Methane Hydrates (Methane Clathrates)

Natural gas deposits aren’t created equal. The natural gas resources known to Dr. M.K. Hubbert when he was writing his 1956 paper was “conventional” natural gas which consisted of mostly methane (CH4) trapped by deep geological formations in a manner similar to oil. This resource was already well understood and well scoped out in that time. It turns out there is another mechanism by which methane is stored in nature. Methane and water can form crystals of methane hydrate when compressed together to about 50 atmospheres at temperatures around 0 Celsius. These conditions may arise beneath permafrost and at ocean bottoms on the continental shelf.[1] Methane hydrate looks like dirty ice. Estimates of the quantity of methane hydrate vary widely. One estimate puts the carbon content in global methane hydrate reserves at three trillion tonnes, much more than in the supply of conventional gas. If all of it could be recovered, the methane from hydrates would last a thousand years at the current rate of exploitation of methane, or one hundred years if methane hydrate replaced all fossil fuels.[1] Other estimates place the amount between 0.5 and 2.5 trillion tonnes.[2]

According to Wikipedia, 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 kg/litre. One litre of methane clathrate solid would therefore contain, on average, 168 litres of methane gas under standard conditions.[2]

No deposit of methane hydrate has yet been developed commercially. Extraction is a tricky procedure. In the case of methane hydrates trapped below permafrost, two methods were tested in Canada’s Mackenzie delta in 2002. The first method, nicknamed “depressurization” involves drilling a conventional wellhead and waiting for the pressure differential from the gravitational pressure of the mass above the deposit against one atmosphere at the wellhead itself to force the methane up the well. The other is to pump steam into the methane hydrate. This second method accelerates the extraction but costs almost as much energy as it liberates, resulting in a poor energy balance.

Canada is not in a hurry to exploit methane hydrates because it has large conventional fossil fuel supplies, but some other countries are keen to try hydrate mining, and they are looking at undersea deposits. Of note are South Korea, which has discovered deposits in the Ulleung Basin, and Japan in the Nankai trough off Honshu. Production may begin in 2015-16.

Some scientists worry about the risk of a spontaneous release of methane into the atmosphere because the hydrates are in a delicate balance. If a deposit is disturbed such that an escape path for methane is created, then the escaping gas decreases the local pressure, which triggers the release of methane from neighbouring crystals. A chain reaction like this can lead to an uncontrollable “methane burp” that will empty an entire large reservoir of methane into the atmosphere. Not only would this mean a large waste of fuel and a large addition of carbon to the total in circulation, but methane is a powerful greenhouse gas in its own right. One idea being considered is to maintain pressure in the hydrate deposits by pumping in CO2 to replace the CH4. This should prevent methane burps and sequester about as much carbon as is being extracted.

The global demand for fossil fuels is so great that the methane in hydrates is likely to be exploited as thoroughly as possible even though it will contribute to and even accelerate global warming. However, it might not be feasible to exploit it as thoroughly as petroleum and conventional natural gas because these conventional hydrocarbons seem to gather in large pools or “fields”, while hydrates are more thinly spread.

It remains to be seen what effect methane hydrates have on the world’s energy situation. The maximum likelihood outcome is that they will be exploited, thus delaying the decline of natural gas fossil fuel, but will contribute to global warming. It is less likely that mining methane hydrates will prove completely unsuccessful. If that were to happen, then hydrates won’t affect our energy picture, but if the permafrosts of northern Russia, Canada, and Alaska melt because of global warming from other causes, that may trigger giant “methane burps” that will accelerate that warming trend.

[1] Ice on Fire, New Scientist, 27 June 2009
[2] http://en.wikipedia.org/wiki/Methane_hydrate, http://en.wikipedia.org/wiki/Methane_clathrate

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