Heat utilization efficiency analysis of gas production from hydrate reservoir by depressurization in conjunction with heat stimulation

The heat utilization efficiency of heat injection is a key factor for the available heat stimulation method in natural gas hydrate, and the heat transfer characteristics during natural gas production from hydrate reservoir require further analysis. In this work, based on the laboratory tests by the Pilot Hydrate Stimulator (PHS), the heat transfer process of hydrate decomposition in the PHS was investigated, and the real-time decomposition rate and the energy utilization efficiency of the hydrate decomposition were quantitatively studied. The experimental results showed that the energy consumption of hydrate decomposition was mainly obtained from the sensible heat hydrate bearing sediments and the heat transfer from the environment in the depressurization process. In the initial stage of heat injection, the energy consumption of hydrate decomposition mainly came from the heat contained in the injected hot water and the heat transfer through the boundaries. In the later stage of heat injection, all of the energy consumption for hydrate decomposition was obtained from heat injection. In this work, the heat transfer coefficient between the reservoir and the environment was first proposed to quantify the heat transfer through the boundary. The results showed that the heat transfer coefficient presented large fluctuations during the initial stage of the depressurization and the period when the reservoir temperature reached the environment temperature in the heat injection process. Meanwhile, the coefficient remained stable at the later stage of heat injection. These results indicated that the change in gas-water hydrate inside the reservoir and the change in reservoir temperature affected the heat transfer coefficient between the reservoir and the environment. Furthermore, the energy utilization efficiency and gas production rate were optimized for hydrate production when the reservoir temperature was controlled within the range close to the environmental temperature. These results may be of great significance for heat injection control during field production from hydrate reservoirs.