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A widespread release of methane from the seabed has been reported in many regions of the world’s oceans over the past decade. However, how methane escapes from these deep formations is poorly understood.
More specifically, scientists are perplexed. Around the world, several on-site observations have shown vigorous columns of methane gas bubbling from these formations in places. The high pressure and low temperature of these deep water environments should create a solid icy layer that should act as a kind of capstone, preventing gas from escaping.
A new study by MIT scientists could reveal how and why gas columns can escape from deep formations, known as methane hydrates. Methane hydrate is an ice-like solid that forms from the methane-water mixture under high pressure and low temperature conditions typical of deep marine environments, often referred to as the hydrate stability zone (HSZ).
Using a combination of deep-sea observations, laboratory experiments and computer models, scientists have discovered phenomena that identify and quantify how the gas is released free from the icy grip of an icy mixture of water and methane.
At first Xiaojing Fu, now at the University of California at Berkeley, saw photos and videos showing methane plumes taken from a NOAA research vessel in the Gulf of Mexico, revealing the bubble-forming process right at the bottom of the sea. It was clear that the bubbles themselves often formed with an icy crust around them and floated upward with their icy shells like tiny helium balloons.
Next, using sonar, the scientists detected similar bubble plumes from a research vessel off the coast of Virginia.
Fu said, “This cruise alone has detected thousands of these plumes,” says Fu, who led the research project when he was a graduate student and postdoc at MIT. We could follow these methane bubbles encrusted with hydrate shells in the water column. It was then that we first understood that the formation of hydrates on these gas interfaces could be a widespread event. “
However, exactly what was happening beneath the seafloor to trigger the release of these bubbles remained obscure. Through a progression of laboratory experiments and simulations, the mechanisms at work gradually became apparent.
Fu said, “Seismic studies of the subsurface of the seafloor in these vent regions show a series of relatively narrow conduits, or chimneys, through which the gas escapes. But the presence of bits of hydrated gas from these same formations made it clear that solid hydrate and gaseous methane could coexist. “
To simulate the laboratory conditions, the analysts used a small two-dimensional configuration, placing a gas bubble in a layer of water between two high-pressure glass plates.
Fu said, “As a gas tries to rise through the seafloor, if it is forming a layer of hydrates when it hits cold seawater, that should stop its progress: it is hitting a wall. So how could that wall not stop it from constantly migrating? “Using microfluidic experiments, they discovered a phenomenon previously unknown at work, which they dubbed crustal fingering.”
“If the gas bubble begins to expand, what we have seen is that the expansion of the gas has been able to create enough pressure to essentially break the hydrate shell. And it is almost as if it is hatching from its shell. But instead of each break freezing again with the reforming hydrate, the formation of the hydrate occurs along the sides of the rising bubble, creating a kind of tube around the bubble as it moves upward. “
“It’s almost as if the gas bubble could chisel its path and that path was walled up by the solid hydrate. This phenomenon that they have observed on a small scale in the laboratory, their analysis suggests, is also what would happen on a much larger scale at the bottom of the sea. “
“That observation was the first time we were aware of a phenomenon like this which could explain how the formation of hydrates does not inhibit the flow of gas, but rather, in this case, would facilitate it by providing a conduit and directing the flow. Without that focus, the gas flow would be much more diffuse and diffuse. “
As the hydrate crust forms, it hinders the formation of more hydrates as it forms a barrier between the gas and seawater. The methane beneath the barrier can, in this way, resist for a long time in its non-frozen gaseous form. The combination of these two phenomena – the focusing impact of hydrate walled channels and the segregation of methane gas from water by a hydrate layer – “goes a long way to clarify why it is possible to have some of this vigorous vent,” due to the formation of hydrates, instead of being prevented by it.
Fu said, “A better understanding of the process could help predict where and when such methane seeps will be found and how changes in environmental conditions could affect the distribution and exit of these seeps. Although there have been suggestions that warm weather could increase the rate of such ventilation, so far there is little evidence of this. “
“Temperatures at depths where these formations occur – 600 meters (1,900 feet) deep or deeper – should experience less temperature rise than would be necessary to trigger a widespread release of the frozen gas.”
MIT Professor Ruben Juanes said: “Some researchers have suggested that these vast underwater methane formations could one day be exploited for energy production. However, there would be major technical obstacles to such use. These findings could help evaluate the possibilities. “
Hugh Daigle, associate professor of petroleum and geosystems engineering at the University of Texas at Austin, said: “The problem of how gas can move through the hydrate stability zone, where we would expect the gas to be immobilized by converting to hydrate, and instead escape to the sea floor, is not yet fully understood. This work presents a probable new mechanism that could plausibly allow this process to occur and seamlessly complements previous laboratory observations with larger scale modeling. “
“In a practical sense, the work here takes a small-scale phenomenon and allows us to use it in a model that only considers larger scales and will be very useful for implementation in future work.”
Journal reference:
- David A. Weitz et al. Crustal fingering facilitates the migration of free gas methane through the hydrate stability zone. DOI: 10.1073 / pnas.2011064117
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