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NATURAL GAS HYDRATES AS A CAUSE OF UNDERWATER LANDSLIDES:A REVIEW

M. PARLAKTUNADepartment of Petroleum and Natural Gas EngineeringMiddle East Technical University06531 Ankara-TURKEY

Abstract

Natural gas hydrates occur worldwide in polar regions, normally associated with onshore and offshore permafrost, and in sediment of outer continental margins. The total amount of methane in gas hydrates likely doubles the recoverable and non-recoverable fossil fuels. Three aspects of gas hydrates are important: their fossil fuel resource potential, their role as a submarine geohazard, and their effects on global climate change. Since gas hydrates represent huge amounts of methane within 2000 m of the Earth’s surface, they are considered to be an unconventional, unproven source of fossil fuel. Because gas hydrates are metastable, changes of pressure and temperature affect their stability. Destabilized gas hydrates beneath the seafloor lead to geologic hazards such as submarine slumps and slides. Destabilized gas hydrates may also affect climate through the release of methane, a “green-house” gas, which may enhance global warming.

1. What are Hydrates?

Hydrates are the members of the class of compounds labeled “clathrates” after the Latin “clathratus” meaning, “To encage”. All hydrate structures have repetitive crystal units composed of asymmetric, spherical “cages” of hydrogen-bonded water molecules. Each cage contains at most one guest (gas) molecule held within the cage by dispersion forces. There is no chemical union between the gas and water molecules. The water molecules that form the lattice are strongly hydrogen bonded with each other and the gas molecule interacts with water molecules through van der Waals type dispersion force.

There are three types of gas hydrate structures: Structure I (sI), structure II (sII), and structure H (sH). Figure 1 shows the shape of these structures. As it was mentioned before, natural gas hydrates consist of hydrogen-bonded lattices with cages in which the gas molecules are trapped. The basic cage, common for all type of gas hydrate structures, is the pentagonal dodecahedron that consists of twelve pentagons joined together as a small ball. At the cross of the pentagon there are oxygen atoms and line of the pentagon is occurred by hydrogen bonds. Five types of cavities shown in Figure 2 have been known; a) 512 b) 51262

c) 51264 d) 435663, and e) 51268.

A. C. Yalçıner, E. Pelinovsky, E. Okal, C. E. Synolakis (eds.), Submarine Landslides and Tsunamis 163-170.@2003 Kluwer Academic Publishers. Printed in Netherlands

Figure 1. The unit cells of structure I (a), structure II (b) and structure H (c).

Figure 2. The hydrate cages; a) 512, b) 51262, c) 51264, d) 435663, and e) 51268.

The 51262 cavity consists of 12 pentagonal faces and 2 hexagonal faces and consists of 24 water molecules. The 51264 cavity has 12 pentagonal, 4 hexagonal faces and consists of 28 water molecules. A 435663 cage consists of three fairly strained square faces, six pentagonal and three hexagonal faces. Finally the bulky 51268 cavity is built of 12 pentagonal and 8 hexagonal faces.

The numbers of the different cavities in the unit cell of the different hydrate structures are tabulated in Table 1.

TABLE 1. The Number of the Cavities in the Hydrate Structures

Structure 512 51262 51264 435663 51268

I 2 6 - - -

II 16 - 8 - -

H 3 - - 2 1

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Figure 3. Yield of 1 m3 of methane hydrate at standard temperature and pressure.

One important feature of gas hydrates is the amount of gas that is stored in hydrate structure. One cubic meter of methane hydrate, if gas molecules occupy all the cavities, yields 164 m3 of gas and 0.8 m3 of water at standard temperature and pressure (Figure 3).

2. Where hydrates are found?

Hydrates are plentiful in nature, both underwater and under permafrost. More than 60 large gas hydrate fields have been revealed to date in oceanic sediments and eight on land (Figure 4).

Hydrates form only under certain temperature and pressure conditions. A phase diagram showing the boundary between free methane gas and methane hydrate for the pure water and pure methane system is given in Figure 5. The addition of NaCl to water shifts the curve to the left. Adding CO2, H2S, C2H6 and C3H8 to methane shifts the boundary to the right and thus increases the area of the hydrate stability field. Stable methane hydrates are found at the temperature and pressure conditions that exist near and just beneath the sea floor where water depth exceeds 300 to 500 meters. Hydrate is also stable in conjunction with permafrost at high latitudes. Hydrates can exist up to depths of about 3100 m below the ocean floor. Below that level heat tends to keep the methane free in the form of gas.

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1 m3 Gas Hydrate

164 m3 Gas 0.8 m3

Water

Figure 4. Map showing worldwide locations of known and inferred gas hydrates in oceanic (solid circle) and in continental regions (solid square) 1.

Figure 5. Phase diagram of methane hydrate 1.There are two processes by which hydrates are formed, organic process and gas venting.

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- Organic process: Most methane gas hydrate is formed from biogenic methane, excreted by bacteria that eat organic matter that has been washed into the ocean. This type of hydrate is concentrated where there is a rapid accumulation of organic detritus and also where there is a rapid accumulation of sediments.

- Venting: Hydrates also form when faults permit natural gas (or other gases) to migrate from deeper inside the Earth’s crust to the surface of the seabed at places with appropriate temperature and pressure levels.

Scientists generally believe that most natural gas hydrate is formed from biogenic methane, produced by bacteria. Hydrates produced by the organic process are generally very pure; they tend to contain only water and methane. Hydrates formed from venting tend to have many gases mixed in, in addition to methane.

Hydrates may exist as outcropping or mounds on the seafloor. Hydrates may also exist in layers separated by sediments. These layers may be largely hydrate, or the layers may consist of sediments mixed in with hydrates so that the sediments are cemented or sealed. Some layers trap free methane beneath them.

3. Hydrates as Geohazard

Hydrates affect the strength of the sediments in which they are found. Areas with hydrates appear to be less stable than other areas of the seafloor. Consequently, it is important to assess their presence in the framework of the construction of underwater structures, for example in relation to military defense and to gas and oil exploration and production. Lack of stability might also be a factor in climate change.

Hydrates can cement loose sediments in the surface layer several hundred meters thick. That might lead one to believe that hydrate stabilizes the seafloor. In fact, the reverse appears to be true. When hydrates are created in loosely consolidated sedimentary rocks, the hydrate will be a cementing material. If the hydrate dissociates, the rock formation becomes unconsolidated and loses its strength. Natural gas hydrates are dangerous during the construction and operation of wells, platforms, pipelines, and other offshore engineering structures. They may even cause tsunamis.

Seafloor slopes of 5 degrees and less should be stable on the Atlantic continental margin. Yet many landslide scars are present on such gentle slopes. The top of these scars is near the top of the hydrate zone, and seismic profiles of these scars indicate that there is less hydrate in the sediment beneath slide scars.

As a result, scientists believe there is a link between hydrates and the occurrence of landslides on the continental margin. Landslides may begin when hydrates at the base of the hydrate layer break down, so that the bottom of the hydrate deposits is no longer semi-cemented but is instead full of free methane. Such a zone would be weak and would likely facilitate sliding (Figure 6).

Slides might also result from the melting of the top of a hydrate layer that is covered by sediment. As the hydrates revert to water and methane, they would likely disturb the sediment and promote shifting. This form of breakdown might occur during drops in sea level, such as occurred during glacial periods when ocean water became isolated on land in great ice sheets.

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Figure 6. Diagram showing seafloor failures and gas release due to hydrate dissociation 2.

Seafloor landslides that result, for instance, from earthquakes can also cause breakdown of hydrate if, as a result, hydrate layers are repositioned so as to reduce the pressure that maintains the hydrate stability.

All of the processes above may interact. The result may be cascading slides, which could result in even further breakdown of hydrate and release of methane to the surrounding water or into the atmosphere.

In the past, hydrates have been associated with significant movement of earth in deepwater ocean environments. Examples include surficial slides and slumps on the continental slope and rise of South West Africa 3, slumps on the U.S. Atlantic continental slope 4, marine slides on the Norwegian continental margin 5, 6 and massive bedding-plane slides and rotational slumps on the Alaskan Beaufort Sea continental margin 7.

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References:

1. Kvenvolden, K.A., (1988) “Methane Hydrate-A Major Reservoir of Carbon in the Shallow Geosphere?”, Chem. Geol., 71, 41-51.

2. McIver, R.D. (1982) “Role of Naturally Occurring Gas Hydrates in Sediment Transport”, AAPG Bull., 66, 789-792.

3. Summerhayes, C.P., Bornold, B.D., and Embley, R.W. (1979) “Surficial Slides and Slumps on the Continental Slope and Rise of South West Africa: A Reconnaissance Study”, Mar. Geol., 31, 265-277.

4. Carpenter, G. (1981) “Coincident Sediment Slump/Clathrate Complexes on the U.S. Atlantic Continental Slope”, Geo Mar. Lett., 1, 29-32.

5. Jansen, E., Befring, S., Bugge, T., Eidvin, T., Holtedahl, H., and Sejrup, H.P. (1987) “Large Submarine Slides on the Norwegian Continental Margin: Sediments, Transport and Timing”, Mar. Geol., 78, 77-107.

6. Bugge, T.S., Befring, S., Belderson, R.H., Eidvin, T., Jansen, E., Kenyon, N.H., Holdedahl, H., and Sejrup, H.P., (1987) “A Giant Three-Stage Submarine Slide off Norway”, Geo Mar. Lett., 7, 191-198.

7. Kayen, R.E., and Lee, H.J. (1991) “Pleistocene Slope Instability of Gas Hydrate –Laden Sediment on the Beaufort Sea Margin”, Mar. Geotechnol., 10, 125-141.

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