An escape route for methane from the seabed

Methane, the main component of natural gas, is the cleanest fuel of all fossil fuels, but when it is released into the atmosphere it is a much more potent greenhouse gas than carbon dioxide. According to some estimates, the seafloor methane contained in ice formations along the continental fringes may equal or exceed the total amount of coal, oil and gas in all other reservoirs in the world. However, how methane escapes from these deep formations is poorly understood.

In particular, the scientists were faced with a puzzle. Observations at sites around the world have shown vigorous columns of methane gas bubbling from these formations in some places, however the high pressure and low temperature of these deep-sea environments should create a solid, icy layer that should act as a kind of capstone, preventing the gas from escaping. So how does the gas come out?

A new study helps explain how and why gas columns can escape from these formations, known as methane hydrates. Using a combination of deep-sea observations, laboratory experiments and computer modeling, the researchers discovered phenomena that explain and predict how the gas breaks free from the icy grip of an icy mixture of water and methane. The findings are reported today in the journal PNAS, in an article by Xiaojing (Ruby) Fu SM ’15, PhD ’17, now at the University of California at Berkeley; Professor Ruben Juanes at MIT; and five more in Switzerland, Spain, New Mexico and California.

Surprisingly, not only does the formation of frozen hydrates fail to prevent methane gas from escaping into the ocean column, it actually facilitates such a leak in some cases.

Initially, Fu 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.

Later, Fu used sonar to detect similar bubble plumes from a research vessel off the coast of Virginia. “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,” he says. “It was then that we first understood that the formation of hydrates on these gas interfaces can be a very common occurrence.”

But what was going on beneath the seafloor to trigger the release of these bubbles remained unknown. Through a series of laboratory experiments and simulations, the mechanisms at work gradually became apparent.

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 hydrate gas from these same formations made it clear that solid hydrate and methane gas could coexist, explains Fu. To simulate conditions in the lab, the researchers used a small two-dimensional configuration, inserting a gas bubble in a layer of water between two high-pressure glass plates.

As a gas tries to rise through the seafloor, Fu says, if it’s forming a layer of hydrates when it hits cold seawater, that should block its progress: “It’s hitting a wall. So how could that wall be? not stop it from continually 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 was able to create enough pressure to essentially break the hydrate shell. And it’s almost as if it’s hatching out of its own shell, “says Fu. 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 sort of of tube around the bubble as it moves upward. “It’s almost as if the gas bubble is able to chisel its own path, and that path is walled up by the solid hydrate,” he says. This phenomenon they observed on 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, he said, “was indeed the first time we were aware of a phenomenon like this that could explain how the formation of hydrates will not inhibit the flow of gas, but rather in this case, facilitate it,” providing a conduit and direct the flow. Without that focus, the gas flow would be much more widespread and distributed.

As the hydrate crust forms, it slows the formation of more hydrate because it forms a barrier between the gas and seawater. Methane below the barrier can therefore persist for a long time in its unfrozen gaseous form. The combination of these two phenomena – the focusing effect of channels with hydrate walls and the segregation of methane gas from water by a hydrate layer – “explains why it is possible to have part of this vigorous release, thanks to the formation of hydrates rather than being prevented by it, “says Juanes.

A better understanding of the process could help predict where and when such methane infiltrations will be found and how changes in environmental conditions could affect the distribution and production of these infiltrations. Although there have been suggestions that warm weather could increase the rate of such ventilation, Fu says there is little evidence of this so far. Note that 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 diffuse release of the frozen gas.

Some researchers have suggested that these vast underwater methane formations could one day be exploited for energy production. While there would be major technical barriers to such use, Juanes says, these findings could help assess the possibilities.

“The problem of how the 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 seabed, is still not fully understood,” says Hugh Daigle, associate professor. of petroleum and geosystems engineering at the University of Texas at Austin, who was not associated with this research. “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,” says Daigle.

The research team included Joaquin Jimenez-Martinez at the Swiss Federal Institute of Aquatic Science and Technology; Than Phon Nguyen, William Carey and Hari Vinaswanathan at Los Alamos National Laboratory; and Luis Cueto-Felgueroso at the Technical University of Madrid. The work was supported by the United States Department of Energy.