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Abstract:
Natural gas hydrates are ice-like crystalline solids consisting of hydrogen bonded network of water molecules and gas molecules, mainly methane. They form at elevated pressure and cold temperature conditions in presence of sufficient amounts of gas and water. Among others, these conditions are fulfilled at continental margins and permafrost regions where natural gas hydrates occur. Within these hydrate-bearing sediments, gas hydrates may occur as cementing solid at the contact area of sediment grains or as pore filling and load bearing hydrate. The morphology of the formed gas hydrate depends on the formation conditions and affects the properties of the host sediment: gas hydrates may support the grain skele-ton, take over effective stress, and considerably increase sediment stiffness even at low con-centrations. Conversely, the dissociation of natural gas hydrates, especially in marine sedi-ments at the continental margins, likely results in the reduction of the mechanical strength of the host sediment, may also induce overpressure due to gas release and may decrease the slope stability. Since continental shelf and slope regions are used by humans for several activities, which largely rely on the mechanical stability of the marine sediment, the knowledge concerning the key factors and processes determining the stability of the weakly consolidated marine slope sediments is crucial. A key factor is the response of natural gas hydrates to potential ocean warming as a result of climate warming or human activities such as the production of methane from hydrate-bearing sediments. Both lead to a decrease of hydrate saturation and, therefore, to a change of the geo-mechanical properties of the host sediment. In the framework of the SUGAR project we developed a ring shear cell and inves-tigated the residual and peak shear strength of hydrate-bearing sediments. Based on litera-ture data and our preliminary results we discuss the dependency of the mechanical proper-ties of hydrate-bearing sediments on the hydrate saturation and the potential consequenc-es.
