Scott D. McCallum

Sediment surface effects on methane hydrate formation and dissociation

Abstract 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 liters), 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 the hydrate zone temperature, while 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 of the pressure vessel used for the experiments. In experiments where hydrates were formed in pure water, temperature and corresponding pressure conditions measured during the temperature plateau matched model-predicted values for hydrate stability in water, thus 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 bentonite in water at 200 mg/l significantly decreased pressures required for hydrate formation relative to formation in pure water alone. On the other hand, hydrate dissociation conditions measured in bentonite and silica suspensions with solids concentrations of 34 g/l did not differ significantly from that of 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.

Influence of water thermal history and overpressure on CO2-hydrate nucleation and morphology

Abstract The onset of gas hydrate nucleation is greatly affected by the thermal history of the water that forms its lattice structure. Hydrate formation experiments were performed in a 72 L pressure vessel by injecting bubbles of carbon dioxide through a 1 L tube at hydrate formation pressures (1.4 to 3.7 MPa) and temperatures (2 to 5 °C). The results revealed that when even a small fraction (e.g., 5-35%) of the water in which the hydrate formed was recently thawed the overpressure for nucleation was reduced by an average of 50% as compared to untreated distilled water. This observation was confirmed by an analysis of variance (ANOVA) test that indicated that recently thawed water required a significantly lower overpressure compared to the untreated distilled water. In experiments where hydrate nucleated at low overpressure (e.g., 0.75 MPa), hydrate formed at the vapor-liquid interface, encrusting the bubbles with less than 1 g of hydrate accumulation in the first minute. When a higher overpressure was required for nucleation (e.g., 1.3 MPa), hydrate was observed to form abruptly not only on bubbles but also from the bulk liquid phase, typically accumulating a mass of more than 100 g in the first few seconds. Our results show that initiation of hydrate formation is strongly influenced by temperature-dependent pre-structuring of water molecules prior to their contact with gas. Although as little as a 5% volume fraction of pre-structured water may decrease the required overpressure, once hydrate formation commences the mass of hydrate accumulation is dependent on the overpressure.