Utilization of water-gas flow on natural gas hydrate recovery with different depressurization modes

Water-gas two-phase flow always exists in the natural gas hydrate production process. It has been confirmed that a suitable water-gas flow rate ratio can efficiently induce hydrate decomposition above hydrate phase equilibrium. Meanwhile, the water-gas flow may help to prevent the ice generation and hydrate reformation that need to be solved in the depressurization process. However, little is known about the synthetic effect mechanism of different depressurization modes and water-gas flow on hydrate decomposition. In this study, the piecewise depressurization, constant depressurization rate, constant gas recovery rate assisted with water-gas flow was used to decompose methane hydrate, respectively. The hydrate decomposition behaviors were investigated via magnetic resonance imaging. The results indicated that the hydrate decomposition characteristics showed spatial dependence, and the decomposition front was moved to the center from the edges along the interface of water and hydrate. Moreover, the higher water-gas flow rate ratio and faster depressurization rate led to a lower energy input (gas and water injection volume) and higher energy recovery rate (hydrate decomposition rate). Compared to sudden depressurization, the ice generation was efficiently avoided and the hydrate decomposition rate was remarkably improved for the all three decomposition modes. By comparison, the combination of piecewise depressurization incorporated with water-gas flow (mode I) was the best mode to recover gas from hydrate reservoirs in our experimental scale.