Methane clathrate (CH4·5.75H2O),
also called methane hydrate, hydromethane, methane ice, fire
ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate
hydrate) in which a large amount of methane is
trapped within a crystal
structure of water, forming a solid similar to ice. Originally thought
to occur only in the outer regions of the Solar
System, where temperatures are low and water ice is common, significant
deposits of methane clathrate have been found under sediments on
the ocean floors
of the Earth.
Methane clathrates are common constituents of the shallow
marine geosphere
and they occur in deep sedimentary structures and form outcrops on the
ocean floor. Methane hydrates are believed to form by migration of gas from
deep 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 cores and
record a history of atmospheric methane concentrations, dating to
800,000 years ago. The ice-core methane clathrate record is a primary source of
data for global warming research, along with oxygen and
carbon dioxide.
Structure and composition
The nominal methane clathrate hydrate composition is (CH4)4(H2O)23,
or 1 mole
of methane for every 5.75 moles of water, corresponding to 13.4% methane by
weight, although the actual composition 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, 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).
Methane forms a structure I hydrate with two dodecahedral
(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 of 20 for methane in aqueous solution. A
methane clathrate MAS NMR spectrum recorded at 275 K and 3.1 MPa
shows a peak for each cage type and a separate peak for gas phase
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
clay. The upper temperature stability of this phase is similar to that of
structure I hydrate.
Methane hydrate phase diagram. The horizontal axis shows temperature
from -15 to 33 Celsius, the vertical axis shows pressure from 0 to 120,000
kilopascals (0 to 1,184 atmospheres). For example, at 4 Celsius hydrate forms
above a pressure of about 50 atmospheres.
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 rocks in polar regions where average
surface temperatures are less than 0 °C; or in oceanic sediment at
water depths greater than 300 m where the bottom water
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 and Alaska in sandstone 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
There are two distinct types of oceanic deposit. The most
common is dominated (> 99%) by methane contained in a structure I clathrate and
generally found at depth in the sediment. Here, the methane is isotopically
light (δ13C
< -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
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
clathrate at depth and pressure, since methane hydrates are more stable in
fresh water than in salt water. Local variations may be very common, 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 comprise high electric resistivity just like 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, 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 hydrocarbons
(< 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 and the Caspian Sea.
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 methanogenic archaea. 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/yr), the
organic carbon content is low (about 1% ), and oxygen is abundant, aerobic
bacteria can use up all the organic matter in the sediments faster than oxygen
is depleted, so lower-energy electron acceptors are not used. But where
sedimentation rates and the organic carbon content are high, which is typically
the case on continental shelves and beneath western boundary current upwelling
zones, the pore
water in the sediments becomes anoxic 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,
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 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.
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 per decade since it was first
recognized that clathrates could exist in the oceans during the 1960s and '70s.
The highest estimates (e.g. 3×1018 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 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 1×1015and 5×1015 m³ (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
geo-organic 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 800 gigatons (see Carbon:
Occurrence).
These modern estimates are notably smaller than the 10,000
to 11,000 Gt C (2×1016 m³) proposed by previous workers 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
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, Siberia, and Northern
Canada.
In 2008, Canadian and Japanese researchers extracted a
constant stream of natural gas from a test project at the Mallik gas hydrate site in the Mackenzie
River delta. This was the second such drilling at Mallik: the first took
place in 2002 and used heat to release methane. In the 2008 experiment,
researchers were able to extract gas by lowering the pressure, without heating,
requiring significantly less energy. The Mallik gas hydrate field was first
discovered by Imperial Oil in 1971-1972.
Commercial use
The sedimentary methane hydrate reservoir probably contains
2–10 times the currently known reserves of conventional natural gas,
as of 2013. This represents a potentially important future source of hydrocarbon
fuel. However, in
the majority of sites deposits are thought to be too dispersed for economic
extraction. Other problems facing commercial exploitation are detection of
viable reserves and development of the technology for extracting methane gas
from the hydrate deposits.
A research and development project in Japan is aiming for
commercial-scale extraction near Aichi
Prefecture 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
approximately 100 billion cubic metres (3.5×1012 cu ft)
of gas.[18]
Bjørn Kvamme and Arne Graue at the Institute for
Physics and technology at the University of Bergen have developed a method
for injecting CO
2 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 (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.
2 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 (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 [was] the 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 50 kilometres (31 mi) from central Japan in the Nankai
Trough, 300 metres (980 ft) under the sea. A spokesperson for JOGMEC
remarked "Japan could finally have an energy source to call its own".
The experiment will continue for two weeks before it is determined how
efficient the gas extraction process has been. 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.
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 (MEG) or methanol, which
act to depress the temperature at which hydrates will form (i.e. common antifreeze).
In recent years, development of other forms of hydrate inhibitors have been
developed, like Kinetic Hydrate Inhibitors (which by far 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. 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; 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
Concept diagram of oil containment domes, forming upsidedown
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 5,000 feet (1,500 m) below sea level
to capture escaping oil. This involved placing a 125-tonne (276,000 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 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
Methane is a powerful greenhouse
gas. Despite its short atmospheric half life
of 7 years, methane has a global warming potential 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
changes. Events possibly linked in this way are the Permian-Triassic extinction event
and the Paleocene-Eocene Thermal Maximum.
Climate scientists like James
E. Hansen predict that methane clathrates in the permafrost
regions will be released consequent to global warming, unleashing powerful
feedback forces which may cause runaway climate change 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 versus liquified natural gas in
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 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 economic.
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