Microstructure of Gas Hydrates in Sedimentary Matrices
Gas hydrates (GH) are found worldwide in marine sediments and permafrost regions. The detection and quantification of gas hydrates present in marine sediments is crucial for safe oil and gas extraction, seafloor stability assessments and for quantifying the impact of GH in climatic change. Therefore...
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| Other Authors: | , |
| Format: | Doctoral or Postdoctoral Thesis |
| Language: | English |
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2016
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| Online Access: | http://hdl.handle.net/11858/00-1735-0000-0028-8792-A https://doi.org/10.53846/goediss-5707 https://nbn-resolving.org/urn:nbn:de:gbv:7-11858/00-1735-0000-0028-8792-A-4 |
| Summary: | Gas hydrates (GH) are found worldwide in marine sediments and permafrost regions. The detection and quantification of gas hydrates present in marine sediments is crucial for safe oil and gas extraction, seafloor stability assessments and for quantifying the impact of GH in climatic change. Therefore, there is a considerable interest in studying the microstructure of gas hydrate-bearing sediments. Although a large amount of research on gas hydrates has been carried out over the years, the micro-structural aspects of gas hydrate growth and the crystallite sizes of hydrate in sediments are still poorly known and understood. The formation process of gas hydrates in sedimentary matrices is of crucial importance for the physical and transport properties of the resulting aggregates. This process has never been observed in-situ at sub-micron resolution. In this study, the nucleation and growth processes of GH were observed using synchrotron X-ray micro-computed tomography at 276 K in various sedimentary matrices such as natural quartz (with and without admixtures of clay minerals) or glass beads at varying water saturation. The process was observed on a timescale of a few minutes to many hours. Both, juvenile water as well as gas-enriched water obtained from gas hydrate decomposition was used in the experiments. Xenon gas was employed to enhance the density contrast between gas hydrate and the fluid phases involved. The nucleation sites can be easily identified and the various growth patterns are clearly established. In sediments under-saturated with juvenile water the nucleation starts at the water-gas interface resulting in an initially several micrometer thick gas hydrate film; the further growth proceeds to form chains of predominantly isometric single crystals. The growth of gas hydrate from gas-enriched water clearly follows a different pattern, via the nucleation in the bulk of liquid producing polyhedral single crystals. A striking feature in both cases is the systematic appearance of a fluid phase film of up to ... |
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