Permeability properties of natural gas hydrate-bearing sediments considering dynamic stress coupling: A comprehensive experimental investigation

As a key exploitative parameter, permeability affects the extraction efficiency and program for natural gas hydrate (NGH). However, few studies have considered the effect of dynamic stress (continuously changing stress) on permeability, let alone permeability evolution during reservoir instability. Given the dynamic stress-seepage coupling process of actual NGH exploitation, this paper upgrades the triaxial apparatus to new equipment that can realize the unified tests of dynamic stress and water permeability. Subsequently, the seabed clay recovered from the South China Sea and quartz sand were used to remold fine-grained hydrate-bearing clayey-silty sediments (HBCSS). Finally, a series of stress-seepage coupling tests were carried out to reveal the permeability characteristics during sediment destabilization and to explain the coupling mechanism. The results show that seepage can reduce the strength and stiffness, and enhance the softening or weaken the hardening of the stress-strain curve. The dynamic stress-influenced permeability exhibits a non-monotonic evolution, which is essentially an expansion or contraction of seepage channels. Stress-strain relationship, volume deformation, and permeability are coupled and have good correspondence. The shear-seepage coupling process can be divided into four stages: initial compression, plastic strain development, plastic expansion and fracture development, plastic failure, and rapid expansion. This paper enriches the existing research from three aspects: permeability properties affected by dynamic stress, mechanical behavior affected by seepage, and stress-seepage coupling mechanism. It has played a significant reference for the safe and efficient exploitation of fine-grained NGH.