Physical and mechanical properties of the overburden layer on gas hydrate-bearing sediments of the South China sea

Gas hydrate exploitation may trigger the destruction of the overburden structure and settlement of the overburden layer on the gas hydrate-bearing sediments. It is extremely urgent to explore the basic laws governing the physical and mechanical properties of the overburden layers before performing commercial exploitations of hydrates. However, relatively few studies have focused on the physical and mechanical properties of the overburden layer (uppermost ~10 m) on the gas hydrate-bearing sediments of the South China Sea. In this research, we studied the physical and mechanical properties of the overburden sediments from this area, including the particle size distribution, the specific surface area, the pore size distribution, the water content, the liquid/plastic limits, and the mineral and chemical compositions of the particles, as well as the strength and deformation characteristics. According to the results, we found that the overburden sediments of the Shenhu area are characterized as well-graded clayey silts and considered to be internally stable and self-filtering. The isothermal adsorption-desorption curve is a Type H3 hysteretic loop, indicating that the shape of the particles occurs primarily in a plate form, and most of the pores are slit-shaped. The plasticity index in this area is relatively lower than that in other areas, which suggests that there is a risk of overburden collapse during the production process of hydrates. The mineral composition of the overburden sediment particles is primarily quartz, plagioclase, calcite and some accessory clay minerals, which can provide guidance for remolding the overburden sediments in the laboratory. The sediments show strain hardening and shear contraction behavior upon shearing; in addition, the established strength criterion will contribute to the formulation of constitutive equations for the overburden sediments, which can be used as essential input parameters in gas hydrate production simulations.