well)
The annulus of an oil well refers to any void between any piping, tubing or casing and the piping, tubing, or casing immediately surrounding it. It is named after the corresponding geometric concept...

 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 what is referred to as a "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
    Shut-in (oil drilling)
    In the petroleum industry, shutting-in is the implementation of a production cap lower than the available output of a specific site. This may be part of an attempt to constrict the oil supply or a necessary precaution when crews are evacuated ahead of a natural disaster....

    " (isolated) may indicate hydrate formation. Following "shut in", the pressure rises as gas diffuses through the reservoir to the bore hole; the rate of pressure rise will exhibit a reduced rate of increase when 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
Deepwater Horizon oil spill
The Deepwater Horizon oil spill is an oil spill in the Gulf of Mexico which flowed unabated for three months in 2010, and continues to leak fresh oil. It is the largest accidental marine oil spill in the history of the petroleum industry...

 in 2010. BP engineers developed and deployed a subsea oil recovery system over oil spilling from a deepwater oil well
Oil well
An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...

 5000 feet (1,524 m) below sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

 to capture escaping oil. This involved placing a 125 tonnes (275,577.8 lb) 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 as much as 85% of the leaking oil but is previously untested at such depths. BP deployed the system on May 7–8, when it failed due to buildup of methane clathrate inside the dome; with its low density of approximately 0.9g/cm3 the methane hydrates accumulated in the dome, adding buoyancy and obstructing flow.

Methane clathrates and climate change


Methane is a powerful greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

. Despite its short atmospheric half life of 7 years, methane has a global warming potential
Global warming potential
Global-warming potential is a relative measure of how much heat a greenhouse gas traps in the atmosphere. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A GWP is calculated over a specific time...

 of 62 over 20 years and 21 over 100 years (IPCC, 1996; Berner and Berner, 1996; vanLoon and Duffy, 2000). The sudden release of large amounts of natural gas from methane clathrate deposits has been hypothesized as a cause of past and possibly future climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...

 changes. Events possibly linked in this way are the Permian-Triassic extinction event
Permian-Triassic extinction event
The Permian–Triassic extinction event, informally known as the Great Dying, was an extinction event that occurred 252.28 Ma ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras...

 and the Paleocene-Eocene Thermal Maximum
Paleocene-Eocene Thermal Maximum
The most extreme change in Earth surface conditions during the Cenozoic Era began at the temporal boundary between the Paleocene and Eocene epochs . This event, the Paleocene–Eocene Thermal Maximum , was associated with rapid global...

.

Climate scientists such as James E. Hansen hypothesize that methane clathrates in the permafrost
Permafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...

 regions will be released as a result of global warming, unleashing powerful feedback forces which may cause runaway climate change
Runaway climate change
Runaway climate change describes a theoretical scenario in which the climate system passes a threshold or tipping point, after which internal positive feedback effects cause the climate to continue changing without further external forcings...

 that cannot be controlled.

Recent research carried out in 2008 in the Siberian Arctic has shown millions of tonnes of methane being released with concentrations in some regions reaching up to 100 times above normal.

Natural gas hydrates (NGH) vs. liquified natural gas (LNG) in transportation


Since methane clathrates are stable at a higher temperature than liquefied natural gas (LNG)
Liquefied natural gas
Liquefied natural gas or LNG is natural gas that has been converted temporarily to liquid form for ease of storage or transport....

 (−20 vs −162 °C), there is some interest in converting natural gas into clathrates rather than liquifying it when transporting it by seagoing vessels
LNG carrier
An LNG carrier is a tank ship designed for transporting liquefied natural gas . As the LNG market grows rapidly, the fleet of LNG carriers continues to experience tremendous growth.-History:...

. 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 economic.

See also

  • Clathrate compound
    Clathrate compound
    A clathrate, clathrate compound or cage compound is a chemical substance consisting of a lattice of one type of molecule trapping and containing a second type of molecule...

  • Clathrate gun hypothesis
    Clathrate gun hypothesis
    The clathrate gun hypothesis is the popular name given to the hypothesis that rises in sea temperatures can trigger the sudden release of methane from methane clathrate compounds buried in seabeds and permafrost which, because the methane itself is a powerful greenhouse gas, leads to further...

  • Clathrate hydrate
    Clathrate hydrate
    Clathrate hydrates are crystalline water-based solids physically resembling ice, in which small non-polar molecules or polar molecules with large hydrophobic moieties are trapped inside "cages" of hydrogen bonded water molecules...

  • Future energy development
  • Long-term effects of global warming
    Long-term effects of global warming
    There are expected to be various long-term effects of global warming. Most discussion and research, including that by the Intergovernmental Panel on Climate Change reports, concentrates on the effects of global warming up to 2100, with only an outline of the effects beyond this...


External links