Changes in Sediment Strength

The formation and decomposition of gas hydrate is believed to exert a significant influence on the mechanical properties of marine sediments. The formation of methane hydrate results in the extraction of water and methane from the pore space. Thus, liquid water and dissolved (and in some cases gaseous) methane are extracted from the pore space and converted into solid gas hydrate crystals. Replacement of the liquid water by solid gas hydrate will increase the shear strength of the sediments. Moreover, there will also be a decrease in the porosity and permeability of the sediments. The formation of gas hydrate within continental margin sediments may be similar to the effects of water ice formation in permafrost areas.

Conversely, when gas hydrates become unstable, they decompose into water plus gas. The effect of transforming solid materials into a liquid (and possibly a free gas phase) will be to decrease the shear strength of the sediments, making the sediments more prone to failure. If gas saturation is exceeded, gas bubbles will be produced as a consequence of gas hydrate decomposition and their presence is likely to further decrease the strength of the sediment.

The process of gas hydrate decomposition will also affect the pore pressures of the sediments (Kayen and Lee, 1991 and 1993). When methane hydrate decomposes in sediments in which the pore waters are already saturated with methane, a volume of both water and methane will be released into the pore spaces that exceed the volume that was previously occupied by the gas hydrate. The net effect is either a pressure increase (if the sediments are well sealed) or a volume increase (if pressure is allowed to dissipate by fluid flow). Thus the decomposition of gas hydrate can cause pressure greater than hydrostatic pressures (the pressure produced by the weight of a column of water above a location, Kayen and Lee, 1991) especially if low-permeability sedimentary layers are available to seal pressure in. The associated increases in pore pressure, dilation (expansion) of sediment volume, and the development of interstitial gas bubbles all have the potential to weaken the sediment (Prior and Coleman, 1984).

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