Underground sequestration has been proposed as a novel method of permanent disposal of harmful gases emitted into the atmosphere as a result of human activity. The method was conceived primarily for disposal of carbon dioxide (CO2, greenhouse gas causing global warming), but could also be applied to CO, H2S, NOx, and chorofluorocarbons (CFCs, which are super greenhouse gases). The method is based on the fact that clathrate hydrates (e.g., CO2×6H2O) form naturally from the substances in question (e.g., CO2) and liquid water in the pores of sub-permafrost rocks at stabilizing pressures and temperatures. The proposed method would be volumetrically efficient: In the case of CO2, each volume of hydrate can contain as much as 184 volumes of gas.

Temperature and pressure conditions that favor the formation of stable clathrate hydrates exist in depleted oil reservoirs that lie under permafrost. For example, CO2×6H2O forms naturally at a temperature of 0 °C and pressure of 1.22 MPa. Using this measurement, it has been calculated that the minimum thickness of continuous permafrost needed to stabilize CO2 clathrate hydrate is only about 100 m, and the base of the permafrost is known to be considerably deeper at certain locations (e.g., about 600 m at Prudhoe Bay in Alaska). In this disposal method, the permafrost layers over the reservoirs would act as impermeable lids that would prevent dissociation of the clathrates and diffusion of the evolved gases up through pores.

Because the natural pressure and temperature conditions in suitably chosen reservoirs would favor the formation of clathrates, no additional energy would be needed, other than the energy for pumping the gases into the reservoirs. There would also be no need to drill holes into the reservoirs: instead, the holes and other infrastructure already in place (and used previously to extract the oil from the reservoirs) would henceforth be used to inject the gases into the reservoirs.

As an additional benefit, pumping of CO2 could help to maintain the pressure necessary for extraction of oil from an adjacent reservoir that had not yet been depleted. At present, natural gas is used for this purpose. The use of CO2 instead of natural gas would make it possible to recover more natural gas as fuel. Moreover, unlike natural gas, CO2 does not pose an explosion hazard.

This work was done by N. Duxbury of Caltech and V. Romanovsky of the University of Alaska at Fairbanks for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category.


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Permanent Sequestration of Emitted Gases in the Form of Clathrate Hydrates

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