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Crustal fingers release methane gas from the seafloor

3 December 2020

A new study reveals how methane bubbles percolate out of the frozen hydrates that entrap them.

A recent estimate puts the amount of methane sequestered in deep-ocean sediments at 2400 gigatons. That’s about a quarter of all fossil-fuel reserves on Earth. The gas is bound up as molecules inside methane hydrates—icelike cages that form when the gas mixes with water at low temperature and high pressure. And yet a plethora of field observations have identified marine regions where gas plumes from such reservoirs are pervasive (see, for example, Physics Today, August 2017, page 21). A long-standing puzzle is why those vents can coexist in conditions where the methane hydrates are supposedly stable enough to prevent the gas’s escape.

An international team led by Caltech’s Xiaojing “Ruby” Fu and MIT’s Ruben Juanes, Fu’s former adviser, have now solved that mystery by experimentally and computationally investigating gas percolation under hydrate-formation conditions. To analyze the flow of the methane through water, the researchers fashioned a Hele–Shaw cell—two parallel glass plates separated by a thin gap (see Physics Today, October 2012, page 15). That geometry allowed them to study the flow driven by an imposed pressure gradient rather than buoyancy. And it restricted the observations to two dimensions, which simplified their study of the gas’s movement.

After injecting a single bubble of xenon (a proxy for methane) into water at high pressure between the plates, the researchers closed the gas port and began depressurizing the cell by drawing out its water. The concomitant expansion of the bubble led to the pressure gradient that drives the gas flow. A thin finger of gas emerged from the bubble, and as it did, hydrate crust solidified along its gas–water interface. But once the crust had formed, the slow diffusion of water and xenon hindered continued growth. Instead of clogging the flow with a thick lid, the hydrate crust remained thin and rigid. Gas pressure repeatedly broke any newly formed crust at the end of the finger and released entrapped gas. Each time, the finger lengthened and meandered around the cell. Eventually, reduced driving pressure arrested the flow, at which point the crust ruptured elsewhere and gave birth to a new branch.

Formation of crustal fingers.
Credit: X. Fu et al., Proc. Natl. Acad. Sci. USA, 2020, doi:10.1073/pnas.2011064117

The figure shows snapshots of the phenomenon, dubbed crustal fingering. The circles mark locations of ruptured hydrate, and the thin white line bordering each finger is newly created crust. Counterintuitively, when the local gas-flow rate is sufficiently high, the crust usually remains free of the fluid pathways and forms rigid channels that facilitate the flow. The channels form a barrier that prevents gas–water mixing across the interface. Those two features—the channeling of gas and the inhibition of further growth—explain the common appearance of plumes. (X. Fu et al., Proc. Natl. Acad. Sci. USA, 2020, doi:10.1073/pnas.2011064117.)

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