Numerical investigation of the three-phase layer production performance of an offshore natural gas hydrate trial production

How to improve the gas production is the most urgent matter for natural gas hydrate exploitation. During the second gas hydrate production test in the Shenhu Area, South China Sea, the concept of three-phase layer production was put forward, and the gas production rate was greatly improved. However, the formation mechanism and specific production performance of three-phase layer is still unclear. Therefore, in this study, we first elucidate the formation mechanism of the three-phase layer and realize its characterization in the model. It is founded that the increase in pore water salinity is the main reason for the stable existence of the three-phase layer. If this phenomenon is ignored, the three-phase layer will be in a pseudo-stable state and simulated gas production will be lower by nearly 15%. Additionally, simulation results show that gas production rate of three-phase layer is 2.5 times that of the gas hydrate-bearing layer, although the pressure drop is only one-third of that applied to the latter. Long-term prediction illustrates that gas production rate halves from its peak within a month due to the poor permeability of clayey silt reservoir. Then, the influence of well location and reservoir stimulation on production performance is further discussed.