Optimization of Horizontal Well Placement for Natural Gas Hydrate Production Considering Seafloor Subsidence

Natural gas hydrate (NGH) has attracted increasing attention as a promising source of clean natural gas, but its exploitation also presents technical challenges and geological concerns. Recent studies and the success of the second NGH production test in the South China Sea suggest that horizontal wells have the potential to enhance the recovery of low-permeability NGH reservoirs. Nevertheless, the rapid depressurization due to horizontal well extraction increases effective stress within the reservoir, triggering reservoir deformation and seafloor subsidence. Additionally, certain NGH reservoirs have been demonstrated to exhibit a multi-layered structure with complex phase conditions, including the hydrate-bearing layer (HBL), the three-phase layer (TPL), and the free gas layer (FGL), introducing additional uncertainties into the production process. Based on geological data from Site W17 in the South China Sea, this study conducts an optimization analysis of horizontal well placement, considering both natural gas production behavior and seafloor subsidence risks. The results indicate that the maximum gas production was obtained when the well placement was in FGL. However, simultaneously, the water production was 1,126% and 570% higher compared to the well placement in HBL and TPL. Furthermore, the maximum seafloor subsidence was observed to reach 0.42 m, 0.19 m, and 0.54 m when the well placement was in HBL, TPL, and FGL, respectively. Production within the TPL layer resulted in less geologic risk while achieving approximate gas production. Therefore, the optimal well placement for horizontal wells is determined to be within the TPL.