Abstract: This paper aims to investigate the mechanical behaviors of natural gas hydrate-bearing silty sediments, which provides an important basis for the production of abundant natural gas hydrate resources deposited in silty reservoirs of the South China Sea. Thus, silty sediments were artificially prepared according to the particle size distribution curve of hydrate cores from the South China Sea; the natural gas hydrate-bearing saturated silty sediments were simulated based on the "hydrate formation water saturation isotropic consolidation" method, followed by a series of triaxial drained shear tests under low effective confining pressure. The study reveals that:(1) at a low effective confining pressure, the failure strength of silty sediments increases almost linearly with the increasing hydrate saturation, while for the sediments containing hydrates of the same saturation, the failure strength under water saturation is significantly lower than that under gas saturation; (2) the natural gas hydrate with a saturation of 40 % contributes to an increase by about 4 times in the cohesion of silty sediments and by about 5.2° in the internal friction angle; (3) as the effective confining pressure increases, the volumetric strain of silty sediments containing hydrates increases at first and then decreases, whereas that of pure silty sediments decreases continuously; the volumetric strain of all silty sediments decreases with increasing hydrate saturation, and the difference in volumetric strain gradually decreases; (4) the hydrate cementation can reduce the initial consolidation degree of silty sediments, while the shearing deformation behaviors of sediments are mainly influenced by the combination of initial consolidation degree, hydrate cementation and effective stress. The conclusion indicates that the cohesion reduction, re-consolidation and shear deformation as result of the decreased hydrate saturation may be the key reason for the deformation of silty reservoirs induced by natural gas hydrates production in the South China Sea.

Key words: natural gas hydrate, silty sediments, water-saturated state, triaxial shear test, strength, deformation behaviors, cohesion

摘要： 为开发中国南海粉质储层中丰富的天然气水合物能源,对水饱和状态下含天然气水合物粉质沉积物进行力学特性研究。根据南海水合物岩心粒径曲线人工配制了粉质沉积物,并基于“先生成—再饱和—后固结”方法制备了水饱和状态下含天然气水合物粉质沉积物,并在较低有效围压条件下开展了一系列三轴排水剪切试验。研究结果表明:①较低有效围压下粉质沉积物破坏强度随水合物饱和度增加呈现近似线性增长,水饱和状态下含相同饱和度水合物沉积物的破坏强度明显低于气饱和状态下其强度;②饱和度为40 % 的天然气水合物使粉质沉积物黏聚力增长了约4倍,内摩擦角则增长了约5.2°;③当有效围压增大时,纯粉质沉积物体应变持续减小,含水合物粉质沉积物体应变则先增后减;粉质沉积物体应变均随水合物饱和度增大而减小,而其体应变差值则逐渐缩小;④水合物胶结作用会降低沉积物初始固结程度,而沉积物剪切变形特性主要受到初始固结程度、水合物胶结作用和有效应力因素综合影响。因此,水合物饱和度降低导致的黏聚力降低、二次固结和剪切变形可能是中国南海天然气水合物开采诱发粉质储层变形的重要原因。

关键词: 天然气水合物, 粉质沉积物, 水饱和状态, 三轴剪切试验, 强度, 变形特性, 黏聚力