Study On Mechanical Properties Of Methane Hydrate And Sediments

Natural gas hydrate has drawn much attention by many researchers, which has been considered as one of the most promising new energy to alleviate the energy crisis. However, the dissociation of methane hydrate during drilling for exploration and production may result in seabed subsidence and deformation of natural gas hydrate sediment strata. Thus it is essential to determine mechanical properties and analyze exploitation safety and slope stability of hydrate reservoir before attempting to exploit these deposits.In this paper, some researchs are concluded as follows:A triaxialtest system containing a pressure crystal device was developed with the experimental conditions to stabilize the hydrate. Moreover, the pressure crystal device was developed to make samples of hydrate and sediments in site conditions.The triaxial shear tests were performed to study the mechanical properties of methane hydrate containing ice. The effect of confining pressure, temperature and strain rate on triaxial strength, deformation modulus and strength parameters are analyzed. The results show that the triaxial strength of methane hydrate containing ice increases with the increase of confining pressure and strain rate. While it increases with the decrease of temperature. Based on the analysis of Mohr-Coulomb criterion, it shows that the variation of triaxial strength of methane hydrate with temperature depends on the variation of cohesion, which increases with the decrease of temperature.According to the result of triaxial compressive tests of methane hydrate samples containing ice, the relationship of the initial yield strain, initial yield strength, initial yield modulus, initial deformation modulus, ultimate deviator stress and damage ratio with confining pressure and temperature are studied. The results show that the initial yield strain, initial yield strength, initial deformation modulus and ultimate deviator stress increase with the increase of confining pressure and decrease of temperature. The initial yield modulus is only affected by temperature. Based on nonlinear elastic Duncan-Chang constitutive model, the constitutive relationship added in damage ratio and temperature was built for methane hydrate containing ice.A series of triaxial constant strain rate tests was performed to study the strength of artificial methane hydrate-bearing clay under various conditions. The variation of deviator stress with axial strain was observed and the strain type was analyzed systematically under different experimental conditions. The results showed that the shear strength ofφ60 (Porosity is 60%) methane hydrate-bearing clay sediments increased in conditions of the enhancement of confining pressure, the enhancement of strain rate and the decrease of temperature. According to Mohr-Coulomb’s criterion, the shear strength of methane hydrate-bearing clay sediments was analyzed. It can be found that both of cohesion and friction angle increased with the decrease of temperature and increase of strain rate.The kaolinite clay and unsaturated methane hydrate was mixed in proportion according to the experimental requirements. The effect of porosity on shear strength and modulus was studied. The stress strain relationship was analyzed under the conditions of different porosity and the expression and constitutive relationship were built. The results showed that the strength increased with decreasing porosity. The porosity has a great influence on the cohesion but not friction angle. The variations of failure strength with porosity mainly were caused by the variations of cohesion.Based on the experimental and analysis results of stress-strain behavior of triaxial tests, this paper proposes the modified nonlinear elastic Duncan-Chang constitutive model, whose coefficients contain the main factors affecting stress-strain behavior.