A model for the diffusive growth of hydrate saturation anomalies in layered sediments
A model for the diffusive growth of hydrate saturation anomalies in layered sediments
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DOI：
10.1029/2011jb008484
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发表时间：
2011-10
影响因子：
--
通讯作者：
A. Rempel
中科院分区：
文献类型：
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作者：
A. Rempel
[1]沉积物颗粒影响气体水合物的相行为，通过增加表面能，其中孔几何形状迫使水合物晶体获得高曲率，并通过润湿相互作用，导致水膜涂覆颗粒表面。这些影响只产生轻微的变化，通过大部分的水合物稳定区的气体溶解度，所以粒径只有适度的影响时，沉积物是均匀的水合物积累的速度。然而，在水合物储层中，沉积物性质的不连续变化是常见的，并且这种地层边界常常与水合物异常相吻合。这些异常现象是地下沉积物性质变化的自然结果。通过考虑沉积物-水合物的相互作用，我展示了组分扩散如何提供水合物尖峰在粗粒度沉积物中的生长，该粗粒度沉积物紧邻无水合物区域（HFR），在更细粒度的沉积物中，溶解度略有升高。在与米兰科维奇旋回相当的时间尺度上，水合物峰的宽度通常小于一米，并且基本上包含所有的水合物，如果没有沉积物的非均质性，这些水合物将占据更大的相邻HFR。只有当峰值达到足够高的饱和度（通常>孔隙体积的90%），溶解度在地层边界上是连续的时，才能在更细粒度的沉积物中形成海波龙。当水合物形成跨越许多孔径的互连骨架时，稳定大部分残余液体的润湿相互作用也可以部分卸载沉积物颗粒接触，并导致分离的水合物结核和透镜体的生长。
[1] Sediment particles affect the phase behavior of gas hydrates, both by increasing the surface energy where pore geometry forces hydrate crystals to attain high curvatures and through wetting interactions that cause aqueous films to coat particle surfaces. These effects produce only slight changes to the gas solubility through most of the hydrate stability zone, so the particle size has only a modest influence on the rate of hydrate accumulation when the sediments are homogeneous. In hydrate reservoirs, however, discontinuous changes in sediment properties are common and such stratigraphic boundaries often coincide with hydrate anomalies. These anomalies are a natural consequence of variations in subsurface sediment properties. By accounting for sediment-hydrate interactions, I show how compositional diffusion supplies the growth of hydrate spikes in coarse-grained sediments immediately adjacent to hydrate-free regions (HFRs) in more fine-grained sediments where the solubility is slightly elevated. Over timescales comparable with Milankovitch cycles, hydrate spikes are typically less than a meter in width and contain essentially all of the hydrate that would have otherwise occupied the much larger adjacent HFR if sediment heterogeneities were absent. Hydrate can form in the more fine-grained sediments only once the spike achieves a sufficiently high saturation level (often >90% of pore volume) that the solubility is continuous across the stratigraphic boundary. The wetting interactions that stabilize much of the residual liquid when hydrate forms an interconnected skeleton spanning many pore diameters can also partially unload sediment particle contacts, and lead to the growth of segregated hydrate nodules and lenses.