In order to know well the range and degree of gas hydrate dissociation during the large-scale exploitation of marine natural gas hydrate, the finite element software COMSOL is used to simulate the electromagnetic monitoring of the natural gas hydrate exploitation through the downhole controlled-source electromagnetic (CSEM) method. By constructing the 3D formation model containing the high-resistivity hydrate reservoir layer, the comparison between the downhole vertical source and the conventional towed horizontal source indicates that the former has advantages on the electric field responses for small source-receiver offsets required for monitoring. Then, the influence of vertical source monitoring system parameters, such as metal casing, vertical source depth, emission frequency, observation errors and noises, on the seafloor electric field responses are analyzed. The simulation results show that the metal casing, vertical source depth and emission frequency have significant influences on the seafloor electric field responses, and especially for the existence of metal casing, the vertical source should be placed below the hydrate reservoir and the low frequency such as 1 Hz of emission electrical current should be considered. The observation errors and noises of the monitoring system would not significantly affect the performance of the monitoring system with metal casing under the condition of small offsets. When the production range of the hydrate reservoir increases, the downhole vertical source monitoring system ensures that seafloor receivers can detect the electric field changes caused by hydrate exploitation and effectively identify the lateral boundary of production area. Hence, it is feasible to use the downhole CSEM method to monitor the dynamic reservoir process during marine hydrate exploitation. 

为掌握海域天然气水合物在大范围开采过程中的分解范围和分解程度, 应用有限元软件COMSOL, 对采用井中可控源电磁方法监测天然气水合物开采过程进行正演模拟。通过构建含高阻天然气水合物储层的三维地层模型, 在比较井中垂直源监测系统相对于传统拖曳式水平源优势的基础上, 分析垂直源监测系统中金属套管、垂直源深度、频率、观测误差和噪声等参数对海底电场响应的影响。模拟结果表明, 金属套管、垂直源深度和电流发射频率对井中电磁监测效果的影响较大, 在有套管时, 垂直源的布设深度应位于天然气水合物储层之下, 并采用1 Hz的低频发射电流; 在小收发距时, 监测系统的观测误差和噪声对有套管监测系统的性能影响不大。天然气水合物开采范围扩大时, 井中垂直源监测系统可以确保海底接收器探测到相应的电场信号变化, 并有效地识别开采范围的横向边界。因此, 采用井中可控源电磁方法对海域天然气水合物开采过程进行储层动态监测是可行的。