329
12
Methane from
Gas Hydrates
12.1 intrOdUCtiOn: What is Gas hydrate
and hOW is it FOrmed?
Clathrate hydrates are solid crystalline “inclusion” compounds, which are formed
when water is contacted with small hydrophobic molecules such as methane, ethane,
H 2 S, and CO 2 [1–7] (Harrison, 2010, pers. comm.) under certain pressure and temperature conditions. When the inclusion compound is a constituent of natural gas,
clathrate hydrates are also referred to as gas hydrates [1–15] (Harrison, 2010, pers.
comm.). The gas (or methane) hydrate composition is in general 5.75 mol of water for
every molecule of methane, although this number does depend on the cage structure of
the water ice. Various molecular structures of gas hydrate and clathrate are illustrated
in Figure 12.1 [2]. The average density of methane hydrate is about 0.9 g/cc. Under
standard conditions, the volume of methane hydrate will be 164 times less than that
of methane gas [1–16] (Harrison, 2010, pers. comm.).
Gas hydrates are formed when natural gas and water are brought together under
suitable conditions of low temperatures and elevated pressures. The formation
depends on (1) the presence of sufficient amount of water, (2) the presence of hydrate
former, and (3) the appropriate pressure and temperature conditions. In a gas hydrate
reservoir, free gas, ice, water, and other components such as ethane, propane, hydrogen sulfide, and carbon dioxide can be found at different temperatures, pressures,
and depth values. Two- and three-phase equilibria curves [5–7,13–16] (Harrison,
2010, pers. comm.) are used for correlation between phases where the amount of
components present plays a significant role; very small and large amounts of water
are not conducive to the formation of hydrates.
The gas hydrates are unstable compounds in which the water molecules form a
sort of cage or lattice around the methane molecules, and the two establish weak
chemical bonds with one another. Methane from methane hydrates must be released
in situ due to the inherent instability of hydrate molecules. The temperature at which
methane hydrate is stable depends on the prevailing pressure. For example, at 0°C,
it is stable under a pressure of about 30 atm, whereas at 25°C, nearly 500 atm pressure is needed to maintain its integrity. The occlusion of other gases within the ice
structure tends to add stability, whereas the presence of salts requires higher stabilizing pressures. Appropriate conditions of temperature/pressure exist on the earth
in the upper 2000 m of sediments in two regions: (1) permafrost at high latitudes in
polar regions where the surface temperatures are very low and (2) submarine continental slopes and rises where not only is the water cold but the pressures are high
(>30 atm). Phase boundary of methane hydrates in permafrost and deep-sea regions
12
Methane from
Gas Hydrates
12.1 intrOdUCtiOn: What is Gas hydrate
and hOW is it FOrmed?
Clathrate hydrates are solid crystalline “inclusion” compounds, which are formed
when water is contacted with small hydrophobic molecules such as methane, ethane,
H 2 S, and CO 2 [1–7] (Harrison, 2010, pers. comm.) under certain pressure and temperature conditions. When the inclusion compound is a constituent of natural gas,
clathrate hydrates are also referred to as gas hydrates [1–15] (Harrison, 2010, pers.
comm.). The gas (or methane) hydrate composition is in general 5.75 mol of water for
every molecule of methane, although this number does depend on the cage structure of
the water ice. Various molecular structures of gas hydrate and clathrate are illustrated
in Figure 12.1 [2]. The average density of methane hydrate is about 0.9 g/cc. Under
standard conditions, the volume of methane hydrate will be 164 times less than that
of methane gas [1–16] (Harrison, 2010, pers. comm.).
Gas hydrates are formed when natural gas and water are brought together under
suitable conditions of low temperatures and elevated pressures. The formation
depends on (1) the presence of sufficient amount of water, (2) the presence of hydrate
former, and (3) the appropriate pressure and temperature conditions. In a gas hydrate
reservoir, free gas, ice, water, and other components such as ethane, propane, hydrogen sulfide, and carbon dioxide can be found at different temperatures, pressures,
and depth values. Two- and three-phase equilibria curves [5–7,13–16] (Harrison,
2010, pers. comm.) are used for correlation between phases where the amount of
components present plays a significant role; very small and large amounts of water
are not conducive to the formation of hydrates.
The gas hydrates are unstable compounds in which the water molecules form a
sort of cage or lattice around the methane molecules, and the two establish weak
chemical bonds with one another. Methane from methane hydrates must be released
in situ due to the inherent instability of hydrate molecules. The temperature at which
methane hydrate is stable depends on the prevailing pressure. For example, at 0°C,
it is stable under a pressure of about 30 atm, whereas at 25°C, nearly 500 atm pressure is needed to maintain its integrity. The occlusion of other gases within the ice
structure tends to add stability, whereas the presence of salts requires higher stabilizing pressures. Appropriate conditions of temperature/pressure exist on the earth
in the upper 2000 m of sediments in two regions: (1) permafrost at high latitudes in
polar regions where the surface temperatures are very low and (2) submarine continental slopes and rises where not only is the water cold but the pressures are high
(>30 atm). Phase boundary of methane hydrates in permafrost and deep-sea regions