The hydrate reservoir in the Shenhu Area of the South China Sea is a typical clayey-silt porous media with high clay mineral content and poor cementation, in which gas hydrate formation and dissociation characteristics are unclear. In this study, the CO2 hydrate saturation, growth rate, and permeability were studied in sandstone, artificial samples, and clayey-silt sediments using a custom-built measurement apparatus based on the low-field NMR technique. Results show that the T2 spectra amplitudes decrease with the hydrate formation and increase with the dissociation process. For the artificial samples and Shenhu sediments, the CO2 hydrate occupies larger pores first and the homogeneity of the sandstone pores becomes poor. Meanwhile, compared with the clayey-silt sediments, CO2 hydrate is easier to form and with higher hydrate saturation for the sandstone and artificial samples. In hydrate dissociation process, there exists a protection mechanism, i.e. the dissociation near the center of hydrates grain is suppressed when gas pressure drops suddenly and quickly. For permeability of those samples, it decreased with hydrate forms, and increases with hydrate dissociation. Meanwhile, with the same hydrate saturation, permeability is higher in hydrate formation than in dissociation.

中文翻译：

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黏土-粉砂沉积物中天然气水合物的原位形成和分解：低场核磁共振研究

南海神狐海域水合物储层为典型的黏土-粉砂多孔介质，黏土矿物含量高，胶结性差，天然气水合物的形成和解离特征尚不清楚。在这项研究中，使用基于低场 NMR 技术的定制测量设备研究了砂岩、人工样品和粘土-粉砂沉积物中的 CO2 水合物饱和度、生长速率和渗透率。结果表明，T2 谱振幅随着水合物的形成而减小，随着解离过程而增加。对于人工样品和神狐沉积物，CO2 水合物首先占据较大的孔隙，砂岩孔隙的均质性变差。同时，与黏土-粉砂沉积相比，对于砂岩和人工样品，CO2 水合物更容易形成并且具有更高的水合物饱和度。在水合物分解过程中，存在一种保护机制，即当气压突然快速下降时，水合物颗粒中心附近的分解受到抑制。对于这些样品的渗透率，它随着水合物的形式而降低，并随着水合物的解离而增加。同时，在水合物饱和度相同的情况下，水合物形成时的渗透率高于解离时的渗透率。