Seafloor subsidence response and submarine slope stability evaluation in response to hydrate dissociation

Gas hydrate dissociation, resulted from temperature and pressure changes in reservoirs, and natural submarine landslides demonstrate spatial consistency. Currently, large-scale production of oceanic gas hydrates is subject to extensive exploration. However, only few studies focus how large-scale hydrate mining affects near-future stability of the submarine slope and landslides. Thus, we used actual exploration data of the hydrate sediment in the Shenhu area in the South China Sea to establish a substantial three-dimensional and multi-field coupled numerical model of the hydrate submarine slope, which take into account dynamic strength changes of the hydrate layer during the hydrate decomposition process. Our numerical study indicated that severe reduction of hydrate sediments strength is the ultimate cause of the seabed settlement and slope instability. When horizontal wells are utilized to produce hydrates on the seabed slope, horizontal wells should be oriented in the vertical slope direction to ensure maximum stability of the slope as well as higher gas yield. Our results also revealed that seafloor subsidence caused by the increased effective stress corresponds to ∼20% of the total subsidence. Excessive production pressure drop (>9 MPa) and heat injection temperature (>80 °C) as well as large production scale (>150 m) will cause sharp decrease in slope stability and trigger submarine landslides. Our study also discussed methods of hydrate recovery and demonstrated benefits of simultaneous implementation of depressurization and the heat injection methods.