Heat transfer in counter-current two phase flow applied to feasibility study of harvesting natural gas from seabed hydrates

The gas hydrate deposits at seabed fall into a special category of gas hydrate resources. This study investigates the technical feasibility of harvesting natural gas from seabed hydrates using a new thermal method called Moving Riser Method (MRM). A mathematical model for heat transfer along the injection pipe in the MRM system was developed. Heat transfer from the injected hot water to the gas hydrate deposit was analyzed. This study concludes that with today's pipe insulation technology water temperature drops only a few degrees from sea surface level to the seafloor level in an insulated 800 m deep vertical pipe. The injected water at seafloor level will be hot enough to dissociate gas hydrate at a commercial rate with an affordable gas consumption rate. The gas production to gas combustion ratio (PCR) is greater than 4. The PCR increases slightly with gas combustion rate. Even the gas production ship stays at the same location for over 40 h with continuous injection of hot water, the water-hydrate boundary will still be within 0.9 m of the hot water injection point. Therefore it is possible to use a gas collector of reasonable size (e.g., 2 m in diameter) to gather all dissociated gas from the hydrate deposit. Result of this investigation shows that harvesting natural gas from gas hydrate at seabed with the MRM is technically viable, economically feasible, and environmentally safe.