Abstract: The shear strength and stiffness characteristics of methane hydrate sediments in seabed reservoirs are crucial for the safe and efficient exploration and exploitation of hydrates. To investigate the impact of changes in occurrence environments on the shear strength characteristics of muddy hydrate-bearing sediments, a series of drained shear tests have been conducted on methane hydrate-bearing muddy silt sediments under varying conditions of saturation (water-saturated and gas-saturated) and confining pressure based on the improved triaxial shear tester for hydrate-bearing sediments. The experimental results show that changes in the occurrence environment in sediments have significant effects on the shear strength and stiffness characteristics of hydrate-bearing muddy silt sediments. Compared to gas-saturated sediments, the shear strength of hydrate-bearing sediments with the same hydrate saturation greatly reduces under water-saturated condition, with a maximum reduction ratio of 52.0%. Although the effect of occurrence environmental changes on the internal friction angle of sediment is limited, the effect on the cohesion of sediment is significant. Compared to the results from gas-saturated sediments, the reduction ratio of the effective cohesion for the water-saturated hydrate-bearing sediments can reach 68.1%. Furthermore, the exponential equation is developed for describing the relation between the effective cohesion and hydrate saturation. Under water-saturated conditions, the deformation modulus of hydrate-bearing sediments decreases significantly, with a maximum reduction ratio of 72.7% for E50. Additionally, the deformation modulus increases linearly with hydrate saturation under water-saturated conditions. Lastly, based on the stress analysis of the skeleton of hydrate-bearing sediments under gas-saturated and water-saturated conditions, the evolution mechanism of the mechanical properties of hydrate-bearing sediments under these two different occurrence environments is preliminarily revealed. The observations can offer fundamental data and methods for recovering the occurrence mechanism of methane hydrate-bearing sediments and assessing the stability of the reservoir.

Key words: methane hydrate, muddy silt, water-saturated, shear strength, deformation modulus