Undrained Mechanical Properties of Overlying Soils of Gas Hydrate Reservoirs in the South China Sea

Gas hydrate resources are abundant in the South China Sea (SCS), primarily occurring in clayey-silty reservoirs. However, the process of natural gas hydrate (NGH) extraction can lead to the destruction of cementation structures, the reduction of the bearing capacity, and the deformation of both the hydrate reservoir and its overlying soil layers (OSLs), ultimately causing engineering geological hazards such as seabed landslides. Therefore, before achieving commercial exploitation of NGHs, a comprehensive grasp of the mechanical characteristics of NGH reservoirs and their OSLs is imperative. However, most of the research on the safety of hydrate exploitation focuses on the mechanical properties of the hydrate reservoirs while overlooking the stability of OSLs, especially the undrained mechanical properties of OSLs. Based on this, this study investigates the undrained mechanical characteristics of the OSLs of NGH reservoirs in SCS under the influence of multiple parameters. The findings suggest that the strength and modulus of the OSLs of NGH reservoirs in the SCS are positively correlated with effective confining pressure, shear rate, and overconsolidation ratio but negatively correlated with porosity. Moreover, an increase in the effective stress and porosity enhances the strain-hardening and shear-contraction characteristics of the OSLs. Therefore, during hydrate extraction, locations within the OSLs characterized by a shallower burial depth, lower OCR, and higher porosity are more susceptible to failure, and it is advisable to select the NGH reservoir with a deeper burial depth, smaller porosity, and higher OCR. The conclusions of this study will contribute to the establishment of the constitutive relationship of the OSLs of NGH reservoirs in the SCS and provide a theoretical basis for reservoir stability during hydrate extraction processes.