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NATURAL GAS HYDRATES AS A CAUSE OF UNDERWATER LANDSLIDES:
A REVIEW
M. PARLAKTUNA
Department of Petroleum and Natural Gas Engineering
Middle East Technical University
06531 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 “greenhouse” 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) 5 12 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
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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 5 1268 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
I
II
H
512
51262
51264
435663
51268
2
6
-
-
16
-
8
-
-
3
-
-
2
1
165
1 m3 Gas
Hydrate
164 m3 Gas
0.8 m3
Water
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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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 semicemented 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 beddingplane slides and rotational slumps on the Alaskan Beaufort Sea continental margin 7.
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