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首页> 外文期刊>Journal of Geophysical Research. Biogeosciences >Formation of natural gas hydrates in marine sediments 1. Conceptual model of gas hydrate growth conditioned by host sediment properties
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Formation of natural gas hydrates in marine sediments 1. Conceptual model of gas hydrate growth conditioned by host sediment properties

机译：海洋沉积物中天然气水合物的形成1.受宿主沉积物特性影响的天然气水合物生长概念模型

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摘要

The stability of submarine gas hydrates is largely dictated by pressure and temperature, gas composition, and pore water salinity. However, the physical properties and surface chemistry of deep marine sediments may also affect the thermodynamic state, growth kinetics, spatial distributions, and growth forms of clathrates. Our conceptual model presumes that gas hydrate behaves in a way analogous to ice in a freezing soil. Hydrate growth is inhibited within finegrained sediments by a combination of reduced pore water activity in the vicinity of hydrophilic mineral surfaces, and the excess internal energy of small crystals confined in pores. THe excess energy can be thought of as a "capillary pressure" in the hydrate crystal, related to the pore size distribution and the state of stress in the sediment framework. The base of gas hydrate stability in a sequence of fine sediments is predicted by our model to occur at a lower temperature (nearer to the seabed) than would be calculated from bulk thermodynamic equilibrium. Capillary effects or a build up of salt in the system can expand the phase boundary between hydrate and free gas into a divariant field extending over a finite depth range dictated by total methane content and pore-size distribution. Hysteresis between the temperatures of crystallization and dissociation of the clathrate is also predicted. Growth forms commonly observed in hydrate samples recovered from marine sediments (nodules, and lenses in muds; cements in sands) can largely be explained by capillary effects. but kinetics of nucleation and growth are also important. The formation of concentrated gas hydrates in a partially closed system with respect to material transport, or where gas can flush through the system, may lead to water depletion in the host sediment. This "freezedrying" may be detectable through physical changes to the sediment (low water content and overconsolidation) and/or chemical anomalies in the pore waters and metastable presence of free gas within the normal zone of hydrate stability.
机译：水下气体水合物的稳定性在很大程度上取决于压力和温度，气体组成和孔隙水的盐度。但是，深海沉积物的物理性质和表面化学性质也可能影响笼形物的热力学状态，生长动力学，空间分布和生长形式。我们的概念模型假定天然气水合物的行为类似于在冰冻土壤中的冰。亲水性矿物表面附近孔隙水活度的降低，以及封闭在孔隙中的小晶体的过多内部能量的结合，可抑制细粒沉积物中水合物的生长。多余的能量可以被认为是水合物晶体中的“毛细管压力”，与孔径分布和沉积物骨架中的应力状态有关。我们的模型预测，一系列精细沉积物中的天然气水合物稳定性基础比在整体热力学平衡计算的温度更低（靠近海床）的温度下发生。毛细管效应或系统中盐的堆积可将水合物和自由气体之间的相界扩展为在总甲烷含量和孔径分布所决定的有限深度范围内扩展的双变量场。还预测了结晶温度和包合物的解离之间的磁滞。从海洋沉积物（结核，泥浆中的晶状体，沙子中的水泥）中回收的水合物样品中通常观察到的生长形式可以用毛细管效应来解释。但是成核和生长的动力学也很重要。就物料运输而言，或者在气体可以冲洗通过系统的情况下，在部分封闭的系统中形成浓的天然气水合物可能会导致宿主沉积物中的水分枯竭。这种“冻干”可以通过对沉积物的物理变化（低水含量和过度固结）和/或孔隙水中的化学异常以及在水合物稳定性正常区域内的亚稳态存在的游离气体来检测。

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