Effect of salinity on the formation of methane hydrate in unconsolidated sediments
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摘要:
深入认识含盐水松散沉积物体系中甲烷水合物生成特征，对精准评价海域天然气水合物资源量有重要意义。本研究利用低场核磁共振原位探测技术研究了盐度对松散砂样中甲烷水合物生成特征的影响。结果表明，松散砂样体系下不同大小孔隙中水合物生成诱导时间存在差异；甲烷水合物更易在小孔隙中生成。随着孔隙水盐度增大，水合物生成诱导时间呈现指数增长。在最初20 min内，随着孔隙水盐度的增大，水合物生成速率先升后降，在盐度为3.0 wt%条件下生成速率最大。在松散砂样体系下的不同位置处，甲烷水合物生成诱导时间的差异性以及孔隙水通过水合物膜的方式转变会引起水合物生成过程中生成速率的变化。沉积物表面影响和水合物的阻隔作用使砂样中部分孔隙水不能转化为水合物，而孔隙水盐度的增加促使这种作用增强，孔隙水最终转化率降低，水合物饱和度减小。
Abstract:In-depth understanding of the characteristics of methane hydrate formation in unconsolidated sediments containing saline solution is of great significance for the accurate evaluation of hydrate resources in marine sediments. In this study, the effect of salinity on the formation characteristics of methane hydrate in unconsolidated sand samples was studied using the low-field nuclear magnetic resonance in-situ detection technology. Results show that there were differences in the induction time of hydrate formation in pores of different sizes in the unconsolidated sands; methane hydrate was more likely to form in small pores. As the salinity of pore water increased, the induction time of hydrate formation increased exponentially. In the first 20 minutes, the hydrate formation rate first increased and then decreased, and the formation rate was the highest under the salinity of 3.0 wt%. At different locations of the unconsolidated sediments, the difference in the induction time of methane hydrate formation and the mode change of water through the hydrate film caused changes in the hydrate formation rate. The large potential energy on sediment surface and the barrier effect caused by the presence of hydrates prevented some pore water in the samples from being converted into hydrates. The increase in the initial salinity of pore water could enhance the barrier effect, decrease the final conversion rate of pore water, and reduce the hydrate saturation.
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图 1 水合物专用低场核磁共振测试装置照片
Figure 1. Photograph of LF-NMR monitoring apparatus for gas hydrate
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图 2 水合物专用低场核磁共振测试装置示意图
Figure 2. Schematic diagram of LF-NMR monitoring apparatus for gas hydrate
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图 3 含水量与T2分布信号总强度之间的关系
Figure 3. Relationship between the total signal intensity of T2 distribution and water content
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图 4 甲烷水合物生成过程中T2分布变化图（实验4）
Figure 4. Variation of the T2 distribution during methane hydrate formation (Experiment 4)
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图 5 甲烷水合物生成过程中T2min与T2max变化（实验4）
Figure 5. Variation of T2min and T2max during methane hydrate formation (Experiment 4)
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图 6 不同初始孔隙水盐度下砂样中甲烷水合物生成过程中的水合物饱和度变化曲线
Figure 6. Variation of hydrate saturation during methane hydrate formation in sand samples with different initial salinities
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图 7 不同孔隙水初始盐度条件下甲烷水合物生成诱导时间变化图
Figure 7. Variation in the induction time of methane hydrate formation in sand samples with different initial salinities
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图 8 不同孔隙水初始盐度下砂样中甲烷水合物生成过程的水合物生成速率变化曲线
Figure 8. Variation of formation rate during methane hydrate formation in sand samples with different initial salinities
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图 9 不同初始孔隙水盐度条件下孔隙水最终转化率与孔隙水最终盐度变化
Figure 9. Variation in conversion ratio and salinity at the end of methane hydrate formation in sand samples with different initial salinities
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图 10 不同孔隙水初始盐度条件下甲烷水合物生成过程的孔隙水盐度变化曲线
Figure 10. Variation of salinity during methane hydrate formation in sand samples with different initial salinities
表 1 砂样的主要参数
Table 1 Major parameters of sand samples
编号 粒径/μm 表观体积/cm3 初始含水量/g 初始含水饱和度/% 孔隙度/% 盐度/wt% 1 100～200 42.41 9.51 49.88 44.97 0.0 2 100～200 42.41 9.40 49.40 44.86 1.5 3 100～200 42.41 9.22 48.62 44.71 3.0 4 100～200 42.41 9.37 49.23 44.87 4.5 
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