“Fire ice” an opportunity and a threat

Scientists have been investigating the mechanical properties of methane hydrates, also known as “fire ice” – ice that contains methane that forms under the sea, typically on the continental shelf, or is buried in permafrost, where they can clog up oil and gas pipelines. A better understanding of its characteristics will help to improve its management, important for climate science as well as possible applications in a future energy source.

With estimates of total methane gas in hydrate form varying from about 3,000 to more than 140,000 trillion cubic meters, some countries have initiated programs for its exploration and exploitation, making the geomechanical properties of gas hydrate-bearing sediments increasingly important.

On melting, methane hydrates release the methane contained inside the ice – however, as the methane was trapped under pressure when the hydrate was formed, a single cubic meter of solid methane hydrate can release up to 160 cubic meters of methane gas, making them a potential energy source. On the other hand, if they melt with the permafrost it could unleash a great deal of methane, which acts as a greenhouse gas.

Although methane hydrates are extremely difficult to study due to the difficulty of obtaining samples, and any that are acquired tend to be highly unstable, researchers from Norway, China and the Netherlands, whose study was reported in Nature Communications [Wu et al. Nat. Commun. (2015) DOI: 10.1038/ncomms9743], have managed to explore the relation between molecular structures and the mechanical stability in both monocrystalline and polycrystalline methane hydrates.

“The cage type, cage occupancy and grain size play an important role in mechanical behaviors of gas hydrates.”Fulong Ning

Using a computer simulation of the two types, the team simulated the effect of forces being applied to the grains, demonstrating how the size of the molecules that make up the natural structure of methane hydrates determines their behavior under mechanical loading or when they are disturbed. They simulation involved to two kinds of stress: tensile and compressive, and showed the factors in the hydrate structure that determined how it reacted to the stresses. As researcher Fulong Ning points out, “the cage type, cage occupancy and grain size play an important role in mechanical behaviors of gas hydrates”.

When the grain size was reduced, the hydrates became stronger and able to tolerate both stresses. However, this was only the case until they reached a certain grain size – after that, the hydrate got weaker. This maximum capacity was when the grain size was about 15–20 nm, the first time that this type of behavior in methane hydrates has been observed as a material, and is similar to the behavior of polycrystalline metals. This grain size-dependent strength and maximum capacity could one day be used to predict, and even prevent, the failure of hydrates.

There was a remarkable difference in mechanical behaviors between the monocrystalline and polycrystalline hydrates, and the dissociation of methane hydrates could be triggered by ground deformation from events such as earthquakes, storms, sea-level fluctuations or even man-made disturbances such as well drilling. The team will now explore the essential mechanical difference between methane hydrates and ice in both single crystal and polycrystalline forms through molecular simulations and micro-experiments, and may then look at the interaction between gas hydrate crystals and sediment grains under the loading condition.