Capillary controls on gas hydrate growth and dissociation in synthetic and natural porous media: PVT, NMR, Neutron Diffraction and SANS

天然气水合物是由水和气体分子在低温高压条件下形成的冰状固体。在水合物结构中，水分子形成大小不等的网笼状空穴，气体分子以压缩的形式被困在其中。20世纪70年代，S认识到，在海底陆坡和北极永久冻土区的次表层沉积物中自然存在大量的甲烷气体水合物。自从这一发现以来，全球对甲烷水合物的兴趣一直在稳步增长，过去十年的研究扩张尤其迅速。推动研究的重要问题包括甲烷水合物作为能源的潜力，二氧化碳作为海底天然气水合物处置的可能性，对海底水合物不稳定与大型海底滑坡之间关系的日益认识，水合物不稳定可能对深水油气平台、管道和海底电缆构成的潜在危险，以及对水合物稳定性、向大气释放甲烷(一种强有力的温室气体)以及全球气候变化的长期考虑。过去，海洋沉积物中天然气水合物的形成和分布模型通常假设，对大宗(不存在沉积物)水-气系统的实验室测量可以直接应用于自然环境。海底钻探已经证实，海底沉积物中的水合物稳定区(BHSZ)的底部通常位于根据大量实验室水合物测量计算的压力和温度条件附近，但有一些地方的水合物稳定区(HSZ)的厚度比预测的要小得多，这表明宿主沉积物以某种方式抑制了水合物的生长和/或稳定性。沉积物可能改变水合物稳定性的机制尚不清楚。气体成分的变化(例如，二氧化碳的加入)可以促进水合物的稳定性，而咸水孔隙水则会抑制水合物。然而，在气体和孔隙水盐浓度合理确定的情况下，当预测和实际的BHSZ不一致时，必须考虑替代的抑制机制。可能改变天然气水合物稳定性并影响其在沉积物中分布的一个因素是孔隙大小和几何形状。众所周知，当限制在狭窄的孔隙中时，流体可能会受到很高的内部(毛细)压力。高毛细压力会导致发生相变的温度/压力条件发生变化，例如液体冻结和熔化。由于含有天然气水合物的沉积物通常以细粒粉砂、泥浆和粘土为特征，通常平均孔径很小，因此以前曾提出将毛细管抑制作为一种机制来解释预测的水合物稳定带与实际水合物稳定带之间的观测差异。这项工作的目的是研究合成沉积物和天然沉积物中的孔隙大小、几何形状、毛细压力与天然气水合物生长和解离条件的关系，并评估毛细管抑制在海底/永久冻土水合物系统中的作用程度。将使用各种实验方法来研究毛细管对水合物从微观(孔隙)到宏观(岩心)生长的影响。新的合成孔隙微观模型将被用来直观地研究孔隙尺度上的水合物晶体生长模式，补充和支持沉积岩心上的大体积、长时间、压力-体积-温度-成分测量，而核磁共振将用于探测流体状态(水合物、水、气体)和孔隙内的分布。实验数据将被结合起来开发一个模型，该模型能够预测水合物的生长和解离条件作为沉积物孔径分布的函数。

Gas hydrates are ice-like solids which form from water and gas molecules at low temperature and high pressure conditions. Within the hydrate structure, water molecules form a network cage-like cavities of varying size within which gas molecules are trapped in a compressed form.In the 1970's it was recognised that very large quantities of methane gas hydrate occur naturally in sediments of the subsea continental slopes and the subsurface of Arctic permafrost regions. Since this discovery, global interest in methane hydrates has grown steadily, with research expanding particularly rapidly over the past decade. Important issues driving research include the potential for methane hydrates as an energy resource, the possibilities for CO2 disposal as gas hydrates beneath the seafloor, increasing awareness of the relationship between seafloor hydrate destablisation and large subsea landslides, the potential hazard hydrate destabilisation could pose to deepwater oil/gas platforms, pipelines and subsea cables, and long-term considerations with respect to hydrate stability, methane (a potent greenhouse gas) release to the atmosphere, and global climate changes.In the past, models for the formation and distribution of gas hydrates in marine sediments generally assumed that laboratory measurements on bulk (no sediments present) water-gas systems could be directly applied to the natural environment. Ocean floor drilling has confirmed that the Base of Hydrate Stability Zone (BHSZ) in seafloor sediments commonly lies close to pressure and temperature conditions calculated from bulk laboratory hydrate measurements, however there are a number of sites where the thickness of the Hydrate Stability Zone (HSZ) is much less than predicted, suggesting that host sediments are somehow acting to inhibit hydrate growth and/or stability.The mechanisms by which sediments may alter hydrate stability are still poorly understood. Variations in gas composition (e.g. the addition of CO2) can promote hydrate stability, while saline pore waters will act to inhibit hydrates. However, where gas and pore water salt concentrations are reasonably well established, alternative mechanisms of inhibition must be considered when predicted and actual BHSZs do not agree. One factor that could potentially alter the stability of gas hydrates and influence their distribution within sediments is pore size and geometry.It is well-established that, when confined to narrow pores, fluids can be subject to very high internal (capillary) pressures. High capillary pressures can result in changes in the temperature/pressure conditions where phase transitions such as liquid freezing and melting take place. As sediments which host gas hydrates are commonly characterised by fine-grained silts, muds and clays, often with quite narrow mean pore diameters, capillary inhibition has previously been proposed as a mechanism to explain the observed differences between predicted and actual hydrate stability zones. The aim of this work is to examine the relationship between pore size, geometry, capillary pressures and gas hydrate growth and dissociation conditions in synthetic and natural sediments, and to assess the extent to which capillary inhibition is a factor in seafloor/permafrost hydrate systems.A variety of experimental approaches will be used to investigate capillary effects on hydrate growth from the micro (pore) to macro (core scales). Novel synthetic pore micromodels will be used to visually study hydrate crystal growth patterns at the pore scale, complimenting and supporting large volume, long-duration, pressure-volume-temperature-composition measurements on sediment cores, while Nuclear Magnetic Resonance (NMR) will be used to probe fluid states (hydrate, water, gas) and distribution within pores. Experimental data will be combined to develop a model capable of predicting hydrate growth and dissociation conditions as a function of sediment pore size distribution.