3. The methane water clathrate compound – treasures of the deep or an ecological timebomb?
Carolyn Koha*, Patrick Hendra
aKing’s College London,
Department of Chemistry,
Strand, London WC2R 2LS
*Corresponding Author: email@example.com,
tel. +44 (0)207 848 2380,
fax +44 (0)207 848 2810.
As long ago as the days of Michael Faraday, it has been known that water and methane (and other small hydrocarbons and ring compounds) form clathrate compounds. What is more recent in their history is the discovery that vast amounts of methane hydrate are present in Polar Regions and beneath the deep sea.
When water freezes into ice, it forms a three dimensional structure with space between the atoms into which methane can become entrapped. The melting point of the clathrate compound, as it is called is above zero, in fact, well above, the temperature rising if pressure is applied. Now, beneath polar ice caps or layers of frozen tundra, or at great depths in the sea, conditions can easily apply where the methane hydrate compound is stable. The consequence is that if methane is generated through the decay of biological material or more rarely, from volcanic action in these zones, the gas does not diffuse away to the atmosphere, but rather it stays in the water saturated sediments.
The methane clathrate remained a laboratory oddity until in the early 1980’s the Russians became aware of just how much of this material was present in Northern Siberia and started to develop methods of extracting it. Methane from this source is now piped to Europe both east and west. Similarly, sources have been exploited in Alaska but the exploitation so far is just the tip of the iceberg (sorry!).
Over the last 20 years an enormous amount of work has been done on the physical and structural chemistry of gas clathrate hydrates, on the thermodynamics and phase relationships and this has pointed geologists and geographers in the direction of where to prospect for deposits. It turns out that almost anywhere in the world where there is really deep sea, the compound is at least potentially present. In the Arctic and Antarctic, enormous amounts are found at shallower depths.
Map showing the distribution of gas deposits (red dots)
Hardly surprisingly, many estimates have been made of the quantities of methane trapped worldwide. They range from what could be present assuming that the compound will always occur where the conditions of temperature and pressure are appropriate, to much more modest estimates based on actual discoveries made so far, but the figures are mind-boggling.
The amount of methane in these gas hydrate deposits varies from location to location. For example, it is estimated that there is around 20 trillion m3 of methane in gas hydrate deposits at 300-1200 m under the sea bottom of the Black Sea, while gas hydrate deposits found in the permafrost of the Mackenzie Delta of Canada are estimated to contain around 0.17 trillion m3 of methane.
Don’t run away with the idea that this latter amount is small. In just one investigation of the Mackenzie River Delta up in the Northwest Territories in Canada, deposits were found at a depth of about 1 kilometre, the layer being ~100m thick and of many square kilometres in extent. There are many, many deposits of this type. In fact, deposits are likely to exist wherever the mean temperature is below 0ºC. As a result the potential for deposits could be found all over this vast region and not just the MacKenzie Delta.
|Estimate method||Permafrost Sources||Under Oceans|
|Based on drilling||1 x 1014m3||1-2 x 1016m3||Kvenvolden et al and separately Makogan 1988|
|Top estimate based on thermodynamics||3 x 1016m3||7 x 1018m3||Dobrynin 1981|
|USA reserve based on drilling||9 x 109m3in Alaska (1998)||Considerable|
To put this in perspective, it is likely that the energy trapped in the methane clathrate easily exceeds that available from coal, oil and conventional natural gas put together. Phew!
At the moment, exploitation is restricted to land based extraction – either, the pressure is deliberately reduced by drilling and the methane is released or heated water or steam is injected and raises the temperature. So far, the technology for releasing the gas from beneath the sea is either not yet available or is not economically worthwhile. In Polar Regions mud beneath 600 feet of water is of interest but in most temperature climates much greater depths are involved so the extraction problem is far from easy.
Assuming people start to explore this resource two environmental consequences are inevitable–
- As economic supplies of methane become available from beneath the seas of the world, Nations could reduce their dependence on oil and from a security point of view, improve their position enormously. Over dependence on a single source of hydrocarbon would be removed and this new source would seem very attractive especially if the source of supply is local and under their own direct control.
- It is almost inevitable that leakage of the methane will occur at the extraction sites and even larger leaks will occur (as they already do) in the gas distribution network. Since methane is 20 times worse than CO2 as a green house absorber, the environmental lobby must already be climbing onto the trampoline!
Currently the concentration of methane in the atmosphere is around 2 ppm rising by ~0.6% per year. This amounts to about 4 x 1010 tonnes of methane or about 3 x 10-4 of the amount in clathrate deposits. So, any significant release of the entrapped clathrated methane could be disastrous. The amount of CO2 in the atmosphere is around 550 ppm, so remembering that methane is 20x more effective than CO2 as a greenhouse absorber, the methane in the atmosphere is currently equivalent to about 40 ppm of CO2 – a non trivial contribution – and its rising!
Now, there is a further quite natural problem to consider, a problem not related to exploitation of this resource. As the temperature of the atmosphere rises, it is an inevitable consequence that marginally stable deposits particularly those under permafrost will simply evaporate off to atmosphere. Similarly, significant rises in the temperature of the sea could destabilise deposits. This is unlikely to occur at great depths because the well known density inversion in water around 4ºC keeps the temperature at depths remarkably consistent. Further, the heat capacity of the oceans is enormous. However, atmospheric changes can cause temporary flows of warm water e.g. off N. California methane hydrate exists at depths >510m and of course is normally stable. In August 1997, the well-publicised El Nino event caused the water to rise temporarily in temperature in excess of the stability limiting value so presumably some methane was released. Similar events have been monitored in the Gulf of Mexico and elsewhere.
There have been sea stories around for centuries describing how ships have been sailing along and then have vanished without trace. No storms, no explosions – just vanished! Some reports have included the sea burning and the ship vanishing. One plausible explanation is the release of methane from the sea bottom. As the gas bubbles rise they reduce the AVERAGE density of the water and hence a ship could simply sink because its displacement x density falls to less than the mass of the ship. If flames derived from electrical equipment, boiler fires etc., are on board the methane could well burn at the sea surface. However, although all this seems plausible, the release of methane may not involve the clathrate and may occur from gas rich oil deposits.
Where does the methane already in the atmosphere come from? No, not the release of ‘frozen’ methane, but rather release from more mundane sources, e.g. agriculture, landfills.
Having skimmed over the science let us go back and develop some of the ideas surveyed so far.
Physical chemistry of methane clathrate
The structure of the compound is based on that of ice. There are various structures of ice i.e. the water phase diagram is complex, various structural forms being present over defined pressure and temperature domains. All have interstitial sites, some of which can be occupied by methane. The methane itself also alters the structure, and hence the phase diagram, by influencing the surfaces between the phases. One typical structure of the clathrate is shown below.
Part of crystal structure of emthane hydrate (purple spheres represent methane molecules, red cylinders represent oxygen atoms, grey cylinders represent hydrogen atoms).
Methane hydrate is a crystalline clathrate compound in which methane gas molecules are trapped in water cages which form part of a distorted ice matrix. Each water cage can only be occupied by one methane molecule. Methane hydrate has a cubic crystal structure consisting of two types of dodecahedral water cages: a small cage which has twelve pentagonal faces and a large cage having twelve pentagonal and two hexagonal faces. This leads to the stoichemistry CH4 + 6H2O CH4.6H2O. The amount of methane incorporated in a methane hydrate compound varies depending on the method of preparation, but can be of the order of 90% of the water cages being occupied by methane gas. The quantity of methane that can be stored in this methane hydrate material is vast, with as much as 180 volumes of methane per volume of the methane hydrate compound.
There is considerable interest in the way that a gas molecule interacts with the host and how it affects the hydrogen-bonded water lattice, and infrared and Raman spectroscopies have been of value. Gas molecules in the hydrate water cages can distort the water lattice as well as modulate the intermolecular vibrational motions of the water lattice. At around 273 K, this crystalline methane hydrate compound is only stable at high pressures in excess of around 3 MPa. Therefore, in situ experimental studies on this compound require specialised high pressure, variable temperature reactors. One way out of this dilemma is to use another molecule which forms a similar clathrate hydrate structure to natural gas, but which does not to require pressurisation. An example of this type of work is given in the submitted paper in Section 2.
THF/water solution (left) before hydrate formation and (right) after hydrate formation.
As is so often the case in any study of this type, vibrational spectroscopy can be of immense value, but its true worth is only revealed when it is supported with X-ray, D.S.C., neutron scatter and other measurements. The Raman spectrum of methane hydrate (shown below) can tell us about the structure of methane hydrate. From the n1(C-H) symmetric stretching vibration of methane we can find out whether methane hydrate has been formed and also what are the relative amounts of methane in the large and small water cages. Methane trapped in a small water cage has a band at higher frequency than methane trapped in a large water cage.
Raman Spectrum of methane clathrate
Perhaps the most significant measurements, because they define the stability parameters for the compound, are those made on pressure/temperature. The isotherms for methane/ice are
Phase diagram of methane hydrate.
Note: Units of pressure x 10-1 atmos.
from which you can see immediately that the compound is likely to be found under arctic tundra and in deep seas. The pressure rises with depth at around 1 atmosphere. (105Ps) every 10m. In tundral deposits, the temperature is a problem. However cold the atmosphere is and however impermeable the frozen tundra, the temperature inevitably rises the deeper you go due to heat conducting from the Earth’s core. Of course, exactly the same happens in alluvial deposits beneath the sea, so the layer of stable material is inevitably of restricted depth. Put another way, the pressure and temperature rises linearly with depth but the diagram above shows that the pressure over the clathrate rises exponentially with temperature. There must always be a limiting depth. As mentioned above, deep seas are remarkably constant in temperature due to the well known density temperature inversion near 4ºC. Thus, given enough depth, the compound should be stable almost anywhere, and it is. It is also worth noting that the DH¼ of the clathrate is high at 54.2KJ mole-1, compared with ice at 6.008 due no doubt to the phase changes solid clathrate → water (l) and methane (g).
Methane hydrate as a fuel in vehicles?
Assuming that methane becomes a popular source of energy then the possibility of using it in ships or even motor vehicles becomes economically very important. Of course, natural gas is already widely used and has been for many years. In the UK, buses and trucks were fitted with huge bags containing methane from anaerobic biocomposition during World War II and some buses today carry compressed methane as a fuel. The problem with compressed or liquefied methane is the cost of storage and the obvious safety implications. Similar doubts exist if H2 was to be developed extensively as a fuel for road vehicles. It has been suggested that methane hydrate supplied as a slush from pumps might provide an answer, first probably for ships but why not in road vehicles.
In the table below, we give some data to explain the attraction.
|Material||Thermal Yield in Btu/cu ft
(Sorry about the units!)
~3000Btu = 1KWhr
1 cu ft. = 28.6l
|CH4gas at 1 atoms.||1012|
|Jet Engine fuel||910,000|
So the tanks in ships, trucks or cars would have to be larger than they are now by about 5 x. However, in a small car the fuel tank is currently ~50l capacity, but the vehicle can well have an internal capacity of 2500-3000l, so this should pose little problem.
We have concentrated so far on the potential of this material to produce energy in the future, but its existence can be a real headache to the petroleum industry. Methane, usually from conventional deep gas fields, is frequently wet and is piped in this condition over long distances. If the pipeline cools down in the winter to low temperatures, the conditions can become just right for the clathrate to form and block the pipe valves or pumps. Although precautions are taken (pipeline heating for example), this is a real problem. The chemical structural work done to date is important in developing methods to inhibit the formation of the blockages and much of the work done using FTIR, X-ray and other structural tools has been driven by urgent needs in this field.
Two main types of chemical inhibitors can be used to control methane hydrate formation: traditional thermodynamic inhibitors or as an alternative low-dosage inhibitors. Thermodynamic inhibitors e.g. methanol or ethylene glycol, have been used widely by the gas and oil industries to prevent gas hydrate formation in gas pipelines, however, the costs involved are huge due to the large volumes of inhibitor required. Low-dosage inhibitors, on the other hand are more attractive since as their name suggests, only small concentrations of these chemicals are needed to prevent hydrates from forming. Current research is concerned on optimising these low-dosage inhibitors. Promising candidates are the alkyl acrylamide polymers amongst others (all of low molecular weight). These materials effect the kinetics of hydrate formation.
How does the methane clathrate influence the future of energy policy and the greenhouse effect. Quite obviously, the availability of relatively pure, sulphur free methane to nations that are energy hungry and available close by and under their own direct control must be very attractive. Obviously too, production of methane this way is incredibly capital intensive but so is nuclear power generation or hydroelectricity. As methane becomes freely available in large quantities from this source, the high thermal yield from burning the gas must start to make wind, solar or wave power generation (where energy yield per dollar is inevitably poor) less attractive.
Thus, there is the prospect that as a consequence of this new source of energy the production of CO2 will inexorably rise. However, methane will provide, as it is already doing, a short-term amelioration of this rise by replacing fuels that generate more CO2 per unit thermal output e.g. lignite, the worst offender, coal and oil. Methane can half the emission, thus the ‘dash for gas’ in the UK – the replacement of coal fired power stations by natural gas fuelled units. But remember, once methane has ousted other carbonaceous fuels the output of CO2is then bound then to rise and rise as more and more energy is required World Wide..
The physical chemistry that entraps CH4 can also clathrate CO2. It has been suggested that this might solve the World’s problem. If the CO2 is removed from the air, compressed and piped to the deep sea it will react with water and freeze as CO2 hydrate, stable at depth at 4ºC. Experiments have been carried out and it is clear that stable CO2 reservoirs could be established at depths of 3600m and probably a lot less.
The snag is that to STABILISE not REDUCE the level of CO2 in the air at the moment we will need to ‘lose’ ~1010 tonnes CO2 a year. The cost would be quite overwhelming. Assuming the World accepts that the polluter pays, the Americans have a little problem!
To finish on a sombre note – we now know that vast amounts of methane are there for the having. Burning the stuff is relatively very clean and its widespread availability must be attractive to politicians. Put another way – politicians may well wring their hands and the environmental lobby may scream, but it is hard to believe that this financial bonanza will stay at the bottom of the sea much longer.
Dr Carolyn Koh is Reader in Chemistry at King’s College University of London and an adjunct professor at Cornell University. She has researched in the hydrocarbon hydrate field for some years and has published extensively.
REF: C.A. Koh & P.J. Hendra,
Int.J.Vibr.Spec., [www.irdg.org/ijvs] 6, 1, 2 (2002)