Study On Deformation And Strength Characteristics Of Gas Hydrate Sediments

Natural gas hydrate is a clathrate crystal compound formed by natural gas and water under high pressure and low temperature.It is widely distributed in deep-sea sediments and permafrost in the land.Methane is the main component of the gas.The natural gas hydrate is burned to form carbon dioxide and water,and it is clean and pollution-free.Therefore,it is considered as a potential ideal new energy in the future,which has the characteristics of wide distribution,shallow burial,huge resource and high energy density.As a strategic energy source with great exploitation value,the exploitation and utilization of hydrates has attracted much attention all over the world,However,the natural gas hydrate on the seabed exists as a solid state,which can cement the sediment particles together,and make the original loose structure of the seabed compact.The decomposition of hydrate in the mining process will lead to submarine landslides,greenhouse effect and marine ecological environment destruction.Therefore,understanding the deformation and strength of gas hydrate deposits is one of the key research contents for exploitation and utilization.In this paper,the samples of gas hydrate sediments were prepared in the laboratory.The hydrate occurrence media refered to the size of the undisturbed samples of hydrate sediments in the sea area of South China Sea trial mining,According to the size of the undisturbed sample,the approximate grain size is used as the occurrence skeleton of hydrate,and then the experiments of istropical compression,unloading springback and triaxial shear are carried out.The main results are as follows:?1?A new type of triaxial testing machine for hydrated sediments containing natural gas is installed and debugged.The system is composed of gas supply system,constant temperature water bath system,pressure loading control system,triaxial shear pressure chamber and data processing system.Pressure tracking pump and axial pump in pressure loading control system provide high pressure liquid to apply confining pressure and axial pressure.The loading mode includes constant pressure,constant current,gradient pressure and tracking pressure mode.The equipment can realize the experiments of hydrate synthesis,isometric compression,unloading springback,triaxial shear and other tests under different conditions,The function of ambient pressure tracking pressure of loop pressure tracking pump keeps the net confining pressure constant during hydrate formation,which ensures the consistency of initial conditions of sample.The volume deformation of sample can also be measured according to the accumulated flow of confining fluid.Finally,triaxial samples were made from clayey sand and carbon dioxide,and hydrate was formed by unsaturated method in laboratory.?2?The saturation of hydrate affects the time of iso-compression consolidation and unloading springback of sediment,and the larger the saturation of hydrate,the shorter the time needed for stability of the test is.Short time-consuming of hydrates-formation indicates that the pore space of sediments is larger and the density of sediments is higher.When the density is high,the volume deformation is small in the compression and springback stages.Hydrate saturation affects the compressibility of sediment.The lower the saturation,the higher the compressibility of sediment sample and the higher the compression index.The saturation of hydrate has little effect on the unloading springback characteristics of sediment,and the springback coefficients of each saturation sample tend to be similar.However,the rebound rate of the samples with high saturation is slightly lower than that of the samples with no hydrate or low saturation.?3?In triaxial shear test,under the same net confining pressure,With the increase of hydrate saturation,the strength and stiffness of CO₂ hydrate deposits increased and dilatancy occurred clayer,the cohesive force of samples also increased,but the internal friction angle was not affected.Under the same saturation,with the increase of confining pressure,the strength of the specimen increases,the strain hardening trend is obvious,and the dilatancy trend decreases.