Occurrence of Natural Gas Hydrates in Sedimentary Basins

Abstract

Clathrates are a special variety of inclusion compound in which the guest molecules fit into separate spherical or nearly spherical chambers within the host molecule, and when the host molecule is water and the guest molecules are largely gases or liquids with low boiling points found in natural gas, the clathrates are termed natural gas hydrates. They are solid compounds, resembling ice or wet snow in appearance, and form both below and above the freezing point of water under specific PT conditions. The water molecules form pentagonal dodecahedra, which can be arranged into two different structures, leaving interstitial space in the form of either tetrakaidecahedra or hexakaidecahedra. Methane and hydrogen sulfide can be accommodated in all the spaces, ethane and carbon dioxide can fit in both the tetrakaidecahedra and the hexakaidecahedra, but propane and isobutane fit only in the hexakaidecahedra. Normal butane, pentane, and hexane are not known to form hydrates. PT diagrams describing the initial conditions for hydrate formation indicate that, relative to methane, all common components of natural gas (except nitrogen and the rare gases) raise the hydrate formation temperature, propane and ethane being the most effective. The presence of dis-solved salts in the water, or nitrogen and rare gases in the natural gas, depresses the temperature of initial hydrate formation.

The most likely way to produce natural gas hydrates in sedimentary basins is through a reduction of temperature, rather than an approach to lithostatic pressures, and the most pertinent situation is that found in regions with relatively thick permafrost sections. Sedimentary basins with extensive areas of relatively thick, continuous permafrost, which may contain potentially commercial occurrences of natural gas hydrates, are limited to the Arctic slope petroleum province of Alaska, the Mackenzie Delta and Arctic Archipelago of Canada, and the northern portion of the West Siberian basin and the Vilyuy basin of the U.S.S.R. Natural gas hydrates have only been well documented in the Messoyakha field in the West Siberian basin, which contains 14 trillion ft³ of potentially recoverable natural gas, partly in hydrate form. That portion of the field with natural gas hydrates contains 54% higher reserves than would be expected based on the assumption that the reservoir rocks were filled with free gas. Conclusive evidence of natural gas hydrates in wells drilled in the arctic regions of North America is lacking, although it is anticipated they will be found. There is need for careful monitoring of wells drilled in permafrost regions, because the nature of natural gas hydrates makes them difficult to detect unless proper precautions are taken. The technology for drilling in natural gas hydrate zone and for the optimum recovery of gas reserves in the form of natural gas hydrates is in its infancy, and more research is required.