Abstract: Drilling and production would exert significant impacts on the geological environment of the gas hydrate-bearing sediments (GHBS), which may cause deformation and failure of GHBS, followed by a series of geotechnical engineering problems. Hence, an underlying understanding of mechanical properties and failure mechanism of GHBS is a prerequisite for safe production from hydrate reservoirs. This paper presents a numerical investigation into mechanical behavior and strain localization in GHBS using distinct element method (DEM). A series of biaxial compression tests on GHBS samples with different hydrate saturations were conducted, whose results showed a satisfactory agreement with experimental results. Besides, strain localization, porosity, particle rotation and bond breakage number were examined to investigate the relationships between micro variables and the evolution of shear band. The research results show that (1) DEM can effectively capture the main features of strength and deformation of GHBS; (2) hydrate saturation affects the pattern of shear band, with two shear bands developed in GHBS specimens with hydrate saturations of 26% and 40%; and with one shear band formed with hydrate saturation of 55%; (3) shear bands start during strain hardening stage, and then fully developed into shear bands in strain softening stage, which is a progressive development of deformation localization; (4) strain localization, porosity, particle rotation field show obvious inhomogeneity within and outside the shear bands; and (5) hydrate cementation has a dual effect on the initiation and development of the shear bands. Microcracks are first generated between hydrate particles due to bond breaks, and crack propagation is hindered by the bonds at sand-hydrate contacts and then motivated by the bond breaks at sand-hydrate contacts. The research results are useful for understanding the mesoscopic mechanism in deformation and failure process of GHBS.

Key words: natural gas hydrate-bearing sediments, discrete element method, triaxial test, shear bands, mesoscopic mechanism

摘要： 钻采工程会影响天然气水合物沉积层的地质环境，这可能导致水合物沉积层的变形和破坏，继而可能引发一系列工程问题。因此，要实现天然气水合物资源的安全有效开发，需要对天然气水合物沉积物的变形破坏特性开展研究。考虑水合物的胶结效应，采用胶结接触模型，用离散单元法模拟了胶结型天然气水合物沉积物试样的三轴试验，通过局部变形、孔隙率、颗粒的旋转及胶结破坏的特征研究了剪切带的萌生发展规律及其细观机理。研究结果表明：①离散元数值试验能有效模拟水合物沉积物的强度和变形特性；②不同水合物饱和度试样的剪切带的模式不尽相同，当水合物饱和度为26%和40%时，趋向于形成两条分叉结构的剪切带，而当水合物饱和度为55%时，趋向于形成单一斜向剪切带；③剪切带萌生于应变硬化阶段，完全形成于应变软化阶段，是局部变形渐进发展的结果；④剪切带内外的局部应变、孔隙率、颗粒的旋转等特征差异明显，具有强烈的局部化特征；⑤水合物胶结对剪切带的萌生和发展具有双重作用，微裂纹首先因水合物颗粒间的胶结破坏而产生，砂土和水合物之间的胶结则限制了裂纹的扩展，而砂土和水合物之间的胶结破坏又促进了裂纹扩展形成剪切带。研究结果对于理解天然气水合物沉积物变形破坏过程中的细观力学机制具有参考价值。

关键词: 天然气水合物沉积物, 离散元法, 三轴试验, 剪切带, 细观机制