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Saturday, July 27, 2013

Methane gas likely spewing into the oceans through vents in sea floor

Could speed up global warming more efficiently than carbon dioxide

by Denise Brehm, Civil and Environmental Engineering, MIT News, September 2, 2009

Scientists worry that rising global temperatures accompanied by melting permafrost in arctic regions will initiate the release of underground methane into the atmosphere. Once released, that methane gas would speed up global warming by trapping the Earth's heat radiation about 20 times more efficiently than does the better-known greenhouse gas, carbon dioxide.

An MIT paper that appeared online August 29, 2009, in the Journal of Geophysical Research elucidates how this underground methane in frozen regions would escape and also concludes 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.

The image above depicts underground methane gas as it begins to invade fine-grain sediment (shown in yellow) by creating a fracture. In the image at right, the blue circles represent pore spaces where the gas has invaded. RUBEN JUANES

"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," said lead author Ruben Juanes, ARCO Assistant Professor in Energy Studies in the Department of Civil and Environmental Engineering.
Ruben Juanes, the ARCO Assistant Professor in Energy Studies. CHRIS CHURCHILL

"This indicates that we may be greatly underestimating the methane fluxes presently occurring in the ocean and from underground into Earth's atmosphere," said Juanes. "This could have implications for our understanding of the Earth's carbon cycle and global warming."

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 percent 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.

But the stability of the hydrate stability zone is climate-dependent.

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," said 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," said Juanes. 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," said Juanes.

This research was funded by the U.S. Department of Energy.

Ruben Juanes

ARCO Associate Professor in Energy Studies

I am a geoscientist with a strong interest in the physics of multiphase flow in porous media.

My research focuses on advancing our fundamental understanding and predictive capabilities of the simultaneous flow of two or more fluids through rocks, soils and other porous materials.

Research in my group combines theory, simulation and experiments that elucidate fundamental aspects of multi-fluid flow, which we then apply for prediction of large-scale Earth science problems in the areas of energy and the environment, including geological carbon sequestration, methane hydrates, and ecohydrology of arid environments.
Loosely speaking, our research areas are:
  • Methane: methane hydrates in nature; methane venting from freshwater sediments
  • CO2: Geological carbon sequestration; capillary and solubility trapping; geomechanics
  • Oil: Enhanced oil recovery; flow instabilities; mixing; flow through fractured media
  • Water: Water infiltration; gravity fingering; ecohydrology of arid environments
I teach courses in soil mechanics (undergraduate), groundwater hydrology (graduate) and computational methods for flow in porous media (advanced graduate). My current teaching schedule is:
  • Spring 2013:
  • Previous semesters:
  • Courses at other institutions
    • PGE383: Computational Geomechanics (UT Austin)
    • PE120: Fundamentals of Petroleum Engineering (Stanford)
    • PE224: Advanced Topics in Reservoir Simulation (Stanford)
    • PE260: Environmental Aspects of Petroleum Engineering (Stanford)
    • PE281: Applied Mathematics in Reservoir Engineering (Stanford)
    • E77: Introduction to Computer Programming for Scientists and Engineers (UC Berkeley)
    • CE100: Elementary Fluid Mechanics (UC Berkeley)
For other academic activities and a complete list of publications, see my complete CV.

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