Mechanical properties research of unconsolidated hydrate-bearing sediments under the effect of clay minerals

The shallow hydrate reservoir in the Shenhu Sea area is mainly composed of clayey silt. Clay mineral has an important impact on the mechanical properties, and the hydrate decomposition aggravates this impact. Therefore, the composition and geological conditions of shallow clay hydrate-bearing sediment in Shenhu sea area are fully considered, hydrate-bearing sediment samples with similar physical properties are synthesized in situ. Then, indoor triaxial mechanical experiments are carried out, and the effect of clay minerals on the mechanical property is analyzed. The results show that the clay content and clay type have an important impact on the mechanical properties of unconsolidated hydrate-bearing sediment. With the increase of clay content, the strain hardening characteristics are prominent, the yield stage is longer, and the plasticity is enhanced. Hydrate-bearing sediment with different clay content shows similar mechanical laws under the influence of hydrate saturation and effective confining pressure. The peak strength, elastic modulus and Poisson's ratio all show a downward trend, but the peak strength and elastic modulus change more obviously. The peak strength changes linearly with hydrate saturation, while nonlinearly with effective confining pressure, especially 0-3 MPa. This is the comprehesive result of clay particle's movement and fragmentation, clay hydration and expansion, affecting hydrate formation and sediment cementation. When the content ratio of montmorillonite/illite decreases, the peak strength and elastic modulus show an increasing trend. Because the frictional resistance and connection strength of illite crystal layer are larger with bigger particle size, weaker hydration and thinner water film. The research can provide reference for drilling and production engineering of natural gas hydrate (NGH) reservoir in the Shenhu sea area.