FROZEN HEAT | Volume 1

1.3 GAS HYDRATE FORMATION, STABILITY, AND OCCURRENCE

Given adequate supplies of gas and water, the fundamental controls on gas-hydrate formation and stability are pressure and temperature. In general, a combination of low tempera-

ture and high pressure is needed to form methane hydrate (Fig. 1.3). Because of Earth’s geothermal gradient – the natural increase of temperature with depth below the ground surface

Stability conditions for gas hydrates

Depth (metres) 0

Depth (metres) 0

gnittes eniraM

gnittes tsorfamreP

Sea surface

Ground surface

Ice freezing temperature

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Stability zone

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Stability zone

Base of permafrost

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Sea oor

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Temperature ºC

Temperature ºC

Figure 1.3: Stability conditions for gas hydrates. Idealized phase diagrams illustrating where methane hydrate is stable in marine and permafrost settings. Hydrate can exist at depths where the temperature (blue curve) is less than the maximum stability temperature for gas hydrate (orange curve). Pressure and temperature both increase with depth in the Earth. Although hydrates can exist at warmer temperatures when the pressure is high (orange curve), the temperature at depth (blue curve) gets too hot for hydrate to be stable, limiting hydrate stability to the upper ~1km or less of sediment. The presence of salt, a gas hydrate inhibitor, shifts the gas hydrate stability curve (orange) to lower temperatures, decreasing the depth range of the gas hydrate stability zone. For seawater, this decrease is approximately 1.1°C (Dickens and Quinby-Hunt, 1994) (Figure modified from Kvenvolden (1988a)).

FROZEN HEAT 16