含水合物沉积物三轴剪切试验与损伤统计分析

天然气水合物开采诱发水合物分解,削弱水合物地层强度,可能导致地层滑动和生产平台倒塌等工程地质灾害,对水合物开采安全性构成严重威胁.深入理解含水合物沉积物力学性质并建立合理的本构关系模型是水合物开采安全性评价的前提条件.在自主研发的含水合物沉积物力学性质测试实验装置上,采用饱和水海砂沉积物气体扩散法制备了含水合物沉积物样品,并开展了系列的排水三轴剪切试验,通过时域反射技术实现了样品中水合物饱和度的实时在线测量;基于复合材料的罗伊斯(Reuss)应力串联模型和沃伊特(Voigt)应变并联模型提出了含水合物沉积物等效弹性模量的细观力学混合律模型,结合损伤统计理论和摩尔-库伦破坏准则改进了含水合物沉积物的本构关系模型.结果表明:随着水合物饱和度的增加和有效围压的减小,应力-应变曲线由应变硬化型变为应变软化型,割线模量和峰值强度均随水合物饱和度与有效围压的增加而提高,黏聚力受水合物饱和度影响明显,而内摩擦角基本不变;提出的等效弹性模量细观力学混合律模型与改进的本构关系模型均具有良好的适用性,模型参数少且物理意义明确. 

TRIAXIAL SHEAR TESTS AND STATISTICAL ANALYSES OF DAMAGE FOR METHANE HYDRATE-BEARING SEDIMENTS

Any perturbation to the thermodynamic equilibrium by exploitation may push out hydrate-bearing sediments (HBS) out of the stability zone, thus inducing hydrate dissociation, loss of cementation, which, in turn, can cause subma-rine landslides and loss of platform foundations during gas extraction operations. Therefore, a thorough understanding of mechanical properties of HBS is of great importance for stability analyses under different environmental conditions. A series of drained triaxial shear tests were carried out on a self-developed apparatus with the samples prepared by gas dif-fusion method, in which the time domain reflectometry technique was used in measurement of hydrate saturations in real time. A meso-mechanical and mixed model for the elastic modulus of HBS was proposed based on the classical series and parallel models, including the parameter of statistical force transfer paths between particles in HBS. A constitutive model of HBS was improved by coupling the statistical damage theory and the Mohr-Coulomb failure criterion. It is shown that the stress-strain curve changes from strain-hardening into strain-softening with the increase of hydrate saturation and the decrease of effective confining pressure;the secant modulus and the peak strength of HBS increase when the hydrate saturation and the effective confining pressure increase; the cohesion of HBS increases obviously with the increase of hydrate content, and the internal friction angle changed little with the increase of hydrate amount; the proposed mixed model for elastic modulus and the constitutive model of HBS are both reasonable and feasible.