The double-edged characteristics of the soaking time during hydrate dissociation by periodic depressurization combined with hot water injection

The combination of depressurization and thermal stimulation is considered as a promising production method of gas hydrates. In this study, the experiments of methane hydrate dissociation by periodic depressurization combined with hot water injection (D&H) were conducted. Each cycle of D&H was consisted of the hot water injection stage, the soaking stage, and the depressurization stage (below the equilibrium pressure). The influences of the soaking time (4 min, 8 min, 12 min) on the hydrate dissociation behavior, the ratio of gas to water, and the energy efficiency were analyzed. The experimental results showed that, in the early period of depressurization combined with hot water injection stage (DHS), most of the consumed heat (70–90 %) during hydrate dissociation was provided by the heat transferred from surroundings. The proportion of the heat of the injected hot water to the consumed heat in the early period of DHS was only 10–30 %, and it quickly increased to 50–60 % in the later period of DHS. The calculation showed that the reduction of the soaking time improved the heat transfer rate from surroundings to the hydrate reservoirs at the soaking stage, thereby increasing the hydrate dissociation rate. However, the reduction of the soaking time decreased the thermal efficiency and the energy efficiency during hydrate production. For the soaking time of 4 min, 8 min, and 12 min, the range of the thermal efficiency were respectively 0.34–0.82, 0.50–1.06, and 0.54–1.20, and the range of the energy efficiency were respectively 6.24–14.89, 8.91–18.82, and 9.62–21.13. Meanwhile, the reduction of the soaking time also decreased the ratio of gas production to water production, which was adverse for hydrate production. This study makes up for the deficiency that the current understanding of soaking time in periodic depressurization combined with heat injection is insufficient. The research shows that the soaking time has a double-edged characteristic, which are helpful for choosing a suitable soaking time during gas hydrate production by periodic depressurization combined with hot water injection in actual field.