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.
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.
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.
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