Mechanical behavior of methane hydrate-bearing sediments with various hydrate saturations and morphologies under a complicated environment

The safe and efficient extraction of methane from marine hydrate-bearing sediments (HBS) necessitates a thorough understanding of their geomechanical behavior, particularly shear strength, which is critically influenced by hydrate saturation and occurrence habit. This study systematically investigates the mechanical properties of hydrate-bearing sandy sediments (HBSSs) under varying hydrate saturations and morphologies, focusing on the effects of hydrate dissociation. Methane hydrates were synthesized in sand using the excess-gas (EG) and frost-seeding (FS) methods. Consolidation-drained triaxial tests were conducted on specimens under gas-saturated, water-saturated, and partially dissociated conditions. Results show that hydrate occurrence habit—cementing, pore-filling, or load-bearing—significantly affects the stress-strain response and shear strength. EG-prepared specimens exhibited higher strength than FS-prepared ones at low saturation due to better cementation, though this difference diminished at higher saturation. Water saturation degraded cementation, reducing shear strength, especially at low saturations where hydrates transitioned to non-cementing habits. Partial hydrate dissociation via depressurization caused progressive strength reduction, shifting behavior from strain-softening to strain-hardening. The residual hydrate had a negligible mechanical influence at high water content. It is suggested that the shear strength of partially dissociated HBSS in gas-rich environments aligns with that of water-saturated specimens prepared by the same method, offering a novel strategy for determining the strength of HBSs in water-rich seabed environments. These findings underscore the need to incorporate dynamic hydrate dissociation into strength models to prevent overestimation of sediment stability during gas extraction.