http://serc.carleton.edu/earthandmind/posts/just_phase_were.html#discussion http://serc.carleton.edu/earthandmind/posts/just_phase_were.html#post12505
I appreciate your timely post, as I've been thinking a lot about the Gulf of Mexico oil spill. I have a question, though. Why isn't the gas in the gas/oil mixture turning into clathrate at the seafloor, as soon as it exits from the pipe and hits the Gulf of Mexico water? I checked the IRI data viewer to see what a plausible seawater temperature would be at that depth (5000 feet or approx 1500m) of the Gulf of Mexico, and it would be about 5-6 deg C (http://iridl.ldeo.columbia.edu/SOURCES/.LEVITUS94/.MONTHLY/.temp/.) If I look at your phase diagram, the mixture near the exit from the broken pipe should be solidly in the hydrate + water stability field (water depth 1500m, temp 6degC). So why isn't there a pile of hydrates building up on the seafloor, like a chimney at a hydrothermal vent? Why aren't we seeing bits of hydrate forming in the web spill-cams like the "smoke" at a hydrothermal vent? Why did hydrates form in the top hat, but not on the seafloor?

Kim]]> Kim Kastens 1276533360 http://serc.carleton.edu/earthandmind/posts/just_phase_were.html#post12505 http://serc.carleton.edu/earthandmind/posts/just_phase_were.html#post12506 Callan Bentley 1276534200 http://serc.carleton.edu/earthandmind/posts/just_phase_were.html#post12506 http://serc.carleton.edu/earthandmind/posts/just_phase_were.html#post12507
One of the early explanations that I read about the blowout suggested that gas hydrates were a cause of the original blowout (as well as a problem for the containment). (http://news.sciencemag.org/scienceinsider/2010/05/gulf-spill-did-pesky-hydrates-tr.html is one source.) I can't find the original source that I read, but if I remember correctly, Robert Bea (quoted in the Science article) described the possibility that, as the concrete casing set, reactions in the concrete released enough heat to raise the temperature of the gas hydrates enough to make methane stable. If that's the case, it's another thing that could be discussed better with phase diagrams.

Are methane hydrates important in blowouts in general? I don't know much about them.]]> Kim Hannula 1276537380 http://serc.carleton.edu/earthandmind/posts/just_phase_were.html#post12507 http://serc.carleton.edu/earthandmind/posts/just_phase_were.html#post12509
Gas hydrates do indeed form as outcrops on the sea floor in areas where there is a high rate of flux of methane from the sediments to the seawater interface, and these look like little ice mounds or upside down stalctites. Here is a picture from NOAA showing an outcrop of gas hydrate at the bottom of the ocean: oceanexplorer.noaa.gov/explorations/03windows/background/plan/media/hydrate2.html. Gas hydrates can also form as free-floating crystals in the water column IF the concentration of methane or carbon dioxide is great enough and you stay in the prescribed P-T range. The situation gets a bit more complicated because the stability field of the gas hydrates is reduced a bit in the presence of saline (sea water) solutions--the same principle as using salt to melt ice on your sidewalk. For the naturally occurring gas hydrate ices that form on the ocean bottom, they are not readily sampled because they dissociate so rapidly once they are heated or decompress (so it's hard to get them to the surface to study in the lab). Note that there is an interesting reversal of temperatures with depth: as you get deeper in the water column the temperature of the water decreases. BUT, when you hit the sediment interface, the geothermal gradient increases the temperature as you get deeper into the sediments. So, there is a narrow window of stability just at the water-sediment interface where the clathrates can form.

For a really excellent overview of how gas hydrates work, see the powerpoint presentation by Megan Madden, Univ. of Oklahoma, that was just presented at the Teaching Geochemistry Workshop (http://serc.carleton.edu/NAGTWorkshops/geochemistry10/program.html and link to the Powerpoint from the program page). Kim, Megan does address the question of the intial blowout and subsequent problems associated with the cap...check out her Powerpoint and see what the Science says!

I forgot to address the question of whether or not the original blowout was somehow related to clathrates. It is indeed the case the solidifying cement is exothermic, and this may have heated up the surrounding sediments to cause any clathrates to dissociate. The phase transition from solid to vapor does indeed cause a rapid, positive change in the volume and this, I think, may be a plausible explanation worth future investigation. When water is heated and flashes to steam, the resulting volume expansion provides the driving force for geysers in nature, and is powerful enough to drive turbines when the energy is harnessed for electricity. Melanie has more to say about this in her presentation.]]> Dave Mogk 1276612020 http://serc.carleton.edu/earthandmind/posts/just_phase_were.html#post12509 http://serc.carleton.edu/earthandmind/posts/just_phase_were.html#post12513
Thanks for the very clear and informative answer. I especially like the concept that putting down the "top hat" changed the system from an approximately open system, where exiting methane was diluted by exchange with the vast ocean, to an approximately closed system where the exiting methane accumulated in sufficient concentration to allow clathrate formation. The concept of closed and open systems can be hard to convey, and here is a vivid example where the boundary of the closed system is concrete and visible and impenetrable, rather than an abstraction. Eventually students will have to grapple with systems where the boundary is nebulous or arbitrary, but it is so helpful to have some concrete example early in the learning progression.

Kim]]> Kim Kastens 1276716720 http://serc.carleton.edu/earthandmind/posts/just_phase_were.html#post12513 http://serc.carleton.edu/earthandmind/posts/just_phase_were.html#post12531
Thanks for the timely and informative post on gas hydrates and for the shout-out! As an additional answer to Kim's question, it also takes time for hydrates (an any other solid phase) to nucleate and grow. In the case of the leak in the gulf of Mexico, the concentration of gas likely decreases rapidly as the plume is dispersed away from the well. So, at the source of the leak the concentration of gas might be high enough to theoretically form hydrate given enough time, but the rate of flow away from high concentrations may be faster than the rate of hydrate formation, so hydrate wouldn't have a chance to form. When they put the first cap on, it held all the gas and oil in one spot for a prolonged period of time increasing the concentration and the time window which lead to hydrate formation.

It's also interesting to compare the current cap which seems to be working to the "top hat" which failed. Why the difference? The current system lets in much less water, but the gas is probably still wet and would likely form hydrates in the pipe if they didn't inject a hydrate-inhibiting chemical, probably methanol or possibly salts which both shrink the stability field for hydrates.

Cheers!
Megan Elwood Madden
http://faculty-staff.ou.edu/E/Megan.E.Elwood.Madden-1/]]> Megan Elwood Madden 1277131020 http://serc.carleton.edu/earthandmind/posts/just_phase_were.html#post12531