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Conditions for Methane Hydrates in Seafloor and Permafrost Sediments a


2378


Te most common type of gas hydrate is methane hydrate and the conditions required for its stability can occur in marine sediments and in permafrost soil. Te phase diagrams (Figure 8.19) redrawn from Kvenvolden and Lorenson (2001) show the physical conditions (temperature and pressure) required for the stability of methane hydrate in the marine environment (top) and the permafrost environment (bottom). First, we discuss the marine setting


(Figure 8.19a). Salty oceanic water can be no colder than about −1.8°C before freezing. Assume that you are in a polar region, where the sea bottom temperature is 0°C. Furthermore, assume that the average temperature increase is 3°C per 100m sediment depth. Te figure then shows that methane hydrate cannot be stable at a water depth of 100m, but it may occur in a seafloor that is 400m below sea level. When drilling at a water depth of 400m, you can expect or hope to find a 370m-thick hydrate layer. Beneath this depth the temperatures will be too high for a formation of gas hydrate, so free gas and water is found. For a case of 1,000m water depth, the hydrate layer will be 600m thick. Obviously, the thickness of the hydrate zone will depend on the temperature gradient. In sediments that display a stronger increase in temperature, which can be the case, for example, at active continental margins (4–6°C per 100m depth), the hydrate zone will generally be thinner. Next, we look at the permafrost


M. Maslin et al. temperature (˚C) 0


–5 0 100 m


400 m 500


zone of GH stability


1000 1000 m 1500 2000


2376 b


gas hydrate (GH) = stable


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http://www.gashydrate.de), showing the physical conditions (temperature and pressure) required for the stability of methane hydrate in a marine environment. Assuming a constant temperature of 0◦C, e.g. in polar regions, methane hydrate cannot be stable at a water depth of 100 m. It may occur in a seafloor, which is more than 400m below sea level. The thickness of the hydrate zone will depend on the temperature gradient. However, with an increasing depth below the seafloor, temperatures get too high for a formation of gas hydrate, so that one can find free gas and water. Given an average temperature increase of 3◦C per 100m sediment depth, when drilling at a water depth of 300 m, we can expect to find a 300m thick hydrate layer. At 1000m water depth, the layer will be 600m thick. If, however, sediments are characterized by a stronger increase in temperature, which can be the case, e.g. at active continental margins (4–6◦C per 100m depth), the hydrate zone will generally be thinner. Gas hydrate has been found in sediments up to 1100m below the seafloor.


M. Maslin et al. case 2


permafrost base at 750 m


–20 0 case 1


permafrost = 100 m


500


gas hydrate (GH) = unstable


–10


temperature (˚C) 0


10 20


permafrost base


= 100 m and for the East Siberian Arctic Seas by Shakhova et al. (2010). This is because


the land temperature can be below −20◦C, but salty oceanic water can be no colder than about −1.8◦C before freezing (Maslin & Thomas 2003). So, if sea level rises and floods permafrost areas, these will encounter a thermal shock of at


case 2


least 20◦C warming, which could lead to considerable break down of gas hydrates. Marine gas hydrate stability can also be affected by pressure changes. Increased


1000


permafrost = 750 m


setting (Figure 8.19b), where temperature gradients are considerably lower than in the ocean. Typically, the temperature can be expected to change by 1.3°C per 100m within the permafrost zone, and with 2°C per 100m in layers below the permafrost zone. Te ambient temperature and the thickness of the frozen layer are therefore of significant importance for the stability of gas hydrate. Consider the case where the base of the permafrost is at a depth of 100m or less. Te figure shows that the physical conditions will not be adequate for the formation of gas hydrate. If the permafrost base is, say, at 750m, the thickness of the gas hydrate zone is 900m. Since the stability of gas hydrates is related to relatively


1500


low temperatures and high pressure, any change in these two parameters can increase or decrease the stability of the gas hydrate. For example, if either the temperature is


272


permafrost base = 750 m


sea level will increase the hydrostatic pressure and will stabilize gas hydrates to a deeper depth, while lowering sea level will reduce the pressure on the sediment causing gas hydrates to become unstable (Paull & Dillon 2001; Paull et al. 2003). Another way of removing pressure from underlying sediments is by marine sediment failure. When the overlying sediment column collapse occurs and sediment moves down slope there is a reduction in weight of the sedimentary


zone of GH stability


1650 m


Phil. Trans. R. Soc. A (2010) 2000


gas hydrate (GH) = stable


Figure 8.19: Methane hydrates in seafloor sediments (a) and permafrost soils (b). Modified from Kvenvolden and Lorenson (2001)


case 1


permafrost base at 100 m


zone of GH stability


1600 m 5 10 15 20


gas hydrate (GH) = unstable


25 100 m 400 m 770 m 1000 m


drilling in 100 m water depth


drilling in 400 m water depth


drilling in 1000 m water depth


than in the ocean (figure 6). For example, the temperature can be expected to change by 1.3◦C per 100m within the permafrost zone, compared with 2◦C per 100m in layers below the permafrost zone. The ambient temperature and the thickness of the frozen layer are of paramount importance for the stability of gas hydrate. If the permafrost base is located at a depth of 100m or less (case 1), the physical conditions will not be adequate for a formation of gas hydrate. The situation is different in case 2, where the permafrost basis is located at greater depth. In polar regions, methane hydrate can occur at depths ranging from 150 to 1650 m.


increased or the pressure is reduced, the gas hydrate will change phase from a solid to a gas and liquid. Gas hydrates are not chemical compounds since the


is unlikely to be achieved in the short term for several reasons: (i) low concentration of gas in the sediment, even though every cubic metre of gas hydrate can produce 164m3 of methane, at atmospheric conditions, the highest concentrations of oceanic gas hydrates quoted are 17–20% by volume of the pore space, which equates to as little as 3 per cent sediment volume, (ii) the conditions for production are far more complicated than in permafrost regions, and (iii) the geohazards involved as well as the impact on the environment are difficult to assess. Small-scale production of methane from permafrost gas hydrate already occurs at the Messoyakh field, in western Siberia (Krason 2000) and has been trialled at Mallik, on the Mackenzie delta, northern Canada (Dallimore et al. 2005).


Phil. Trans. R. Soc. A (2010)


sequestered molecules are never bonded to the lattice. Te formation and decomposition of hydrates are first-order phase transitions. However, the detailed formation and decomposition mechanisms are still not well understood on a molecular level.


water depth (m) depth below ground (m)


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