Abstract:

Hydrate distribution concentration and occurrence modes in porous media have been investigated widely and concerned constantly by hydrate academic community. It was acknowledged that there was not a consistent argument about these topics notwithstanding heaps of scholars had conducted numerous works using diverse precise favorably instruments and respective methods. Unlike visual tools like X-CT (X-ray computed tomography), MRI (magnetic resonance imaging), SEM (scanning electron microscopy) is a splendid way to characterize directly surface morphology information of samples due to high resolution and depth of field, etc., which caters completely to hydrate crystalline and porous media size limitation. In this article, we use cryo-cold field SEM (cryo-CFESEM) to capture the texture of ice and tetrahydrofuran (THF) hydrate, glass beads, and observe the distribution modes of ice and hydrate in porous media. Ice shows mainly hexagonal and spherical shape, which the latter is more than the former in quantity. Moreover, there are a small number of atypical polycrystalline attributed to sintering and/or Ostwald ripening. THF hydrates are block-like, irregular crystals adhering to a few ice crystals on their surface and differ notably in size of the two. This is because THF hydrates forestall the mass transfer of ice-water-vapor system so that it is difficult to form polycrystalline structure of ice without reaching the condition for sintering and/or Ostwald ripening. Additionally, both of them have different growth habits-ice obeys the “pore-filling” and “coating” pattern but packs together tightly and disseminates discretely while THF hydrate size is larger than ice, sprawling densely as well as exhibiting “patchy” mode through secondary electron imaging and energy disperse spectroscopy (EDS) qualitative analysis. Moreover, the operation condition of EDS characterizing hydrate samples is determined, which provides a powerful guide for the application of EDS.

Key words: tetrahydrofuran hydrate, ice, glass beads, occurrence behavior, cryo-CFESEM

摘要：

水合物在多孔介质的分布含量和赋存模式是当前水合物界研究的热点话题之一。许多学者利用不同的高精密仪器和独立的测量方法做了大量的工作，但仍然没有得到一致的结论。与X射线计算机显微断层技术（X-CT）、核磁共振成像（MRI）等可视化仪器不同，扫描电镜（SEM）具有高分辨率、高景深等特性，是一种可直观地表征样品表面形貌信息的优良工具，而一般的水合物晶体和多孔介质粒径完全在扫描电镜的可视化精度范围。本文利用低温冷场扫描电镜对冰和四氢呋喃（THF）水合物及其玻璃珠进行成像，冰多为六角形和圆球形，且在数量上圆球形冰晶多于六角形冰，还存在少数的由于烧结或者奥斯特瓦尔德熟化产生的多晶不规则结构。而THF水合物为块状、不规则的晶体，其表面上存在少数的冰，两者尺寸相差较大。这是由于THF水合物阻断了冰-水-蒸汽系统的传质，使得烧结/奥斯特瓦尔德熟化的条件很难实现，也因而很难看到冰的多晶结构。此外，观察到冰、水合物在多孔介质内的分布模式也不相同，冰为明显的“孔隙填充”型和“包裹”型，且分布较分散，但THF水合物通过二次电子成像观察和能谱仪（EDS）定性表征，分布密度比较集中，接近“斑块”型。给出了利用EDS表征水合物样品的条件，为EDS在水合物方面的应用提供了有力的指导。

关键词: 四氢呋喃水合物, 冰, 玻璃珠, 赋存行为, 低温冷场扫描电镜