Hydrate morphology and mechanical behavior of hydrate-bearing sediments: a critical review

Abstract: Natural gas hydrate is a promising energy resource in the future because of its little contamination and huge reserve. However, gas exploitation may induce large deformation and failure of the seabed due to a reduction in the stiffness and strength of the hydrate-bearing sediment (HBS). Therefore, it is essential to investigate the mechanical behavior of the HBS for safe and efficient gas exploitation. Additionally, it is widely acknowledged that the hydrate morphology inherently affects the mechanical behavior of the HBS. This paper aims to critically synthesize the information on the hydrate morphology and mechanical behavior of the HBS available in the literature to facilitate the application of the research result into engineering practice and provide guidance for future investigation. Hydrate morphology is identified firstly both in natural and synthesized HBS. The similarities and differences of the hydrate morphology in the HBS synthesized using the excess-gas and excess-water methods are highlighted. The available experimental data on the small-strain stiffness, strength, and stress–strain behavior are critically selected and grouped into two categories based on the synthesizing methods. It has been creatively discovered that most mechanical parameters (e.g., bulk modulus, shear modulus, cohesion, dilation angle) share a concave power relationship with the hydrate saturation S_H for the HBS synthesized using the excess-water method. While it is a convex power relationship for the bulk modulus, shear modulus, and dilation angle, and a linear relationship for the cohesion c when the HBS is synthesized using the excess-gas method. These observations contribute to establishing the conceptual model reflecting the particle-level failure mechanism of the HBS synthesized using different methods. Afterward, the creep behavior of the HBS, the reported constitutive models, the associated advantages and limitations of each model, and the mechanical response during hydrate dissociation (e.g., depressurization, thermal stimulation, carbon dioxide replacement), are summarized and discussed. It is expected that the state-of-the-art review can deepen our understanding of the mechanical behavior of the HBS and assist in the design of gas extraction programs without triggering potential geohazards. Article highlights: Similarities and differences in the hydrate morphology of the HBS synthesized using theexcess gas and excess water methods are clarified.The experimental data on stiffness, strength and stress strain in the literature arecritically selected and synthesized.The influence of hydrate morphology on the mechanical behavior of the HBS iscomprehensively analyzed.The creep behavior and mechanical response to hydrate dissociation are summarized.The constitutive equations on stiffness, strength and stress strain are summarized andevaluated.