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

合成和天然多孔介质中天然气水合物生长和解离的毛细管控制：PVT、NMR、中子衍射和 SANS

基本信息

批准号：
EP/D052556/1
负责人：
Bahman Tohidi
金额：
$ 36.41万
依托单位：
Heriot-Watt University
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2006
资助国家：
英国
起止时间：
2006 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FD052556%2F1
关键词：
Capillary controls gas hydrate growth

项目摘要

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.

气体水合物是在低温高压条件下由水和气体分子形成的冰状固体。在水合物结构中，水分子形成一个大小不等的网状笼状空腔，气体分子以压缩的形式被困在其中。在20世纪70年代，人们认识到大量的甲烷气体水合物天然存在于海底大陆斜坡的沉积物和北极永久冻土区的地下。自这一发现以来，全球对甲烷水合物的兴趣稳步增长，过去十年的研究扩展尤其迅速。推动研究的重要问题包括甲烷水合物作为能源的潜力，二氧化碳作为天然气水合物在海底处置的可能性，提高对海底水合物不稳定与大型海底滑坡之间关系的认识，水合物不稳定可能对深水石油/天然气平台、管道和海底电缆造成的潜在危险，以及水合物稳定性方面的长期考虑，过去，海洋沉积物中天然气水合物形成和分布的模型通常假设，对大量（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底部（BHSZ）通常接近根据大量实验室水合物测量计算的压力和温度条件，但有一些地点的水合物稳定区（HSZ）厚度远小于预测，这表明宿主沉积物在某种程度上抑制了水合物的生长和/或稳定性。沉积物改变水合物稳定性的机制仍然知之甚少。气体成分的变化（例如添加CO2）可促进水合物稳定性，而含盐孔隙沃茨将起到抑制水合物的作用。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BHSZ不一致时，必须考虑替代抑制机制。孔隙的大小和几何形状是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。高毛细管压力可导致温度/压力条件的变化，其中发生相变，例如液体冻结和熔化。作为沉积物的主机天然气水合物的特点通常是细粒度的粉砂，泥浆和粘土，往往具有相当窄的平均孔径，毛细管抑制以前已被提出作为一种机制，解释预测和实际的水合物稳定区之间观察到的差异。本文旨在研究天然和合成沉积物中孔隙大小、几何形状、毛细管压力与水合物生长和分解条件之间的关系，并评估毛细管抑制作用在海底/冻土水合物系统中的作用程度，采用多种实验方法从微观（孔隙）到宏观（岩心）研究毛细管对水合物生长的影响。新的合成孔隙微观模型将被用来直观地研究水合物晶体生长模式在孔隙尺度，补充和支持大容量，长时间，压力-体积-温度-组成的测量沉积物岩心，而核磁共振（NMR）将被用来探测流体状态（水合物，水，气体）和孔隙内的分布。实验数据将结合起来，开发一个模型，能够预测水合物的增长和分解条件作为沉积物孔径分布的函数。