Study on Gas Hydrate Formation and Dissociation Characteristics for Hydrate Slurry Transportation

Abstract Gas hydrates are ice-like crystalline solids consisting of water and gas molecules such as methane and carbon dioxide. Slurry transportation method has been proposed to convey natural methane hydrates mechanically excavated from deep-sea sediments. While conveyed in a form of slurry from seafloor to sea surface, the gas hydrates can be partially dissociated due to changes in pressure and temperature, which provides a solid-gas-liquid multiphase flow. Further changes in pressure and temperature can also form the hydrates again, which can cause plugging in pipes. In this study, the characteristics of gas hydrate formation and dissociation in gas-seawater systems were investigated by visual observations using a probe-type microscope camera to develop an evaluation model for slurry transportation of gas hydrates. In experiments, gas hydrates were formed from gas and synthetic seawater at high-pressure and low-temperature conditions in a pressure chamber and sequentially dissociated by batchwise de-pressurization. The experimental result implies that enough retention time and non-equilibrium conditions such as oversaturation are required to form massive gas hydrates. On the other hand, the hydrate dissociation proceeds to maintain the phase equilibrium against pressure deviations from the hydrate stable region induced by external actions.