Literature review on physical properties of gas hydrates and implications for sediment stability

This literature review was undertaken as a preliminary study as part of an Ocean Margins Link research project, involving BGS, the University of Leicester and Geotek, into the characterisation of the physical properties of gas hydrates.
The bulk of global organic carbon is taken to be stored in methane hydrates below the ocean floor. These ice-like solids, formed within sediments, comprise ‘cages’ of water molecules enclosing gas that are stable at high pressures and low temperatures. Recent work suggests that destabilisation of sediment hosted hydrates could trigger catastrophic slope failures on ocean margins (Mienert et al.,1998). The liberation of ‘greenhouse’ gases, such as methane, within the hydrate cages, during destabilisation may have significant effects on the efficiency of global thermohaline circulation.
Current research and observations from recovered samples indicate that hydrates form a variety of fabrics within the sediment, and that such fabrics are significant in influencing hydrate stability, (Malone,1985; Ivanov et al.,1998). The influence of the pressure and temperature history on the gas-hydrate distribution in laboratory sediments is being investigated. Pressure vessels are being developed to enable laboratory quantification of the effect of hydrate fabrics. Current investigations are concentrating on developing techniques to grow significant quantities of hydrates within sediments under controlled conditions. It is intended that this will lead to the study of the physical properties of these hydrate/sediment assemblages.
The effect of hydrate on submarine slope stability is still poorly understood. Although it is possible to use standard geotechnical models to calculate Factors of Safety against slope failure, it is likely that these will be inadequate for reliable risk assessment of submarine slopes containing gas hydrate as there is huge uncertainty in physical properties of hydrate bearing sediments. The solid hydrate and the gas and water evolved from disassociating hydrate will probably change the way the sediment responds to static (e.g. weight) and dynamic (e.g. earthquake) loading and a more rigorous method of slope stability analysis will be required for more accurate risk assessment.