Simulation research on solid fluidization exploitation of deepwater superficial layer natural gas hydrate reservoirs based on double-layer continuous pipe

In this paper, a new method for exploiting shallow marine natural hydrate reservoirs is proposed using a double-layer continuous pipe. In order to research the multiphase flow behavior and hydrate decomposition characteristics during the hydrate mining process, a non-isothermal transient gas-liquid-solid multiphase flow model is established considering the coupling effect of multiphase flow, heat transfer and hydrate phase transition. The model's accuracy is verified by comparisons with laboratory and field data. Numerical simulation results show that the hydrate decomposition is slow in the first 2 h of mining, thus the volume fraction of each phase in the pipe changes little; with the increase of time, the volume fraction of each phase in the pipe changes significantly until it reaches a stable state after about 5 h of mining. When the mining rate exceeds 15 kg/s, the hydrate particles will not be fully and effectively transported, and a short time aggregation phenomenon will occur. Increasing the temperature of the injected seawater is beneficial to promote the hydrate decomposition, which in turn increases gas production. While, increasing the wellhead backpressure and seawater displacement facilitates in reducing gas production. The research findings will be of reference value for gaining insight into studying complex multiphase flow behaviors involving hydrate phase transition and provide a new approach to exploiting subsea shallow natural gas hydrate reservoirs.