Methane Hydrates

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Another unconventional gas source that has attracted increasing attention in recent years are methane hydrates (Fig. 4.14). Gas hydrates are naturally occurring crystalline, ice-like solids in which the gas molecules are trapped by water in cagelike structures called clathrates. Although many gases can form hydrates in nature, methane hydrate is by far the most common. Various clathrate compounds

Figure 4.14 Methane hydrate. A molecule of methane is trapped in a cage made out of water molecules (Source: NETL).

were discovered and studied in laboratories beginning in the 1800s. However, as no natural occurrences were known, the subject remained a purely academic curiosity until the 1930s, when solid hydrates were found to be plugging natural gas pipelines. In the 1960s, "solid natural gas" or methane hydrate was observed as a naturally occurring constituent in Siberian gas fields. Since then, gas hydrates have been found worldwide in oceanic sediments of continental and insular slopes, deepwater sediments of inland seas and lakes, and in polar sediments on both continents and continental shelves of the seas (Fig. 4.15). In permafrost Arctic regions, gas hydrates can be present at depths ranging from 150 to 2000 m, but most occurs in oceanic sediments hundreds of meters below the sea floor at water depths greater than 500 m.

Whilst the exact size of the world's methane hydrate deposits remains the subject of debate, largely due to the lack of field data, there is a consensus that the overall amount is huge. The estimates of methane hydrates, at the present time, are in the order of 21000 trillion cubic meters (Fig. 4.16). This represents more than 100 times our conventional proven gas reserves! However, in the consideration of methane hydrates as a future energy source, knowing the overall amount of methane hydrates present is only a part of the question. As is the case for the production of oil from the tar sands in Alberta, very large investments in research and development will be necessary to resolve significant technical issues before methane hydrates can be considered as an affordable methane source. Furthermore, the gas hydrate reserves are likely to represent only a small fraction of the gas hydrate resources because the deposits are too dispersed or the gas concentration is too low to be economically recovered. Regardless, the amounts of

Methane Hydrates Worldwide
Figure 4.15 The distribution of methane hydrate, worldwide. Source: Lawrence Livermore National Laboratory.

Fossil fuels (coal, oil natural gas), 5000

Fossil fuels (coal, oil natural gas), 5000

10000 Source: USGS

Figure 4.16 Distribution of organic carbon on Earth reservoirs (Gigatons). (Source: USGS).

10000 Source: USGS

Figure 4.16 Distribution of organic carbon on Earth reservoirs (Gigatons). (Source: USGS).

natural gas hydrates seems to be so enormous, that even the exploitation of a small part of the resources could provide all of our energy needs for centuries to come. Japan and India - two countries with large energy needs but limited energy resources, as well as the United States and Canada - are pursuing methane hydrate recovery research programs. For the time being, methane hydrates remain however, a potential but distant energy source.

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