Strength properties and strength criterion of unconsolidated hydrate-bearing sediment under the effect of clay minerals

The Shenhu sea area is characterized by substantial natural gas hydrate (NGH) resources, where clay minerals exert a considerable influence on the mechanical properties of the surrounding sediments. This influence is further amplified during hydrate decomposition. Given this context, the composition and geological characteristics of the shallow, clay-rich, hydrate-bearing sediments specific to the Shenhu sea area was investigated. Subsequently, a series of carefully controlled triaxial mechanical experiments were performed on in-situ synthesized hydrate-bearing sediments, which determined the variations in strength parameters under different clay contents, and facilitated the development of an effective strength criterion through multiple regression analysis. The results indicate that: (1) Clay minerals significantly affect the mechanical behavior of hydrate-bearing sediments. An increase in clay content is associated with a decrease in peak strength. Under the influence of hydrate saturation and effective confining pressure, hydrate-bearing sediments exhibit similar mechanical responses across varying clay contents. The peak strength demonstrates a linear relationship with hydrate saturation while a non-linear relationship with effective confining pressure. (2) Cohesion and the internal friction angle generally decrease with increasing clay content and decreasing hydrate saturation. However, variations in cohesion are more pronounced and follow a more consistent pattern, which are characterized as a function of clay content and hydrate saturation. (3) Based on the Mohr-Coulomb criterion, a high-precision strength criterion for unconsolidated hydrate-bearing sediments is established incorporating the effects of clay content, hydrate saturation, and effective confining pressure. This research provides a valuable reference for the design and optimization of NGH drilling and production operations in the Shenhu sea area.