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Hydrates (or clathrates) of hydrocarbons

A pipeline in a cold climate region

Oil companies have known about methane hydrate since the 1930s, when they began using high pressure pipelines to transport natural gas in cold climates. Pipelines were noted to be obstructed by ice-like crystals, even though temperatures were higher than zero degrees.

Blocks of methane hydrate obstruct pipeline

After several years, the obstructing substance was found to be not ice but methane hydrate, a crystalline compound formed by water and methane molecules. Notwithstanding scientists have been working in the years to reduce formation of hydrates, the problem still exists and it affects the costs. In fact, before entering the pipeline, water must be carefully removed, since formation of chunks of methane hydrate will impede the flow of gas.

Methane hydrate. The methane molecule is schematized in red

What are, then, hydrocarbon (methane) hydrates? When a non polar molecule, such as a hydrocarbon, enters water, some of the hydrogen bonds among water molecules must be broken to make room to accommodate solute molecules. New hydrogen bonds are formed, that can compensate partially or totally the broken hydrogen bonds. Each solute molecule is entrapped in a cage, ice-like structure, that is rigid and ordered because of the organization of water molecules. Such a structure, referred to as "clathrate cage", is constituted by a well defined number of water molecules held together by hydrogen bonds.

Burning methane hydrate
Methane hydrate looks like water ice, but its behaviour is very different. It is known as "burning ice", because it burns when a lighted match is put to it.

In the right picture, a model is shown of the hydrate crystal structure. Water molecules form cages, and almost all cages are filled with methane molecules indicated by yellow balls.

Model of the crystal structure of methane hydrate

The map shows the distribution of known gas hydrate deposits. Most methane hydrates occur in marine sediments on continental margins

The greatest amounts of methane hydrates exist in ocean floor sediments, where they are stable at water depths greater than 300 meters. Bacteria in submarine sediments consume organic material and generate methane. Under high pressure and low temperature conditions, namely in the deep, methane hydrate (clathrate) forms. Deposits of methane hydrate on the ocean floors are estimated to contain about ten thousand billion tons of carbon, about twice the amount of carbon to be found in all carbon, oil and natural gas layers on Earth. Methane hydrates, then, are an immense resource of methane gas, and they could be exploited as fuels. That could be a potential solution for energy problems of our planet in the transition towards renewable energy sources.

To date, however, exploiting the energy stored in methane hydrates is a challenging engineering and environmental problem. In fact, aside from the difficulties of deep-sea operations, nowadays technologies do not protect against the risk of putting large amounts of methane in the air and in the sea. That would accelerate the global warming of Earth, since the ability of methane greenhouse effect is estimated to be twenty times higher than that of carbon dioxide.