Multistage Triaxial Shear Behavior of Methane Hydrate-Bearing Sands: Insights from the Discrete Element Method

Natural gas hydrate (NGH), recognized as a potential clean energy resource, poses significant geomechanical challenges during exploitation. A triaxial shear test serves as a critical tool for evaluating the mechanical properties of hydrate-bearing sediments (HBS) to ensure safe exploitation. This study employs the discrete element method (DEM) to compare the mechanical behaviors of HBS subjected to multistage and single-stage triaxial loading conditions. Key findings reveal that (1) shear-induced volumetric dilation exhibits a positive correlation with hydrate saturation but an inverse relationship with confining pressure; (2) multistage tests yield comparable failure strength to single-stage tests yet exhibit a reduced elastic modulus due to the irreversible structural damage accumulated in prior loading stages; (3) at elevated hydrate saturations (>35.5%), pronounced particle slip occurs following stage II loading, suggesting premature shear band formation prior to complete reconsolidation; (4) force chain analysis at σc′ = 3 MP and εa = 20% indicates analogous contact force distributions between multistage and single-stage specimens, despite their different loading histories. A multistage triaxial test offers an efficient approach for characterizing strength and dilatancy properties of HBS, though its applicability for stiffness evaluation remains limited due to progressive structural damage effects. The findings provide fundamental insights for optimizing NGH exploitation strategies while mitigating geomechanical risks.