Triaxial Tests on Water-Saturated Gas Hydrate-Bearing Fine-Grained Samples of the South China Sea under Different Drainage Conditions

Abstract

More than 90% of hydrates worldwide are accumulated in fine-grained deposits, particularly in the South China Sea. Hydrate exploitation may lead to the geomechanical instability of the hydrate reservoir; thus, a sound understanding of the strength and deformation properties of the deposits is needed before hydrate exploitation. The in situ drainage conditions and fluid flow are very important factors that will influence these characteristics. To comprehend the strength and deformation of hydrate-bearing fine-grained deposits and assess the effects of drainage conditions on mechanical properties, consolidated drained (CD) and undrained (CU) triaxial tests were conducted on water-saturated gas hydrate-bearing fine-grained samples (HBFSs) of the South China Sea. The soil index characteristics were measured and showed that the mean particle size of clayey-silty samples was approximately 7 μm. The drained triaxial experiments demonstrated that the strength and deformation properties were dependent on the effective confining stress and hydrate saturation. The influence of hydrate saturation on the strength of the gas-saturated sample was much greater than that of the water-saturated sample. The undrained triaxial experiments for HBFSs demonstrated the development of positive excess pore water pressures. A comparison of the above results showed that undrained samples yielded more easily and were closer to the critical state than the drained samples and that the boundary surface of the HBFSs was almost not influenced by the drainage conditions. The triaxial testing results revealed that the drainage conditions and boundary condition played critical roles in determining the strength and deformation characteristics of the hydrate-bearing fine-grained deposits.