Study On Strength And Deformation Behaviors Of Methane Hydrate-Bearing Sediments

Methane hydrate has attracted global attention due to its widespread occurrences and large potential as an energy resource. During the production of methane from hydrates, hydrate dissociation may induce a variety of geological disasters, such as subsea landslides, borehole collapse, destruction of engineering structures and methane gas leakage. In this paper, according to the related mechanical problems during production, the constitutive relationship of methane hydrate-bearing sediments was studied, based on a series of experimental research, in order to provide a theoretical basis and technical support for safe extraction of methane hydrates.A series of triaxial compression tests were conducted to investigate the effects of temperature, confining pressure and hydrate content on the mechanical properties of permafrost-associated methane hydrate-bearing sediments (prepared by mixed volume sample preparation method), using a specifically designed low-temperature high-pressure triaxial testing device (TAW-60). This paper originally discovered that the failure strength of methane hydrate-bearing sediments and methane hydrate containing ice presented a slow descending trend with increase of confining pressure under high confining pressures, obtained the shear strength, cohesion and internal friction angle of methane hydrate-bearing sediments, proposed a piecewise linear strength criterion and a nonlinear strength criterion under high confining pressures, verified the applicability of modified Duncan-Chang model for permafrost-associated methane hydrate-bearing sediments under high confining pressures.The methane hydrate-bearing specimens were also prepared by in situ forming method, using a temperature-controlled high pressure triaxial apparatus of Yamaguchi University. The effects of thermal recovery method and depressurization method on the mechanical properties of methane hydrate-bearing sediments were studied. The results indicate that:the thermal recovery method will cause the failure of methane hydrate-bearing sediments when the axial load is higher than the strength of methane hydrate-bearing sediments after dissociation; the depressurization method will not cause collapse of methane hydrate-bearing sediments during depressurization. However, water pressure recovery may lead to failure under certain conditions. Also, the effects of temperature, pore pressure, effective confining pressure and hydrate saturation on the mechanical properties of gas-saturated methane hydrate and CO2hydrate-bearing sediments were studied, and a comparison was made with water-saturated methane hydrate-bearing sediments. The results indicate that:(1)failure strength, and stiffness of gas-saturated specimens are higher than those of water-saturated specimens, and the gas-saturated specimens present more markedly shear dilation and strain-softening behavior;(2)it is feasible to use CH4-CO2replacement technology for the production of methane from hydrate reservoirs in mechanical considerations.Based on the modified Cam-Clay model, subloading surface theory and obtained experimental data, a new elastoplastic constitutive model for methane hydrate-bearing sediments was proposed, considering the effects of methane hydrate on the cohesion and shear dilation.