Page 283 - Handbook Of Multiphase Flow Assurance

282 10. Research methods in flow assurance

Environmental aspect of gas hydrates is related to the greenhouse effect of methane and
carbon dioxide. Hydrates of these two gases can exist on earth at subsea T, P. If a tempera-
ture of the sea bottom increases above methane hydrate stability limit, it causes hydrate to
release methane. Methane has a greenhouse effect which is 21 times stronger than that of car-
bon dioxide (Englezos, 1993a). As more methane is released, the greenhouse effect increases
temperature further which may cause a runaway global warming (Leggett, 1990). The 1996
Conference in Brussels on the Global Environment has reiterated that natural hydrates of
methane can contribute to a greenhouse effect (Sloan, 1996b).
Offshore gas hydrate deposits may release natural gas if the sea water temperature at the
bottom rises above hydrate stability limit. It was shown that a model ship floating on a water
will sink if the water becomes foam because of the natural gas bubbles released from de-
composing hydrate (Makogon, 1996a,b). A series of photographs in Fig. 10.49 shows such a
process. In the image 1 the ship is floating on the water. In image 2 the gas starts to rise from
a system of pipes on the bottom of the pool simulating natural gas discharge. The ship is
still afloat. In image 3 the ship has sunk. The ship model is 2 m long. Ocean can be in contact
with gas hydrate (Makogon, 1982) which may release gas upon dissociation as shown further
(Fig. 10.50).
Existence of gas hydrates in space is reasonable to expect at low temperatures. It was pro-
posed that the core of the Halley's comet consists of clathrate hydrate when an anomalous
behavior of eruptions was observed at the surface of the comet (Makogon, 1987).
Three forms of clathrate hydrates which are studied the most in the western hemisphere
are known as structure I (sI), structure II (sII) and structure H (sH). All these hydrates contain
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a common basic “building block” water cavity—the 5 cavity which has 12 pentagonal faces.
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The 5 cavity is composed of 20 water molecules held together by hydrogen bonds.
Hydrogen bonding is weaker than chemical bonding. The magnitude of hydrogen bonds
is about 5 kcal/mol (Sloan, 1990) for bonding between water molecules, which is weak com-
pared to hundreds of kcals/mol for covalent bonds. However, in the laboratory it is important
to remember that hydrogen bonds are stronger than van der Waals bonds. Organic solvents
containing chlorine atoms, like chloroform, can hydrogen bond to compounds containing
carbonyl groups, like acetone, in a fairly exothermic reaction which can heat the system to
spontaneous combustion.
Properties of hydrates as solids are not very different from those of ice. The specific volume
of water increases by 26% upon transition to a hydrate phase, while such an increase for ice
is only 9%. The specific volume of gas changes in such transition by several orders of magni-
tude. The electric conductivity of hydrate is lower than that of the initial solution. The sonic
velocity in hydrate is 60–100% higher than that in gas saturated rock (Makogon, 1985). These
properties of gas hydrates provide effective means for surveying gas hydrate deposits.
Several applications for gas hydrates described earlier (Makogon, 1981, 1985; John et al.,
1994) are based on the change of properties of hydrate forming gases in the hydrate state. Gas
molecules are packed much closer together in hydrate than in gaseous state.
Storage and main-line transportation of natural gas in the hydrate state may be profitable
in some regions of the world (Makogon, 1970; Smirnov and Dyachenko, 1989; Gudmundsson
and Borrehaug, 1996). The original idea belongs to Mr. Kalina (Makogon, 1996a) A method
of container and briquette transportation of gas in hydrate state from Siberia to Europe was
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ruled out because it required a rate of hydrate formation of 11 m of solid hydrate per second.