Sediment surface effects on methane and natural gas hydrate formation and dissociation

The effects of sediment surfaces on methane hydrate formation and dissociation were investigated using colloidal suspensions and new experimental methods developed for a large volume (72-L), temperature-controlled pressure vessel. Hydrates were formed by bubbling methane gas through test solutions at temperatures and pressures within the hydrate stability field. Hydrate formation was visually detected by the accumulation of hydrate-encrusted gas bubbles. To measure hydrate dissociation conditions, the pressure vessel was warmed while temperature was monitored within a zone of previously-formed hydrate-encrusted gas bubbles. Hydrate dissociation was indicated by a distinct plateau in temperature within the hydrate zone, at the same time that temperatures of the gas and liquid phases within the vessel continued to rise. The "dissociation plateau" appears to be a phenomenon that is unique to the large volume pressure vessel used for the experiments. In experiments where hydrates were formed in pure water, temperature and pressure conditions for the temperature plateau matched model-predicted values for hydrate stability in water, confirming the validity of this new method for measuring hydrate dissociation conditions. Formation and dissociation conditions were measured for methane hydrates in colloidal suspensions containing bentonite. Hydrate formation experiments indicated that the presence of 200 mg/L bentonite in water significantly decreased pressures required for hydrate formation relative to formation in pure water.On the other hand, hydrate dissociation conditions measured in 34 g/L bentonite, silica,and calcium carbonate suspensions and a humic acid suspension with a concentration ofIg/L did not differ significantly from that of water. Dissociation experiments in a 3.4%sodium chloride solution showed a 2 °C negative effect in hydrate stability. Dissociation Conditions in natural gas hydrates in a 34 g/L bentonite suspension matched model predictions of stability in pure water. These results are relevant to the origin and stability of natural gas hydrate deposits known to exist in deep permafrost and marine sediments,where the effects of sediment surfaces are largely unknown.