Strength And Deformation Characteristics Of Hydrate-bearing Sediments

Gas hydrate is a crystalline compound formed from methane and water under low temperature and high pressure.Gas hydrate plays a load-bearing role in the reservoir,and the exploitation process will reduce the strength and change the stress state of MHBS,which will lead to seabed landslides,subsidence and other seabed geological disasters.Therefore,a comprehensive understanding of the strength characteristics of MHBS is critical to ensure the successful commercial exploitation of gas hydrates and to maintain the safety and stability of drilling platforms.In this paper,according to the sediment characteristics of Shenhu Sea area of South China Sea and Nankai Trough of Japan,fine grained soil and sandy soil were used as host sediments in the manual allocation test,and MHBS samples were prepared by gas-rich method and ice seed method.A series of isotropic compression tests and triaxial compression tests were carried out,and the following conclusions were obtained:(1)Fine-grained soil samples containing hydrate were prepared by gas-rich method,and a series of isotropic compression tests were conducted on them.The experimental results show that MHBS samples exhibit completely different isotropic compression curves under different hydrate saturation,temperature and pore pressure.With the increase of hydrate saturation and pore pressure and the decrease of temperature,the compressibility of MHBS decreases significantly.Hydrate saturation does not affect the compression index,but significantly reduces the rebound index and increases the yield stress.The compression index of MHBS is independent of temperature and pore pressure,but with the decrease of temperature and pore pressure,the rebound index weakens and the yield stress increases.Phase parameters are effective variables to characterize the conditions of temperature and pore pressure,and can be used to characterize the effects of temperature and pore pressure on the isotropic compression characteristics of MHBS.(2)Hydrate saturation,effective confining pressure and initial dry density can significantly affect the mechanical properties of MHBS.A series of consolidation and drainage triaxial compression tests were carried out on MHBS.The results show that the cementation effect of hydrate can enhance the strength and stiffness of the samples.With the increase of effective confining pressure,the strain hardening degree increases gradually,and the resistance to deformation also increases.The initial dry density has little effect on the strength of MHBS,but can significantly enhance the stiffness of MHBS.Duncan-Chang model can simulate the mechanical behavior of MHBS under different hydrate saturation,confining pressure and initial dry density.(3)Ice seed sampling method was used to simulate the occurrence model of seafloor hydrate pore filling,and undrained triaxial compression and heating induced dissociation tests were conducted on the sand samples of hydrate sediments.It is concluded that high hydrate saturation increases the strength and negative excess pore pressure.High initial effective confining pressure enhanced MHBS strength but inhibited negative excess pore pressure.The effective internal friction Angle and cohesion increase with the increase of hydrate saturation.In addition,unlike ordinary soil,the effective undrained strength index of MHBS is not the same as the drainage strength index.Hydrate dissociation results in positive excess pore pressure axial compression and volume expansion under undrained conditions.The larger deviator stress promotes the volume expansion of MHBS,but hinders the increase of excess pore pressure in the process of hydrate dissociation by thermal stimulation.