MIT Researchers Elucidate Model for Escape of Underground Methane in Frozen Regions; Current Rate of Escape May be Much Faster Than Earlier Believed
|Left: underground methane gas as it begins to invade fine-grain sediment (shown in yellow) by creating a fracture. Right: the blue circles represent pore spaces where the gas has invaded. Graphic / Ruben Juanes, MIT. Click to enlarge.|
Researchers at MIT have elucidated how underground methane in frozen regions—e.g., gas hydrate accumulations in ocean sediments and permafrost—escapes. Their findings, published online 29 August in the AGU Journal of Geophysical Research, also suggest that methane trapped under the ocean may already be escaping through vents in the sea floor at a much faster rate than previously believed.
Some scientists have associated the release, both gradual and fast, of subsurface ocean methane with climate change of the past and future. Methane is about 21 times more powerful at warming the atmosphere than carbon dioxide (CO2) by weight (see box below), according to the US EPA.
The sediment conditions under which this mechanism for gas migration dominates, such as when you have a very fine-grained mud, are pervasive in much of the ocean as well as in some permafrost regions. This indicates that we may be greatly underestimating the methane fluxes presently occurring in the ocean and from underground into Earth’s atmosphere. This could have implications for our understanding of the Earth’s carbon cycle and global warming.—lead author Ruben Juanes, the ARCO Assistant Professor in Energy Studies in the Department of Civil and Environmental Engineering
Methane, the primary component of natural gas, is more abundant in the Earth’s atmosphere now than at any time during the past 400,000 years, according to a recent analysis of air bubbles trapped in ice sheets. Over the last two centuries, methane concentrations in the atmosphere have more than doubled. It is estimated that about 60% of global methane emissions are tied to human activities like raising livestock and coal-mining, with the rest tied to natural sources such as wetlands, decomposing forests and underground deposits known as methane hydrates.
In the hydrate phase, a methane gas molecule is locked inside a crystalline cage of frozen water molecules. These hydrates exist in a layer of underground rock or oceanic sediments called the hydrate stability zone or HSZ. Methane hydrates will remain stable as long as the external pressure remains high and the temperature low. Beneath the hydrate stability zone, where the temperatures are higher, methane is found primarily in the gas phase mixed with water and sediment.
The stability of the hydrate stability zone is climate-dependent.
|Underground methane gas invading fine-grain sediment (shown in yellow) by creating a fracture, as predicted by the Jain and Juanes model. Blue circles represent pore spaces where the gas has invaded. The maroon lines indicate compressive forces between sediment grains. The video shows that the network of compressive forces changes drastically with the evolution of the fracture. The green lines indicate tension between grains, caused by capillary forces that hold the grains together. The network of tension forces also changes with time, as the gas invades the sediment. Video / Ruben Juanes and Antone Jain, MIT.|
If atmospheric temperatures rise, the hydrate stability zone will shift upward, leaving in its stead a layer of methane gas that has been freed from the hydrate cages. Pressure in that new layer of free gas would build, forcing the gas to shoot up through the HSZ to the surface through existing veins and new fractures in the sediment.
A grain-scale computational model developed by Juanes and recent MIT graduate Antone Jain indicates that the gas would tend to open up cornflake-shaped fractures in the sediment, and would flow quickly enough that it could not be trapped into icy hydrate cages en route.
Previous studies did not take into account the strong interaction between the gas-water surface tension and the sediment mechanics. Our model explains recent experiments of sediment fracturing during gas flow, and predicts that large amounts of free methane gas can bypass the HSZ.—Ruben Juanes
Using their model, as well as seismic data and core samples from a hydrate-bearing area of ocean floor (Hydrate Ridge, off the coast of Oregon), Juanes and Jain found that methane gas is very likely spewing out of vents in the sea floor at flow rates up to 1 million times faster than if it were migrating as a dissolved substance in water making its way through the oceanic sediment—a process previously thought to dominate methane transport.
Our model provides a physical explanation for the recent striking discovery by the National Oceanic and Atmospheric Administration of a plume 1,400 meters high at the seafloor off the Northern California Margin. [Gardner et al., Eos]—Ruben Juanes
|The undersea plume. Credit: Gardner et al. Click to enlarge.|
This plume, which was recorded for five minutes before disappearing, is believed not to be hydrothermal vent, but a plume of methane gas bubbles coated with methane hydrate.
The Jain and Juanes paper in the Journal of Geophysical Research also explains the short-term consequences of injecting carbon dioxide into the ocean’s subsurface, a method proposed by some researchers for reducing atmospheric greenhouse gas. Juanes found that while some of the CO2 would remain trapped as a hydrate, much would likely spew up through fractures just as methane does.
It is important to keep both methane and carbon dioxide either in the pipeline or underground, because the consequences of escape can be quite dangerous over time.—Ruben Juanes
This research was funded by the US Department of Energy.
Jain A. K., R. Juanes (2009), Preferential Mode of gas invasion in sediments: Grain-scale mechanistic model of coupled multiphase fluid flow and sediment mechanics, J. Geophys. Res., 114, B08101, doi: 10.1029/2008JB006002
Gardner, J. V., Malik, M. A., Walker, S. (2009) Plume 1400 Meters High Discovered at the Seafloor off the Northern California Margin, EOS Transactions, American Geophysical Union, Vol. 90, No. 32, pp. 275 - 275