Interactions Between Methane Gas and Hydrate Beneath Methane Plumes Offshore Svalbard

Abstract:

Plumes of methane bubbles have been observed over a number of years emanating from the seabed on the western continental margin of Svalbard. The occurrence of these bubble plumes close to the outcrop of the hydrate stability zone led to the hypothesis that some of the methane is derived from dissociation of hydrate caused by an observed 30 year warming trend in the ocean bottom water at around 400 m depth. A further hypothesis suggests that the methane seepage has been occurring for 3000 years and that the formation and dissociation of gas hydrate has focussed the flow of methane to the site of the plumes. In this modelling study, a range of numerical models have been built to examine particular aspects of the Svalbard system to improve our understanding of the migration of methane in the sediments beneath the plumes and how hydrate formation and dissociation may influence methane flow. The choice of equation for the hydrate stability curve is shown to be a significant factor in determining the location of the hydrate stability zone in relation to the plumes, and could therefore affect the interpretation of methane migration pathways. Numerical models are used to examine the effect of seasonal ocean temperature fluctuations on the flux of methane at the seabed. The release of methane is shown to be affected by the period of the temperature cycles compared to the rate of methane supply. Finally 2D models of methane flow at the base of the hydrate stability zone are used to examine how methane migration might be affected by the presence of hydrates. The models consider the degree to which methane migrates upslope beneath the hydrate stability zone compared to entering the hydrate stability zone and forming hydrate. The results obtained from these modelling studies build on previous work by a number of authors to improve the conceptual model of methane migration at the edge of the hydrate stability zone.