Research On The Mechanical Characteristics Of Gas Hydrate-bearing Sediments

The natural gas hydrate has become a new energy receiving worldwide extensive attention for the advantages of large reserves, clean etc. There are a lot of permafrost hydrate in Qinghai-Tibet Plateau, the Qilian Mountain tundra, the Greater Xing’an Mountains and many other areas of China. The replacement of CH4 from methane hydrate by using CO2 is regard as a promising method for both natural gas extraction and CO2 sequestration, and it is feasible in permafrost. However, it is noteworthy that the exploration of natural gas hydrate may result in the damage of the stability of the reservoir, which will cause geology hazard and climate hazard, such as seabed landslides, greenhouse effect. Thus, it is essential to determine the mechanical properties of hydrate-bearing sediments and the effect on the mechanical properties of mineral deposits hydrate production before commercial exploration of these deposits. This paper will study the mechanical properties of hydrate-bearing sediments during and after the exploration, focusing on CH4- CO2 replacement.In this paper, we study the effects of CO2 Hydrate volume ratio and confining pressure on CH4 and CO2 mixture hydrate-bearing sediments, using a high-pressure and low-temperature triaxial testing apparatus; establish the relationship between CO2 hydrate volume ratio α, confining pressure σ3 and the failure strength of hydrate-bearing sediments σm; also establish the relationship between CO2 hydrate volume ratio a and cohesion c. The result shows, the destruction process of specimen can be divided into three parts, elastic deformation, strain hardening, and yield. The maximum deviator stress of specimen increases linearly with CO2 hydrate volume ratio under different confining pressure. For given proportion of CO2 hydrate and temperature, the strength of mixture hydrate-bearing sediments increased as confining pressure increased when which was less than 5MPa, while it was just the opposite when confining pressure was greater than 5MPa. The cohesion of specimen increases linearly with CO2 Hydrate volume ratio, while the friction angle changes little and its value is very small. Cohesion plays a leading role on the shear strength of the specimen.Besides, we study the effects of confining pressure on creep properties of CO2 hydrate-bearing sediments; establish the calculation formula of the rheological initial time and the creep speed which is associated with the experimental parameters such as temperature, porosity, creep loading coefficient and so on. The result shows, creep curves of specimen appear logarithmic form, and shows significant attenuation creep characteristics. The rheological initial time, rheological initial strain and steady-state creep rate decrease exponentially with the increase of confining pressure. The increase of confining pressure can reinforce the resistance of creep, which is particularly evident at low confining pressure and feeble at high confining pressure.At last we study the change of sub-microstructure of CH4 hydrate-bearing sediments in triaxial creep process using an inspeXio SMX-225CT Microfocus X-Ray CT System made by SHIMADZU. The result shows, the specimen compress elastically before 5% of strain; the morphology and distribution of hydrate changes little; the cemented structure is integrity; the height of specimen recover after the remove of axial load.5%-15% of strain is plastic deformation, and the center of the specimen bulge outward; there exist the migration of particles and deformation of broken hydrate. The internal density of the sample is bigger than external density at 15% of strain; the specimen shows expansion trend with the compression; the internal porosity of specimen grow poorly and there is not obvious fault zone.