Temperature effects in deep-water gas hydrate foam

Abstract

This study focuses on heat and mass exchange processes in hydrate foam during its formation from methane bubbles in gas hydrate stability zone (GHSZ) of the Lake Baikal and following delivery of it in open container to the lake surface. The foam was formed as a result of methane bubble collection with a trap/container. The trap was inverted glass beaker of diameter of 70 mm and 360 mm long. Open bottom end of the beaker used as enter for bubbles ascended from the lakebed. At a depth of 1400 m all bubbles which fed to the trap were transformed here into solid hydrate foam. The sensitive thermometer was mounted in the middle of the trap and recorded the temperature inside trap. The fate of the bubbles in the trap was recorded by video-camera. Hydrate front, i.e. the border between forming hydrate foam and pure water, propagated slowly from the top to the bottom of the trap during bubbles collection. The temperature in the trap was slightly larger (by 0.05 o C) than the temperature of ambient water.However as soon as the front crossed the location of temperature sensor, the temperature jump by ~+1.1 o C was recorded. After that during 3 hours the temperature exponentially relaxes to an asymptotic magnitude which however exceeded the temperature of ambient water by 0.3 o C. Thus hydrate foam formation occurs with heat dissipation with maximal intensity on hydrate front. The dissipation does not stop within hydrate volume at the constant depth of 1400 m during at least of 3 hours. After bubbles collection the trap with the foam was ascended from the lakebed to the surface, initially within GHSZ and then above GHSZ. During ascend within GHSZ with velocity about 0.375 m/s two remarkable features were observed. First one is displacement of the water from the trap by gas which was released from the foam sample under the decrease of the hydrostatic pressure. It was found that gas expansion follows to Boyle-Mariotte law in which initial volume of gas is equal approximately to volume of the foam sample. The second feature is continuous decrease of the temperature in the foam up to a level of negative magnitude in a depth interval of 1400 -750 meters. Above 750 m temperature decrease was changed by small growth. However once the trap ascended above top boundary of GHSZ at a depth of 380 m, the temperature fell sharply. Falling to a negative values -0.25 o C, the temperature sharply stabilized and did not changed further until the trap reached the surface. The decreasing of the temperature during the ascent is due to the cooling of gas as a result of the performing of the work against the forces of hydrostatic pressure. The temperature drop at the boundary of the GHSZ is due to the absorption of heat by the decomposition of hydrate. The keeping of the foam temperature at the constant level -0.25 o C above GHSZ reduces hydrate decomposition and is exhibition of effect of selfconservation of hydrate.