Study On The Statics And Dynamic Mechanical Properties Of Natural Gas Hydrate-bearing Sediments

Natural gas hydrate was widespread on the continental margins and permafrost in the world. Its reserves are huge and it has the potential to replace the traditional fossil energy. However, because gas hydrates are metastable, changes of pressure and temperature may easily affect their stability, which will result in a decrease in geomechanical strength and deformation of gas hydrate reservoirs, affect the safety and stability of engineering facilities, and even induce geologic disasters such as landslide. Therefore, the study on the mechanical properties of gas hydrate reservoirs has become one of the important research projects during the process of exploitation for natural gas hydrate.By using the static and dynamic triaxial experiment system for natural gas hydrate, this paper discusses the mechanics problems during the exploitation of gas hydrate. The experimental work for the remolded gas hydrate sediment cores has been carried out, the strength and deformation criterion has been established, and the constitutive equation has been set up, providing the theoretical references and technical methods to evaluate the stability of gas hydrate reservoirs. The main work is as follows:To meet the high confining pressure and low temperature formation conditions of gas hydrate reservoir, the dynamic triaxial test apparatus for natural gas hydrate DDW-600 has been developed. The present work is the first in China to study the mechanical properties of the remolded methane hydrate-bearing marine sediments. By comparing the mechanical properties of the methane hydrate-bearing kaolin sediments, it proves the feasibility to use kaolin clay instead of gas hydrate sediment core material in the South China Sea. Considering the effects of effective confining pressure, temperature and dissociation time on the mechanical properties of hydrate-bearing sediments during hydrate dissociation, the stress-strain relationship before and after hydrate dissociation has been studied, the mathematical expression of the M-C criterion related to the hydrate dissociation time has been proposed, and the Duncan-Chang constitutive model of considering the influence of temperature and hydrate dissociation time has been established.The creep tests during hydrate dissociation have been carried out, and the effects of confining pressure, temperature and stress level on the initial strain, the rheological initial time, the rheological initial strain, the failure time and the failure strain of methane hydrate-bearing sediments have been studied. Based on the experimental data, the strain model considering hydrate dissociation has been established, which can describe the creep deformation characters of methane hydrate-bearing sediments during hydrate dissociation, and provide the theoretical basis to simulate the interaction between hydrate dissociation, underwater structure and gas hydrate deposits.By simulating the seismic load, the dynamic stress-strain relationship and the effects of confining pressure, temperature and porosity on the dynamic strength have been first studied. Based on the Mohr-Coulomb strength criterion, the mechanism of the influence of temperature and porosity on the dynamic strength of methane hydrate-bearing sediments has been revealed. The results indicate that increasing temperature will reduce the dynamic strength by reducing the soil cohesion and internal friction angle, while increasing porosity will reduce the dynamic strength mainly by reducing the soil internal friction angle. In addition, the study on the dynamic elastic modulus of methane hydrate-bearing sediments shows that the maximum dynamic elastic modulus will increase with increasing confining pressure and reduce with increasing temperature or dynamic strain; the lower the porosity, the larger the maximum dynamic elastic modulus. By extracting the skeleton curves of methane hydrate-bearing sediments under different confining pressures, it has been found that the skeleton curve shapes are the same as the stress-strain curves shapes from static compressive tests.Based on the experimental data of the dynamic mechanical properties of natural gas hydrate-bearing sediments, a viscoelasticity constitutive model was proposed. Considering the dynamic mechanical properties of natural gas hydrate-bearing sediments and the effects of temperature and confining pressure on the modulus attenuation and damping ratio, the value of temperature and confining pressure was introduced to the model's parameters of E and A, and the traditional formulas of the dynamic elastic modulus and damping ratio was modified. By comparing with the experiment results, the reliability and applicability of this model was validated.