甲烷水合物三维离散元模拟参数反演初探

Preliminary investigation on parameter inversion for three-dimensional distinct element modeling of methane hydrate

含填充型水合物的砂性能源土可视为特殊的散粒体材料(砂粒和水合物颗粒混合物),具有明显的非连续特征。在离散元中若采用团粒(胶结成团的颗粒组)模拟填充水合物颗粒则需合理确定团粒结构内颗粒间胶结模型参数。为此,基于前人的室内纯水合物三轴试验资料进行离散元建模与参数反演。结果表明,宜采用松散且颗粒间摩擦系数较小的试样模拟水合物块体,当颗粒间摩擦系数小于等于0.04时,可确保无胶结试样的内摩擦角小于室内试验获得的纯水合物内摩擦角。胶结刚度只需在较小范围变化即可反映相同温度不同围压条件下的弹性特性,且微观刚度参数与胶结强度参数的相互作用较小,可以假定二者相互独立。通过选取不同的微观胶结强度值进行不同围压下的三轴压缩试验,建立微观胶结强度参数与宏观参数(内摩擦角和黏聚力)之间的关系,从而确定与室内试验强度特性相符合的微观胶结强度值,实现甲烷水合物三轴试验离散元模拟;由体变规律可知,甲烷水合物在发生剪胀前均存在一个初始的体积收缩阶段,且剪胀特性随着围压的减小而呈现增强趋势。通过微观变量颗粒接触方向组构的分布图可知,随着轴向应变增大,颗粒间接触主方向朝竖直方向偏转,表现出明显的各向异性特性。随着轴向应变的增大,颗粒间胶结残余率变小,表明试样逐步破坏。

Marine sandy sediments containing pore-filling type methane hydrate particles can be considered as a class of special granular materials which present apparent discontinuity characteristics.To numerically simulate such materials,the distinct element method(DEM) can be used by modeling methane hydrate particles as groups of spheres cemented together and filled into the pores of soil skeleton.The model parameters for inter-particle bonds within an individual hydrate particle are investigated through parameter inversion against the existing laboratory triaxial compression(TC) test results of methane hydrate blocks under various confining pressures.The results indicate that a loose packing with low inter-particle friction needs to be used for the simulated methane hydrate block.When the inter-particle friction coefficient is equal to or less than 0.04,the friction angle obtained from the unbounded sample is less than that of the experimental tests.The bond stiffness varying in a very small range can adequately capture the elastic behavior of methane hydrate under different confining pressures at the same temperature.Because the interaction between stiffness parameters and bond strength parameters is small,it is assumed that the two types of parameters should be independent.The relationships between micro bond strength parameters and macro parameters(internal friction angle and cohesion) are established by conducting TC tests on choosing different micro bond strength parameters.The methane hydrate shows volume contraction,then dilatancy.And the characteristic dilatancy increases with the decrease of the confining pressure.With the increase of the axial strain,the grain contact direction deflects to the vertical direction and shows obvious anisotropic characteristics.The percentage of the intact bonds decreases and the sample is gradually destroyed with the increase of the axial strain.