Study On The Mechanical Properties Of Methane And Carbon Dioxide Hydrate-bearing Sediments

Owing to their enormous worldwide reserves, natural gas hydrate has been considered to be a new energy source with the biggest potential to alleviate the global energy crisis, however, the exploration of hydrate could cause submarine geologic disasters, aggravate global greenhouse effect and destroy marine ecological balance. Therefore, in this study, a low-temperature and high-pressure triaxial apparatus, which was developed by Dalian University of Technology, was used to research the effect of hydrate decomposition on triaxial compression strength and creep compression strength of CH4hydrate-bearing sediments, to provide some significant deformation data for safe exploration of natural gas hydrate. The experimental results indicated that the effect of hydrate decomposition on triaxial compression strength and creep compression strength of CH4hydrate-bearing sediments depended largely on decomposition temperature. The influence of hydrate decomposition on triaxial compression strength and creep deformation of CH4hydrate-bearing sediments were not very obvious, when the decomposition temperature was less than and not very close to the melting point of ice; When the decomposition temperature was close to the melting point of ice, hydrate decomposition greatly reduced triaxial compression strength and increased creep deformation. However, the influence of hydrate decomposition on creep rate was not very obvious under a small constant load, creep rate of sediments were eventually tend to zero.In addition, because it could sequestrate greenhouse gas CO2at the bottom of the sea accompany with the production of natural gas hydrate, and was expected to maintain the stability of gas hydrate-bearing sediments, CH4-CO2replacement method has therefore given rise to a heated discussion. Therefore, in this study, the triaxial compression tests of CO2hydrate-bearing sediments were also conducted, then compared the experimental results with the results of triaxial compression tests of CH4hydrate-bearing sediments, which were conducted in our previous work, to provide a preliminary evaluation of mechanical safety for CH4-CO2replacement method, and laid a solid foundation for later work. The experimental results indicated that both the strength of CH4and CO2hydrate-bearing sediments depended largely on temperature, confining pressure, strain rate and porosity, and the strength varied with all of the influence factors in a nearly identical fashion. The strength increased while the strength increment ratio decreased as the temperature decreased; when the confining pressure was less than5MPa, the strength increased near-linearly as the confining pressure increased, however, the strength displayed a downward trend as the confining pressure further increased; the strength clearly increased with increasing strain rate; the strength increased with decreasing porosity, and the relationship between the strength and porosity is near-linear. However, the strength of CO2hydrate-bearing sediments was appreciably larger than the strength of the CH4hydrate-bearing sediments under the same conditions, the differences in the hydrate physical properties, which were caused by the differences of forming conditions or guest molecule characteristics, may cause the strength difference between CH4and CO2hydrate-bearing sediments:according to the comparison results, the preliminary conclusion could be draw that the mechanical stability of natural gas hydrate-bearing sediments could be maintained by using the CH4-CO2replacement method to recover methane from the sediments after recovering the methane gas.