The Microscopic Failure Mechanism of Hydrate-Bearing Sand Under Direct Shear: Insights from 3D DEM Simulation

Gas production in hydrate reservoirs can lead to large deformations, causing wellbore or platform instability, reservoir subsidence, and submarine landslides. However, current research on the mechanical responses of hydrate reservoirs under large deformations is not thorough. Benefiting from direct shear tests on hydrate-bearing sands, this study employed the discrete element method to conduct further microstructure characterizations and failure mechanism analysis on calibrated numerical specimens. The results indicate that specimens’ responses under shear loading can be generally divided into elastic, yielding, strain-softening, and residual-deformation stages, where specimens with higher hydrate saturation exhibited higher peak and residual strengths, larger volumetric dilation, more bond breakage, and a larger coordination number. The local stress concentration and dominant force chains evidently formed and developed during the yielding and strain-softening stages, and the shear bands were clearly observed by distinct particle translations and rotations around the shear surface. The widths and angles of the shear bands decreased with increasing hydrate saturations because of the concentrated strong force chains. The local porosity within the shear band increased with a large shear displacement and higher hydrate saturation, contributing to the macro dilation of the specimen. A tensile-dominated failure mode of the specimens during shear loading was observed, and parts of the tensile failure may gradually transform into shear failure with increasing hydrate saturation, due to particle cluster rotation suppressed by the hydrates. These findings provide microscopic mechanical insights for hydrate reservoirs under large shear deformations.