Key properties of methane hydrates found in permafrost and on the continental shelf illuminated
Methane hydrates are a kind of ice that contains methane, and that form at certain depths under the sea or buried in permafrost. They can also form in pipelines that transport oil and gas, leading to clogging. Yet methane hydrates are nearly impossible to study because it is very hard to get samples, and the samples themselves are highly unstable in the laboratory.
Methane hydrates are extremely difficult to study, and could either be an important energy source or a source of methane, a greenhouse gas that is 20 times more potent than CO2. These reasons led a Norwegian, Dutch and Chinese research team to explore the mechanical properties of this poorly understood substance.
Credit: Geir Mogen, NTNU
A team of scientists from Norway, China and the Netherlands has now shown how the size of grains of the molecules that make up the natural structure of methane hydrates determines how they behave if they are loaded with weight or disturbed.
That could have important implications for everything from climate science to their use as a future energy source, said Zhiliang Zhang, a professor at the Norwegian University of Science and Technology and founder of the university's Nanomechanical Lab.
"If we have basic knowledge about the mechanical properties of methane hydrates, we can use this information so that we manage them properly," Zhang said. "How methane hydrates behave can have a big impact on safety, environmental issues and climate change."
Poorly understood and unstable
Methane hydrates have been known since the 1930s, when natural gas companies found that their pipelines were sometimes clogged by a kind of ice composed of water and methane. Methane hydrates were later found in permafrost in the 1960s, and in the oceans, commonly on the edges of the continental shelves, but only at certain ocean pressures and temperatures. They are also thought to be found on other planets, including Mars.
When methane hydrates "melt", they release the methane trapped inside the ice, but because the methane was first trapped under pressure when the hydrate was formed, one cubic metre of solid methane hydrate will release 160 cubic metres of methane gas. That makes them either a potent energy source, or if they melt as the permafrost melts, a potent source of methane, which will act as a greenhouse gas.
But mining methane hydrates as an energy source, an option that is being explored by Japan among others, is technically difficult. Their location on the soft, sediment-loaded edges of the continental shelves makes them difficult to mine, and when they are disturbed, their crystal structure may suddenly dissociate and release the methane trapped inside.
This mechanism has been suggested as one reason why the largest landslide known to humankind, the Storegga Slide, was so destructive. The Storegga Slide took place about 8000 years ago, from an underwater location off the west coast of southern Norway.
The slides - there were three in total¬- sent a wall of water roaring out across the North Sea and Norwegian Sea. The evidence of the passage of the tsunami wave in Scotland that shows the wave reached heights of 3-6 metres in that region. One hypothesis for the slide was that an earthquake caused the methane hydrates in the region to become unstable and to explosively release their gas.
Computer simulations show surprising behaviour
Researchers at NTNU's Nanomechanical Lab and from the university's Department of Chemistry and their collaborators in China and the Netherlands are interested in understanding the relation between molecular structures and the mechanical stability of materials. Methane hydrates, with their ice lattice structure containing trapped molecules of methane, pose an intriguing three-dimensional and practical problem from this perspective.
In a paper published in the 2 November edition of Nature Communications, corresponding author Zhang and his colleagues describe how they used a computer simulation of two types of methane hydrates, monocrystalline hydrates and polycrystalline hydrates, to see what would happen if they were compressed or if pressures on the hydrates were suddenly released.
The researchers built their computer models using common molecular models for ice/water and methane, arranged as either monocrystalline or polycrystalline grains, and simulated the effect of applying forces to the collection of grains.
Maximum capacity found
The simulated hydrate structures were subjected to two different kinds of stress: tensile stress, or the forces they would experience as they were pulled apart, and compressive stress, or the forces they would experience if they were squashed by weight.
The simulations showed that the size of the crystals--what researchers call the grain size--that made up the hydrate structure had a great deal to say in terms of how the structure reacted to both kinds of stresses.
In both tensile and compression stress simulations, the surprising finding was that as the grain size got smaller, the hydrates first got stronger, able to tolerate both compression and tensile stress--but only until they reached a certain grain size. If the researchers conducted simulations on grain sizes smaller than those identified as the turning point, the hydrate actually got weaker.
The maximum capacity of the hydrates appears when the grain size is around 15 to 20 nm. This resembles the behaviour of polycrystalline metals, such as copper. However, this is the first time that researchers have seen this type of behaviour in methane hydrates as a material. The grain size-dependent strength and maximum capacity that the researchers found can be used in predicting and preventing the failure of hydrates in the future.
Instability can be triggered
This unexpected rapid weakening of the crystal structure as the grain size gets smaller has important implications for any work in areas where hydrates are found.
The researchers reported that the dissociation of methane hydrates can be triggered by the ground deformation caused by "earthquakes, storms, sea-level fluctuations or man-made disturbances (including well drilling and gas production from hydrate reservoirs)."
"This has an impact on these big issues," Zhang said. "Here we have taken one step forward, but of course there is a lot more work to be done."
Zhang said the researchers plan to continue their collaboration and are currently at work on a center of excellence application to the Research Council of Norway.
Reference: Jianyang Wu, Fulong Ning, Thuat T. Trinh, Signe Kjelstrup, Thijs J.H. Vlugt, Jianying He, Bjørn H. Skallerud and Zhiliang Zhang. Mechanical instability of monocrystalline and polycrystalline methane hydrates. Nature Communications. DOI: 10.1038/NCOMMS9743
Zhiliang Zhang | EurekAlert!
Nagoya physicists resolve long-standing mystery of structure-less transition
21.08.2017 | Nagoya University
Scientists from the MSU studied new liquid-crystalline photochrom
21.08.2017 | Lomonosov Moscow State University
Whether you call it effervescent, fizzy, or sparkling, carbonated water is making a comeback as a beverage. Aside from quenching thirst, researchers at the University of Illinois at Urbana-Champaign have discovered a new use for these "bubbly" concoctions that will have major impact on the manufacturer of the world's thinnest, flattest, and one most useful materials -- graphene.
As graphene's popularity grows as an advanced "wonder" material, the speed and quality at which it can be manufactured will be paramount. With that in mind,...
Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.
Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...
For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.
While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...
An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.
The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...
A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.
Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...
16.08.2017 | Event News
04.08.2017 | Event News
26.07.2017 | Event News
22.08.2017 | Power and Electrical Engineering
22.08.2017 | Medical Engineering
22.08.2017 | Awards Funding