DEM simulation on undrained shear characteristics of natural gas hydrate bearing sediments

Natural gas hydrate (NGH), commonly found in deep-sea sediments, has been identified as a promising energy resource due to its abundance in nature, high energy density and clean combustion, and it is crucial for exploring the underlying mechanical properties of natural gas hydrate bearing sediments (NGHBS)for gas recovery. This study aims to investigate the undrained shear behaviors of NGHBS using the discrete element method (DEM). The constant volume method was adopted to simulate the undrained conditions. A series of DEM simulations of biaxial compression tests were performed and then the macromechanical behaviors of NGHBS were presented and verified, and finally mesomechanical mechanisms during shear were examined. The results highlighted that (1)the DEM simulation can effectively reflect the macromechanical properties of NGHBS under undrained conditions; (2)the average mechanical coordination numbers of all samples decrease initially and then increase until it reaches a relatively stable value during shear, and NGHBS samples show greater variances in coordination number compared with sand samples because of the hydrate cementation effect; (3)the force chain network is composed of nearly circular force chains at initial state and then featured by column-like structures and finally consists mostly of buckling force chains; (4)the fabric anisotropy is closely related with the direction of external loading, and the principal directions of fabric anisotropy of contact normals and normal contact forces are parallel with the direction of external loading. Besides, the values of normal contact forces are about an order of magnitude higher than that of the tangential contact forces.