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.
Unconventional gas: shale gas
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The melting of the ice cells, not only brings about the release of methane gas, but it also produces another important substance: fresh water