Due to the vast amount of recoverable natural gas predicated (~3,000 TCM) in natural gas hydrate on earth, natural gas hydrate has the potential to become the next generation of unconventional source of fuel. Recently years, laboratory researches are still underway to advance our understanding of the theory and technology for natural gas hydrate exploitation. The Pilot-Scale Hydrate Simulator (PHS), a three-dimensional 117.8 L pressure vessel, was applied to study the methane hydrate dissociation using depressurization with different depressurization rates in the sandy sediment. The volume of the vessel is big enough to simulate the field-scale gas production from hydrate reservoir. The production behaviors and heat transfer characteristics during hydrate dissociation in sandy sediments with different depressurization rates were compared and investigated. The experimental results indicate the influence of depressurization rate on hydrate dissociation using depressurization method. The lower depressurization rate leads to the larger amount of hydrate dissociation during depressurizing stage, because the fluid flows in this stage enhance the heat convection in the sediment causing a higher heat transfer rate from surroundings. In addition, the lower depressurization rate causes the lower water production rate, which benefits for the gas production from hydrate reservoir. On the other hand, fast pressure dropping may lead to ice formation and secondary hydrate formation in the pipeline. However, the higher depressurizing rate leads to a higher gas production rate in the depressurizing stage. Therefore, an optimized depressurizing rate for hydrate dissociation with the highest energy efficiency is performed.
Keywords Nature gas hydrate, Depressurization rate, Sandy sediment, Large-scale experiment, Heat and mass transfer