Research On The Mechanical Properties Of CH₄-CO₂ Hydrate Sediments

With huge reserves, widely distributed, clean combustion, and many other advantages, natural gas hydrate is recognized as one of the most potential alternative energy sources. But in the process of their exploitation, the phase equilibrium conditions of gas hydrate is destroyed, which lead to the shear stress reservoir damage, it will further affect the whole stability of the geological structure and exploiting engineering facilities, prone to underwater landslides, tsunamis and other natural disasters; The exploitation process may reveal a large amount of methane gas escape into the atmosphere, has potential influence to the climate warming. Therefore, before the exploitation of natural gas hydrate, need to give fully research and evaluation about the mechanical properties of hydrate deposits.On lots of exploitation methods of natural gas hydrate, as a new way of exploiting, the CH4-CO2 displacement method aroused people’s widespread attention, because this method not only can be stored in the bottom of the sea of CH4 gas replacement for human using, but also can seal the greenhouse effect of CO2 gas at the bottom of the sea, at the same time can still maintain the stability of hydrate sediments. In order to provide adequate theoretical basis of mechanical properties changes for CH4-CO2 displacement method in exploiting hydrate gas from sediments. This paper was carried out by the following experiments:Firstly, make use of self-developed low temperature and high pressure triaxial shear apparatus did a series of shear experiments of natural gas hydrate sediments which formed by CH4 hydrate and CO2 hydrate, analysis and compare mechanical properties with the changes of the content of two kinds of hydrates. Results show that under the same shear condition, with the increase of CO2 hydrate content in mixed hydrate sediment, sediment strength increase, the strength of the mixture of hydrate sediments always than pure CH4 hydrate sediment strength, and less than CO2 hydrate sediment strength, according to More coulomb criterion analysis, can be found that cohesion with little influence on the changes in concentrations of CO2 hydrate, internal friction angle along with the increase of CO2 hydrate content showed a trend of obvious increase.In addition, the mechanics properties of CH4-CO2 hydrate sediments along with the confining pressure, temperature, strain rate, and porosity are studied, the results show that when confining pressure is round 5MPa, the strength with the increase of confining pressure increase-reinforcing effect; when the further increase of confining pressure, strength present a downward trend- weakening effect; with the decrease of temperature, the strength of sediment increased, but the breakdown strength growth rate gradually decreased; With the increase of strain rate, the destruction of the sediment strength increased obviously; sediment strength are increased with the decrease of the porosity. According to the result of comparing three kinds of hydrate sediments strength can get a preliminary conclusion:use of CH4-CO2 displacement method in exploiting CH4 gas in the seabed sediment, after replacement, sediment can still maintain the stability of the original mechanical properties.Finally, based on two different particle size of quartz sand and kaolin hybrid generation of CH4 hydrate sediment mechanical property to do the research, the experimental results show that with the increase of grain size more coarse quartz sand content, the destruction of the methane hydrate sandy sediment strength.