Study on creep characteristics of clayey silt natural gas hydrate-bearing sediments

The natural gas hydrate reservoirs in the South China Sea are predominantly composed of argillaceous silt sediments, with the trial production area mainly consisting of fine-grained particles and clay minerals, notably montmorillonite and illite. The type and content of clay minerals directly influence the material composition and particle size distribution of the host sediments, thereby significantly affecting their mechanical properties. Furthermore, clay minerals exhibit pronounced plasticity, and their presence markedly alters the creep characteristics of the reservoir, exacerbating reservoir deformation and potentially leading to reservoir failure. Therefore, exploring the mechanism of clay minerals influence on the mechanical properties and creep behavior of hydrate deposits is an important basis for evaluating the stability of clayey silt reservoirs. Based on this background, this study deeply analyzes the influence of clay mineral type and content on the creep characteristics of clayey silt hydrate sediments to provide theoretical support for the safe exploitation of hydrates.

The failure strength of clayey silt hydrate-bearing sediments was obtained through experiments to determine the creep test conditions. Subsequently, the triaxial creep experiment under the stable presence of the hydrate was carried out to study the effects of clay mineral species and clay content on the creep characteristics of the sediments under different stress levels. The research results indicate that hydrate-bearing sediments containing montmorillonite exhibit greater initial and final deformation compared to those containing illite, and the creep rate of montmorillonite-bearing sediments is consistently higher than that of illite-bearing sediments. Clay content is negatively correlated with both sediment deformation and initial creep rate, and hydrate-bearing sediments with lower clay content require greater strain to achieve a constant creep rate. Stress level is positively correlated with sediment deformation and significantly influences the relationship between strain rate and strain. Montmorillonite-bearing hydrate sediments demonstrate stronger sensitivity to loading stress. Additionally, the long-term strength of hydrate-bearing sediments containing illite and montmorillonite at medium and low clay contents was determined.

Triaxial creep experiments were carried out to study the effects of clay mineral species, clay content and stress level on the creep characteristics of the silty silt sediment during hydrate decomposition. The results of the creep experiments during hydrate dissociation reveal that the creep strain and rate of montmorillonite-bearing sediments are generally higher than those of illite-bearing sediments, consistent with the deformation trends observed under stable hydrate conditions. For both illite- and montmorillonite-bearing hydrate sediments, the creep rate increases rapidly at the initial stage of the experiment and then decreases until it stabilizes in the later stage. As the illite content increases, the initial creep strain decreases, while the final creep strain increases. In montmorillonite-bearing sediments, both the initial and final creep strains increase with higher clay content. Additionally, an increase in clay content leads to a higher peak creep rate, particularly evident in montmorillonite-bearing sediments. With increasing stress levels, the initial and final strains of both types of sediments increase, with more pronounced changes in montmorillonite-bearing sediments. Under high stress levels, the peak creep rate increases, and a larger axial strain is required to achieve stability.