Methane clathrate Totally Explained

Everything about Methane Clathrate totally explained

Methane clathrate, also called methane hydrate or methane ice, is a solid form of water that contains a large amount of methane within its crystal structure (a clathrate hydrate). 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 Earth.
   Methane clathrates are common constituents of the shallow marine geosphere, and they occur both in deep sedimentary 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 cores, and store a record 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.
   At higher pressures, methane clathrates remain stable at temperatures up to 18 °C. The average methane clathrate hydrate composition is 1 mole 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 liter of methane clathrate solid would therefore contain, on average, 168 liters of methane gas (at STP).
   Methane forms a structure I hydrate with two dodecahedral (20 vertices thus 20 water molecules) and six tetradecahedral (24 water molecules) water cages per unit cell. The hydration value of 20 can be determined experimentally by MAS NMR. A methane clathrate spectrum recorded at 275 K and 3.1 MPa 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 Na-rich montmorillonite 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 (for example < 2000 m depth). Furthermore, necessary conditions are found only either in polar continental sedimentary rocks where 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 lakes may host gas hydrates as well, for example the freshwater 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's 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, as the δ¹³C values of clathrate and surrounding dissolved methane are similar.
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. Methane 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. The methane in clathrates typically has a bacterial isotopic signature and highly variable δ¹³C (-40 to -100‰), with an approximate average of about -65 ‰ .Kvenvolden, 1993; Dickens et al., 1995; 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 per decade since it was first recognized that clathrates could exist in the oceans during the 1960s and 70s. The highest estimates (for example 3 m³) were based on the assumption that fully dense clathrates could litter the entire floor of the deep ocean. However, 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 lies between 1 and 5 m³ (1 quadrillion to 5 quadrillion). 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 don't rule out their economic potential, but a lower total volume and apparently low concentration at most sites In August of 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..

Hydrates in natural gas processing

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 can't 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 risk of massive increases in pressure as the methane is released, and the potential for the hydrate to let go with high velocity is exposed to a high pressure differential.
It is generally preferable to prevent hydrates from forming or blocking equipment. This is commonly achieved by removing water, or by the addition of ethylene glycol (MEG) or methanol, which act to depress the temperature at which hydrates will form. In recent years, development of other forms of hydrate inhibitors have been developed, being Kinetic Hydrate Inhibitors (which dramatically slow the rate of hydrate formation) and anti-agglomerates, which don't prevent hydrates forming, but do prevent them sticking together to block equipment.

Methane clathrates and climate change

Methane is a powerful greenhouse gas. Despite its short atmospheric lifetime of around 12 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, the Paleocene-Eocene Thermal Maximum.

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

Since methane clathrates are stable at a higher temperature (−20 vs −162 °C) than LNG, there's some interest in converting natural gas into clathrates rather than liquifying it when transporting it by seagoing vessels. Accordingly, the production of NGH from NG at the terminal would require a smaller plant than LNG would.

Methane clathrates in popular fiction

The book Mother of Storms by John Barnes offers a fictional example of catastrophic climate change caused by methane clathrate release.
   Another book is The Life Lottery by Ian Irvine, in which unprecedented seismic activity triggers a release of methane hydrate, reversing global cooling. Clive Cussler's Fire Ice also mentions methane hydrate. It tells of how a Russian mining industrialist wants to detonate a bomb into three pockets off of the American coast creating large tsunamis. The intent was to swamp Boston, Charleston, and Miami.
   In the anime Ergo Proxy, a string of explosions in the methane hydrate reserves wipes out 85% of life on Earth.
   In the German bestseller The Swarm (Der Schwarm), an undersea intelligence known as the Yrr heats methane hydrate deposits to cause tsunamis in the North Sea.
   In the movie Stealth, high-end fighter planes use Pulse detonation engines which use methane hydrate as the fuel.
   In The Great Sea Battle, an episode of, the Ultrasaurus was able to fend off an attack from the Death Stinger by using depth charges to ignite an undersea pocket of methane hydrate.
   In, an island by the name of Yorioyajima was said to have sunk into the bottom of the sea 300 years ago following an earthquake, which had separated the methane and water molecules of the methane hydrate holding the island up, forming the sea ruin which the story revolves around.
   In the anime Code Geass R2, the protagonist uses a methane hydrate deposit to fend off a naval attack by the Empire of Brittania, capsizing the entire fleet.

Further Information

Get more info on 'Methane Clathrate'.

External Link Exchanges

Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:

<a href="http://methane_clathrate.totallyexplained.com">Methane clathrate Totally Explained</a>

Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned.