Influences of Pore-Occupying Media on Strength of Hydrate-Bearing Sediments by Triaxial Tests

Marine hydrates occupy the pores of sediments, altering the composition of pore-occupying media and pore structure, leading to significant changes in the reservoir strength, which is crucial for hydrate extraction site selection and engineering safety evaluation. By conducting high-pressure, low-temperature triaxial tests on hydrate-bearing sediments (HBS) and hydrate-free sediments, the impact of pore-occupying media types, the proportion of different occupying media, and effective confining pressure on the strength of sediments was investigated. The results show that the type of pore-occupying media significantly affects the strain hardening and softening characteristics of sediments. The deformation resistance of HBS increases with the increment of hydrate content, however, the peak strength of HBS does not increase with hydrate content until a critical hydrate content is reached. In contrast, the peak strength of hydrate-free sediments increases with the content of soil particles with no critical content observed. Both hydrates and soil particles significantly affect the effective cohesion of sediments, but through different mechanisms: hydrates enhance cohesion through cementation, while soil particles enhance it through interlocking friction. As the effective confining pressure increases, the pore-filling effect of hydrates on sediment strength gradually weakens compared to soil particles. When soil particles in the pores are gradually replaced by hydrates, the strength of HBS first decreases and then increases, with a critical replacement ratio where the strength development tendency changes. This critical replacement ratio increases as the effective porosity decreases, reaching 76% when the effective porosity is 38%.