Volume change and creep behaviors related to stress-phase transition path in methane hydrate-bearing silty sand

The volume change behavior of natural gas hydrate-bearing sediment is essential as it influences settlement, strength, and stiffness, which directly affect the stability of hydrate reservoirs during hydrate extraction or in response to environmental changes. The volume change is influenced not only by stress but also by the formation and dissociation of hydrates. This study adopted a customized apparatus for one-dimensional compression tests, allowing independent control of gas pressure and effective stress. Tests were conducted on samples with different hydrate saturations along various temperature-gas pressure-effective stress paths, yielding some conclusions related to compressibility and creep. An unusual phenomenon was observed under low-stress conditions: hydrate formation led to shrinkage rather than expansion. Three potential mechanisms behind this occurrence were discussed. As hydrate saturation increases, the yield stress rises while the compression and swelling indexes remain minimally affected. After hydrate dissociation, the compression curve of hydrate-bearing sediment drops to that of hydrate-free sediment. Once hydrate is formed, the compression curve of hydrate-free sediment gradually approaches that of hydrate-bearing sediment during the subsequent loading. Under low-stress conditions, the creep of both hydrate-free and hydrate-bearing sediments is very weak. However, when stress increases, significantly beyond the yield stress, the creep of both sediments increases significantly, with hydrate-bearing sediment exhibiting much greater creep than hydrate-free sediment.