Unsaturated water flow-induced the structure variation of gas hydrate reservoir and its effect on fluid migration and gas production

Multi-phase (gas, saturated water, and unsaturated water) seepage is bound to exist in natural gas hydrates (NGHs) production process. The effect of water flow, especially for unsaturated water flow, on the permeability variation of hydrate reservoir and gas production behavior do not appear well understood. In this study, the unsaturated water flow in hydrate-bearing sediment and hydrate-free sediment is simulated by controlling water flow velocity. The hydrate phase distribution was monitored using visualization magnetic resonance imaging system. The variation of temperature, pressure, and gas production rate during the unsaturated water flow were analyzed. The results show that the changing trend of the pressure difference presented the three stages for the hydrate-free sediment. In contrast, the pressure difference followed a five-stage change for the hydrate-bearing sediment in the unsaturated water flow process due to the hydrate dissolution. The hydrate dissolution caused an increase in permeability (maximum of log10(Kr) was 0.5) and formed the obvious unsaturated water flow channel. Moreover, the higher water flow velocity, which increased the chemical potential difference between hydrate phase and water phase, accelerated the MH dissolution, and further induced the faster increase rate of permeability and gas production. Surprisingly, when the unsaturated water flow velocity was improved to 15 mL/min from 0.5 mL/min, the gas production rate increased by 35 times. Furthermore, the average gas production rate was mainly determined by the unsaturated water flow velocity, it was changing in a linear fashion with the increasing water flow velocity. The findings could provide new knowledge on the strategy design on NGHs production with high efficiency.