It is essential to understand the in-situ geomechanical properties of natural gas hydrate-bearing reservoirs for hydrate drilling, the design of reservoir stimulation and production schemes, geologic disaster prevention, and environmental impact assessment. In this way, it serves as the basis for the development and utilization of gas hydrate resources. In 2016, the Guangzhou Marine Geological Survey conducted the fourth natural gas hydrate drilling (Expedition GMGS4) in the north of the South China Sea, during which the effective stress of gas hydrate-bearing reservoirs in the study area was ascertained and gas hydrate-bearing pressure cores were collected. The effective stress was determined to be 700–800 kPa by in-situ piezocone penetration tests. The saturation, permeability, and shear strength of gas hydrates in the pressure core samples were measured under the in-situ stress state using the shipboard multi-sensor core logger (MSCL), pressure core analysis and transfer system (PCATS), and PCATS Triaxial system. Based on this, the distribution characteristics and the relationship of the permeability and gas hydrate saturation of the gas hydrate-bearing reservoirs were explored. Furthermore, the effects of effective stress and gas hydrate saturation on shear strength of gas hydrate-bearing clayey-silt sediments were mainly analyzed, as well as the stress–strain characteristics of gas hydrate-bearing sediments. The results are as follows: (i) the gas hydrate saturation and reservoir permeability in the study area are 0.7%–55% and 10–10mD, respectively, indicating low-permeability reservoirs; (ii) the gas hydrate saturation and reservoir permeability have obvious anisotropy in horizontal and vertical directions; (iii) the undrained shear strength of gas hydrate-bearing sediments is 0.24–0.52 MPa, and the shear strength of fine-grained sediments is more sensitive to the changes of in-situ stress. In the case that deviatoric stress continues increasing, micro-fractures may occur inside gas hydrate-bearing sediments, and this can lead to drastic changes in the strength of gas hydrate-bearing reservoirs. Therefore, it is necessary to take into account the effects of micro-fractures in gas hydrate-bearing reservoirs on engineering safety during long-term hydrate production.

中文翻译：

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南海神狐海域天然气水合物沉积物地质力学特征

了解天然气水合物储层的原位地质力学特征对于水合物钻井、储层增产开发方案设计、地质灾害防治、环境影响评价等具有重要意义。从而为天然气水合物资源的开发利用奠定基础。2016年，广州海洋地质调查局在南海北部进行了第四次天然气水合物钻探（GMGS4），查明了研究区含天然气水合物储层的有效应力，并收集压力芯。通过原位压电锥渗透试验确定有效应力为 700-800 kPa。利用船载多传感器岩心记录仪（MSCL）、压力岩心分析和传输系统（PCATS）以及PCATS三轴测量仪，在地应力状态下测量压力岩心样品中天然气水合物的饱和度、渗透率和剪切强度。系统。在此基础上，探讨了含天然气水合物储层渗透率与水合物饱和度的分布特征及关系。此外，重点分析了有效应力和天然气水合物饱和度对含天然气水合物粘土粉砂沉积物抗剪强度的影响，以及含天然气水合物沉积物的应力应变特征。结果表明：（i）研究区天然气水合物饱和度为0.7%~55%，储层渗透率为10~10mD，属于低渗透储层；(ii) 天然气水合物饱和度和储层渗透率在水平和垂直方向上具有明显的各向异性；(iii) 含天然气水合物沉积物的不排水剪切强度为0.24~0.52 MPa，细粒沉积物的剪切强度对地应力的变化更为敏感。在偏应力继续增大的情况下，含天然气水合物沉积物内部可能会出现微裂缝，导致含天然气水合物储层强度发生剧烈变化。因此，在水合物长期开采过程中，需要考虑天然气水合物储层微裂缝对工程安全的影响。