Natural Gas Hydrates: Overview

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Natural gas hydrates are a curious kind of chemical compound called a clathrate. Clathrates consist of two dissimilar molecules mechanically intermingled but not truly chemically bonded. Instead one molecule forms a framework that traps the other molecule. Natural gas hydrates can be considered modified ice structures enclosing methane and other hydrocarbons, but they can melt at temperatures well above normal ice.

At 30 atmospheres pressure, methane hydrate begins to be stable at temperatures above 0 C and at 100 atmospheres it is stable at 15 C. This behavior has two important practical implications. First, it's a nuisance to the gas company. They have to dehydrate natural gas thoroughly to prevent methane hydrates from forming in high pressure gas lines. Second, methane hydrates will be stable on the sea floor at depths below a few hundred meters and will be solid within sea floor sediments. Masses of methane hydrate "yellow ice" have been photographed on the sea floor. Chunks occasionally break loose and float to the surface, where they are unstable and effervesce as they decompose.

The stability of methane hydrates on the sea floor has a whole raft of implications. First, they may constitute a huge energy resource. Second, natural disturbances (and man-made ones, if we exploit gas hydrates and aren't careful) might suddenly destabilize sea floor methane hydrates, triggering submarine landslides and huge releases of methane. Finally, methane is a ferociously effective greenhouse gas, and large methane releases may explain sudden episodes of climatic warming in the geologic past. The methane would oxidize fairly quickly in the atmosphere, but could cause enough warming that other mechanisms (for example, release of carbon dioxide from carbonate rocks and decaying biomass) could keep the temperatures elevated.

There are three types of methane hydrate structure. They all include pentagonal dodecahedra of water molecules enclosing methane. This geometry arises from the happy accident that the bond angle in water is fairly close to the 108 degree angle of a pentagon. Generally, the dodecahedra are slightly distorted so that three dodecahedra can share an edge. This requires a dihedral (inter-face) angle of 120 degrees, whereas the dihedral angle of a true dodecahedron is 116.5 degrees. Between the dodecahedra are other cages of water molecules with different shapes. In practice, not all cages are occupied by hydrocarbons, but occupancy rates of over 90 per cent occur.

In all the diagrams below and on the linked pages, each vertex is occupied by an oxygen atom and the midpoint of each edge is a hydrogen atom. This atom is attached to one oxygen as part of a water molecule and hydrogen bonded to the other. In the diagram at left one cage is shown with oxygen atoms in blue and hydrogen in red. A methane molecule is shown inside one of the cage skeletons.

Structure I

Structure I is cubic. A dodecahedral cage is centered at the corners of the unit cell and a rotated dodecahedron is in the center of the cell. The dodecahedra are linked by 14-faced cages that consist of hexagonal ends and 12 pentagons. In the diagram below dodecahedra are in magenta. The central rotated dodecahedron is hidden but its counterpart in the next unit cell above is shown at top.

Structure II

Structure II is also cubic. 16-faced cages consisting of 12 pentagons and 4 hexagons are arranged tetrahedrally (left side of diagram below). The interstices are filled by dodecahedral cages (right, below).

 

H Structure

The H (for hexagonal) structure consists of three cages: dodecahedra, cages with 4, 5 and 6-sided faces, and "barrels" consisting of 12 pentagonal and 8 hexagonal faces. The "barrels" can hold large hydrocarbon molecules. They are surrounded by a hexagonal net of the 4-5-6 cages, and layers of these cages alternate with hexagonal nets of dodecahedra.

References

Henriet, J.-P., Mienert, J., 1998; Gas hydrates : relevance to world margin stability and climate change, London : The Geological Society, Geological Society special publication no. 137, 338 p.

Kleinberg, Robert; Brewer, Peter, 2001; Probing gas hydrate deposits.  American Scientist. vol. 89; no. 3, Pages 244-251. 

Holder, Gerald-D (editor); Bishnoi, P. R. (editor), 2000; Gas hydrates; challenges for the future. Annals of the New York Academy of Sciences. 912; New York Academy of Sciences. New York, NY, United States. Pages: 1039.

Paull, Charles K. (editor); Dillon, William P. (editor), 2000; Natural gas hydrates; occurrence, distribution, and detection. Geophysical Monograph 124, American Geophysical Union. Washington, D.C., United States. Pages: 315. 

Haq, Bilal U., 1998; Gas hydrates; greenhouse nightmare? Energy panacea or pipe dream? GSA Today. vol. 8; 11, Pages 1-6. Geological Society of America (GSA). Boulder, CO, United States

Smelik, Eugene A.; King, H. E. Jr., 1997; Crystal-growth studies of natural gas clathrate hydrates using a pressurized optical cell. American Mineralogist. vol. 82; 1-2, Pages 88-98. Mineralogical Society of America. Washington, DC, United States.


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1 August 2003, Last Update 14 Dec 2009

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