Methane hydrates

Energy Natural gas

Natural gas is the last fossil source of energy to be used on a global scale. For over a century, when the gas was discovered in areas that were far from the places where it could be utilized, it was preferably burnt at the gas well or freed into the atmosphere, because harnessing it in a pipeline and making it travel for many kilometres was too expensive. The situation has changed over the past forty years and today natural gas ranks third in world consumption of energy and is the fossil source with the best growth prospects.

At the beginning of the Nineties, in marine geology research environments, word began to spread about a particular substance present under the ocean floor: it was the first information on methane hydrates that had up to then received scarce attention because they were considered not much more than a mere geological curiosity and lacking any commercial value.

The so-called biogenic methane is released during decomposition processes of organic matter and accumulates within sediments where it may concentrate and eventually rise towards the surface. If the surface is a sea bed, the gas that is released mixes with the cold water of the deep sea and forms a sort of ‘ice’. The water molecules crystallize forming a ‘cage-like’ structure inside which methane molecules are trapped. On freezing, the water squeezes the gas and the mixture’s density increases greatly. Chemically, methane hydrates are made up of a molecule of methane and 6 molecules of water (CH46H2O) and belong to the ‘clathrate’ family, that comprises compounds whose crystalline solids occur when water molecules form cells closed in a ‘cage-like’ structure. For this process to take place, two simultaneous factors are necessary: a low temperature (-15°C) and high pressure all around (20 bar, that corresponds to a sea depth of a little less than 200 m), in addition to an abundant supply of methane and water molecules, of course.

Where do you find them?

Due to the particular conditions required for these compounds to form and remain stable, their presence is limited to three environments: ocean floors, terrains covered by permafrost and the deeper polar ice.

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An original behaviour

Methane hydrates, made as they are of ice ‘cages’ that trap gas molecules, are stable compounds only when simultaneous very low temperature and high pressure conditions occur. If the temperature increases or the pressure decreases, the ice melts and methane is released in its gaseous form.

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Methane: a ‘clean’ fuel

Among fossil fuels, currently methane seems to be the one that will be exploited more and more in the near future, thanks to its relative abundance and thanks to the fact that it is relatively ‘clean’. Its molecule is made up of 4 atoms of hydrogen and one of carbon (CH₄).

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What is known about methane hydrates, about their behaviour and their distribution along with the geological data of the past, suggest caution in the race to utilize these compounds for the production of methane. The exploitation of these enormous energy reserves could temporarily solve many energetic problems and help in the difficult transition phase between the utilization of fossil fuels and the use of renewable energy sources. However, the environmental risks connected to an indiscriminate exploitation that does not respect the environment seem very high. Commercial exploitation must therefore be postponed till when state of the art technologies can protect us from the most serious risk: the release of great quantities of methane into the sea and the atmosphere.

Moreover, the possible risk of triggering off great underwater landslides on a vast scale must not be forgotten.

The presence of compounds with such particular and unstable characteristics, that are so sensitive to the slightest variation in temperature keeps us on guard as far as man’s contribution to the greenhouse effect is concerned: it is true that geological data show that climate changes on a large scale and environmental and climatic ‘crises’ at a planetary level have occurred in the geological past without any involvement on man’s part, but our behaviour could give a decisive contribution to sparking off these processes that, once started, could prove to be irreversible.

Limits of methane hydrates

The exploitation of such quantities of natural gas is not possible today: present day technologies are not yet able to collect the hydrates so as to extract the gas, without losing it in the environment. The first problem that has to be solved is that of finding the deposits.

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Hydrates and climate changes

Methane is much more opaque to infrared radiation than CO₂ and consequently it produces a greenhouse effect 20 times greater than that of carbon dioxide. It is a gas whose effect on the atmosphere is much more dangerous than that of CO₂.

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A look at the past

Some geological evidence proves that there have been climatic ‘crises’ on a large scale that have modified the distribution of creatures living on Earth. Recent geological and paleontological researches seem to indicate that in at least one of these crises the role played by methane hydrates could have been very important.

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Methane hydrates

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