Is This Just a Phase We're Going Through?

David Mogk
Author Profile
published Jun 10, 2010

On May 9, 2010, the day that BP was making its first attempt to cap its renegade oil well, I was attending a short course to learn a new thermodynamic modeling program. Conversation at this workshop with other geochemists inevitably turned to the environmental disaster (debacle) that was unfolding. Even before the first capping attempt was made, it was the consensus of the participants that the exercise was doomed to failure based on first principles of thermodynamics. We were all acquainted with the carbon-oxygen-hydrogen (C-O-H) chemical system which encompasses a wide range of fluids that commonly occur in the Earth system (e.g. water, carbon dioxide, methane). Carbon, oxygen and hydrogen undergo a series of chemical reactions in response to changing physical conditions such as temperature and pressure. These chemical reactions produce phase changes that result in changes in the state of matter: volatilization or vaporization reactions that cause liquids to form gases (e.g. water vapor or steam) or crystallization reactions that result in liquids condensing to form solid materials in crystalline form (e.g. freezing water to form ice). One of these crystallization reactions forms a curious group of solid ice compounds known as gas-hydrates (aka "clathrate" compounds).

Phase diagram showing the water depths (and pressures) and temperatures for gas hydrate (purple area) stability. Figure from NOAA Ocean Explorer

These physical and chemical relationships are often represented on phase diagrams which show the stability of different phases (liquids, gases and solids) as a function of pressure and temperature (see Figure 1). Phase diagrams allow us to predict the behavior of chemical systems in changing physical conditions, and by extension, to describe the processes that are likely to occur (e.g. crystallization, melting). At a given pressure and temperature, a system of a known composition will necessarily form a unique set of phases at equilibrium (chemical species, like gases, liquids or solids of fixed composition). Any time the system is perturbed (i.e. change in pressure and/or temperature), the system will typically reorganize itself by reacting to form new stable phases. Given the composition of the fluids in and near the damaged oil well, and the ambient temperature (cold bottom waters) and pressure (from the weight of the overlying water column) on the floor of the Gulf of Mexico, the formation of solid gas hydrate ices in the neck of the capping device was completely predictable. (See the Wikipedia entry for clathrate hydrate for a description of how clathrate-hydrates are prone to form in and plug up pipelines!).

Gas hydrates are well-known to Earth scientists (and should be well-known to chemical and petroleum engineers). These unusual materials have chemical formulae that combine water and another gas species (e.g. carbon dioxide or methane) to form a solid ice phase: xH2O.yCO2 or xH2O.yCH4. These compounds typically have a freezing point of between 0 and 10oC, which is obviously higher than the melting point of pure water (0oC or sea water -3.8oC). Many geochemists have used these phase relations in the study of fluid inclusions, tiny bubbles of liquid and vapor trapped in minerals, to determine the composition of rock-forming fluids that must have been present at the time of crystallization of the host mineral. Gas hydrates may occur in some permafrost and sub-glacial environments, particularly near the polar icecaps. And, gas hydrates have been extensively studied as one of the possible phases present in the ices of other planets such as Mars and moons of Jupiter and Saturn, and may be an important surficial reservoir for water (as a precursor or required component of life) on these planets.

Burning methane hydrate. Image from (http://menlocampus.wr.usgs.gov/50years/accomplishments/gashydrate.html) US Geological Survey, "Burning Ice–Worldwide Occurrence of Gas Hydrate".

Gas hydrates are particularly well-known to the oil and gas industry as well. Methane gas hydrates are known to occur globally in the sediments deposited on continental shelves. This is the source of the famous "flaming ice" crystals (see figure 2). As these gas hydrate ices dissociate upon heating, methane gas is released and thus is available for combustion. Gas hydrate deposits on the continental shelf have generated great excitement as a potential target for natural gas exploration and exploitation. See the Department of Energy's National Methane Hydrates R&D site for more information about methane hydrate deposits..

However, the challenge has been to recover the methane from the buried ices because methane is rapidly released from the system upon heating and decompression and cannot readily be confined for efficient and economical extraction. Natural methane seeps are commonly observed on continental shelves, and are most likely the result of dissociation of the gas hydrate ice in response to warming of ocean water or decompression. The rate at which methane is released from these seeps is a matter of great concern because of the high contribution that methane makes to global warming as one of the primary "greenhouse gases". This may also be part of a positive feedback process, as the oceans get warmer more methane is released to the atmosphere which results in even further warming.

A model of a gas hydrate structure showing the "cage" of water molecules surrounding a gas molecule. Image from the Dept. of Energy National Methane hydrates R&D Program

Gas hydrates have another important relationship to the oil and gas industry: as a potential reservoir for carbon sequestration. Gas hydrate ices have an unusual crystal structure in which the water molecules form a "cage" and the related gases are trapped within these cages (see Figure 3). One strategy for carbon sequestration is to pump carbon dioxide into sediment strata that are known to contain the gas hydrate ices. Carbon dioxide infiltrates the gas hydrate structure, capturing the CO2 in the "cage" and driving out the methane, which can then be recovered as an energy source–two treats for the price of one!

But getting back to the BP oil disaster:

· The failure of the first attempt to cap the well was absolutely predictable. The expected behavior of the fluids at the ambient conditions on the ocean floor were available to the scientists and engineers who should have been working on the problem. By ignoring fundamental chemical principles, the futile capping exercise resulted in an unfortunate waste of time, energy and resources. This response was hastily designed, ill-conceived, and merely provided the illusion that significant action was being taken.

· Science is often the first casualty in attempts to control public perception through the media. Unfortunately, science was ignored in the early stages of this disaster (and largely, this stance is continuing as BP resists attempts to quantify the flow rate of the escaping oil). "Spin" through the media will not make the situation right. The fundamental laws of Nature will not be violated, despite the wishful thinking and hype of corporate representatives. Saying it is "so" (repeatedly and glibly) will not in any way make it "so". And the laws of Nature are not democratic–we cannot simply vote to decide whether or not the laws of thermodynamics, fluid dynamics or gravity should be changed or suspended. When will Science be appropriately integrated into policy (and economic) decision-making?

· We simply need more people asking hard questions of the corporate executives and government agencies based on a solid scientific foundation. This is why we need a scientifically literate public. And this is why scientific curricula in all disciplines must be built on the broad-based foundations of fundamental scientific principles. A good place to start would be exercises in reading and interpreting simple graphical representations of scientific data (Figure 1). Application of critical thinking skills wouldn't hurt, either.

· For the Earth Science curriculum, this is just one example of why students need to understand the properties and processes of Earth materials, including application of phase equilibria that can be applied to a wide variety of Earth processes–both natural and anthropocentric. You never know when a handy phase diagram may point the way to an appropriate strategy or solution (or prevent you from pursuing the impossible). This is why we need to teach the fundamentals of phase equilibria as a central tenet of the geoscience curriculum. Phase relations are not restricted to the geochemistry of "hard rock" problems, and can readily be applied to a wide array of environmental issues (the BP oil spill is only one example; water quality, mine reclamation, are other examples)–the principles transcend the specific disaster du jour.

I hope this is just a phase we're going through. Like a petulant teenager rejecting parental authority, isn't it time for corporate executives and policy makers ro respect and abide by the Laws of Nature? Obey your mother (Earth, that is)!

Related Resources and References

Please visit our module on Teaching Phase Equilibria for more information on phase diagrams and phase equilibria..

See the powerpoint presentation by Megan Elwood Madden, University of Oklahoma on Stability of Methane Clathrates that was presented at the 2010 Teaching Geochemistry Workshop (link from the workshop program).

See the Department of Energy's National Methane Hydrates R&D site for more information about methane hydrates.

See: Milkov et al., 2000, Gas Hydrates at Minimum Stability Water Depths in the Gulf of Mexico: Significance to Geohazard Assessement, Gulf coast Association of Geological Societies Transactions, Volume L for an example of a geologic study of gas hydrates.

Dickens, G.R., Quinby-Hunt, M. 1994. Methane hydrate stability in seawater. Geophysical Research Letters 21, 2115-2118.

Elwood Madden, M.E., Ulrich, S.M., Onstott, T.C., Phelps, T.J., 2007. Salinity-induced hydrate dissociation: A mechanism for recent CH4 release on Mars. Geophysical Research Letters 34 CiteID L11202.




Is This Just a Phase We're Going Through? --Discussion  

Hi Dave,

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

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That's a great question, Kim.

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Thank you for the explanation, Dave. I've been thinking a lot about the spill, as well.

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.

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This post was editted by Kit Pavlekovsky on Jul, 2012
Hi Kim and Cailan, Thanks for the question about natural occurrences of clathrates in sea water and on the sea bed. Three things have to come together to cause precipitation of clathrates (or any other compound): you need to have the correct pressure and temperature conditions AND you have to have the correct chemical components to make the phases of interest. If methane is at low concentrations in ambient seawater, even at the correct temperature and pressure you can't form gas hydrates (clathrates)--you have to have the right combination of chemical reactants. So, when the original cap was put on the well, a micro-environment (closed system) was created that locally greatly increased the methane concentration in the confined space, so at the temperatures and pressures on the sea floor of the Gulf of Mexico the gas hydrates had the right conditions to form. This is well known in the pipeline literature as well.

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.

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Hi Dave,

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

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Dave-

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/

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