Research On The Mechanical Properties Of Methane Hydrate Interbedded With Clayey Sediments

Natural gas hydrate is regarded as the new energy to replace the traditional fossil energy in the future because of its advantages,such as high energy density,rich reserves,being clean and so on.Scientists from many countries do research on it.Along the edge of the continental shelf in the South China Sea and the East China Sea,there is a large amount of natural gas hydrate distribution.It is also found in the permafrost regions such as Qilian Mountain,Greater Khingan Mountains and the Qinghai-Tibet Plateau.Natural gas hydrate reaches the equilibrium state and is stable in the reservoir in nature.A small disturbance which destroys its equilibrium state can result in its dissociation which will lead to a series of geological disasters and environmental problems if the dissociation is not controlled.Therefore sufficient scientific research should be done before the safe and efficient exploitation of natural gas hydrate.There are various forms of its occurrence such as being lumpy,lamellar in sediment fractures,being deposited in the pores of fine silty sand and being interbedded with Clayey Sediments.In this paper,three series of tri-axial experiments was conducted on methane hydrate interbedded with clayey sediments,using a high-pressure and low-temperature tri-axial testing apparatus,in order to provide theoretical reference for future mining.Firstly,the effects of different confining pressures and different tilted angles of layers on the mechanical properties of methane hydrate interbedded with clayey sediments were studied.Experimental results show that the tri-axial shear stress-strain curves of different confining pressures and different tilted angles are roughly the same,which can be divided into four stages: approximate elastic deformation,strain hardening,strain softening and secondary hardening.The deviator stress of samples increases first and then decreases and increases again with the increasing of axial strain.At about 3% of the strain,the peak of deviator stress occurs,and the valley occurs at about 7% strain.The failure strength of the sample increases with the increasing of confining pressure,when the confining pressure is under 5 MPa,while the confining pressure is greater than 5 MPa,the failure strength decreases with confining pressure increasing.We have calculated the value of the internal friction angle and cohesion of sediments according to Mohr-Coulomb Theory and found that all the internal friction angles are about 10°.The failure strength and cohesion of samples are the smallest when the tilted angle of layers is 10°.It is inferred that the sample is the easiest to be destroyed under the tri-axial test when the tilted angle of layers is about 10°.Secondly,the effects of the thickness of the hydrate layer on the mechanical properties of methane hydrate interbedded with clayey sediments were studied.We have got the fitting formula of the strength and the thickness of hydrate layer under different confining pressure,and calculated the value of the internal friction angle and cohesion of sediments.The mathematical expression between the cohesion and the thickness of the hydrate layer is given.The results show that the failure strength decreases when the thickness of the hydrate layer increases.The effects of the order of layers on the mechanical properties of methane hydrate interbedded with clayey sediments were studied at last.Based on the analysis and experimental results,we can find that the failure strength of samples is the biggest when the hydrate layer is in the middle of samples.