Methane hydrates have drawn the attention of climate scientists in the past decades due to the potency of methane as a greenhouse gas and the widespread occurrence of hydrates both in terrestrial and marine environments, which, if destabilised, could enhance global warming. This study aims to investigate how much impact sediment type has on modeled methane escape at the feather edge of stability for methane hydrates in the high latitude Northern Hemisphere (45° to 75° N). This area is characterised by cool bottom-water temperatures leading to a shallow gas hydrate stability zone (GHSZ), and has been disproportionally influenced by contemporary seawater warming. Calculations were performed to establish the depths of the upper and lower boundaries of the feather edge of the GHSZ. These limits were used to estimate seafloor areas covered by three select sediment types that have different petrophysical properties - hemipelagic clay, calcareous ooze and siliceous ooze. Modeling of methane flux for 300 years following a 3°C warming during the first 100 years was performed using TOUGH + HYDRATE for each of the three sediment types. The sediments behaved significantly differently, with siliceous ooze releasing the most methane gas, and calcareous ooze releasing the least. Estimates of total methane gas release were also performed on the areas covered by the three sediments between latitudes 45° to 75° N, and showed that, over the course of 300 years, up to 5 times the current methane concentration in the atmosphere could become susceptible to leaving methane hydrate reservoirs.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20324号 / 工博第4261号 / 新制||工||1660(附属図書館) / 京都大学大学院工学研究科社会基盤工学専攻 / (主査)教授 木村 亮, 教授 三村 衛, 准教授 木元 小百合 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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Reducing atmospheric CO2, a main source of greenhouse gas, has been accentuated recently. One focus is capture, separation, and sequestration of industrial CO2. As a hydrate former, CO2 forms hydrates at moderate temperatures and pressures. This phenomenon could be utilized to capture and separate CO2 from flue gases, and also has the potential to sequester CO2 in the deep sds. This research investigated the CO2-N2 separation efficiency of gas hydrates; it investigated the sequestration potential of CO2 hydrates in ocean sediments. The catalytic effect of surfactants in these processes was investigated. A fluorosurfactant FS-62 was mixed with SDS at 100ppm/1000ppm was found to best catalyze CO2 hydrate formation, giving a high formation rate of 0.1239 mmole of occluded gas/minute-about 2.87 times the base case with no surfactant. FS-62/SDS was verified to increase the separation efficiency of N2-CO2 gas which formed a mixture gas hydrate. In a two-stage process, a desirable separation efficiency was obtained. A total CO2 removed from the gas mixture of 67.7% was obtained. In a series of experiments simulated under ocean sediment environments, the biosurfactants Emulsan and Rhamnolipid showed favorable catalysis of CO2 hydrate formation. Also, the chemical structure of the porous media was found to have some influence on the hydrate formation rate. For a quiescent system, the displacement of natural gas from hydrate by injecting CO2 occurred at a low level and would not be a practical process. In the case of displacing CH4 from hydrate with CO2, no displacement would occur. This research work showed that a potentially cost effective hydrate separation technology applied to N2-CO2 gas, representative of a flue gas, can be improved by adding surfactants. It was found that biosurfactants give some beneficial effect on CO2 hydrate formation in sediments and might be used to assist CO2 sequestration in sediments or to displace natural gas from hydrates already in sediments.

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Ce travail de thèse présente les résultats obtenus lors de l'étude des mécanismes élémentaires à l'origine de la forte conduction protonique mesurée dans le cas de clathrates hydrates d'acides forts. Une méthodologie combinant diffusion neutronique, résonance magnétique nucléaire et simulation de dynamique moléculaire "ab-initio" a permis de modéliser les différents processus dynamiques impliqués, se produisant sur des temps allant de la nanoseconde à la femtoseconde. Le modèle proposé explique la forte conduction de ces systèmes aqueux par la délocalisation à longue distance de leurs protons résultant d'un mécanisme de type Grotthuss gouverné par la relaxation des molécules aqueuses environnant les protons en excès. / This work shows the results obtain about the study of elementary mechanisms behind the high protonic conduction of strong acids clathrate hydrate. A method using quasiélastic neutron scattering and pulse field gradient NMR experiments both with DFT molecular dynamic simulations allowed to establish a model which describe the several dynamical processes involve occuring on timescales from nanosecond to femtosecond. The model deduced explain the high conduction property of ionic clathrate hydrate by a delocalization of their protons following a grotthuss type mecanism managed by the relaxation of water molecules around the excess protons.

Dans les applications industrielles et lors des opérations de transport du gaz naturel, la présence d'eau sous forme liquide ou en phase vapeur peut entraîner la formation d'hydrates provoquant le colmatage des unités industrielles et des lignes de conduites et il est indispensable de définir précisément les seuils de déshydratation à réaliser, afin d'éviter la formation d'hydrates. Cela est réalisé à l'aide d'un modèle thermodynamique qui prédit la stabilité des hydrates, en fonction de la température, de la pression et de la composition du gaz.Les modèles thermodynamiques classiques, développés uniquement sur la base de données expérimentales de formation d'hydrates en présence d'eau liquide, surestiment fortement la température de dissociation des hydrates en l'absence d'une phase aqueuse.Dans le but de définir un modèle thermodynamique capable de représenter convenablement les équilibres de phases vapeur-hydrate et prédire ainsi la température de dissociation des hydrates que l'on soit en présence ou en l'absence d'eau liquide, nous avons mis au point une méthodologie originale pour la détermination de la température de dissociation des hydrates de corps purs et de mélanges en l'absence d'eau liquide. Cette méthodologie, basée sur le suivi de la teneur en eau de phase vapeur, en fonction de la température par coulométrie Karl Fischer, a permis la détermination de la température de dissociation de plusieurs hydrates simples et mixtes à des teneurs en eau et pressions différentes ainsi que les quantités d'hydrates formées dans ces conditions.Sur la base de ces nouvelles données, nous avons défini un modèle thermodynamique basé sur l'utilisation de l'approche de Dharmawardhana pour le calcul de la fugacité de l'eau dans l'hydrate vide,le potentiel de Kihara pour le calcul de la constante de Langmuir et l'équation d'état CPA (Cubic Plus Association) pour la modélisation des phases fluides. Nous avons montré que l'utilisation de l'équation d'état CPA, capable de prendre en compte l'auto association de l'eau apporte une amélioration très significative.Le développement d'un flash biphasique hydrate-fluide nous a permis de calculer les quantités d'hydrates mixtes formées et de les comparer à nos données expérimentales.

The objective of this study is to determine hydrate formation conditions of methane- hydrogen sulfide mixtures. During the study, an experimental work is carried out by using a system that contains a high-pressure hydrate formation cell and pressure-temperature data is recorded in each experiment. Different H2S concentrations and both brine and distilled water are used in the experiments and the Black Sea conditions, which are suitable for methane-hydrogen sulfide hydrate formation are examined. Considering the pressure-temperature data obtained, hydrate equilibrium conditions are determined as well as the number of moles of free gas in the hydrate formation cell. The change in the number of moles of free gas in the hydrate formation cell with respect to time is considered as a way of determining rate of hydrate formation. Effects of H2S concentration and salinity on hydrate formation conditions of methane-hydrogen sulfide mixtures are also studied. It is observed that an increase in the salinity shifts the methane-hydrogen sulfide hydrate equilibrium condition to lower equilibrium temperatures at a given pressure. On the other hand, with an increase in H2S concentration the methane hydrogen sulfide hydrate formation conditions reach higher equilibrium temperature values at a given pressure. After the study, it can be also concluded that the Black Sea has suitable conditions for hydrate formation of methane hydrogen sulfide mixtures, considering the results of the experiments.

Dissociation of gas hydrates has been the primary concern of the oil and gas industry for flow assurance, mainly in an offshore environment. There is also a growing interest in the rapid formation of gas hydrates for gas storage, transport of natural gas and carbon sequestration. In this thesis, we experimentally measure the kinetics of formation and dissociation of methane hydrates and the effect of various anionic and cationic surfactants such as sodium dodecyl sulfate (SDS), cetyl trimethylammonium bromide (CTAB) and alpha olefin sulfonate (AOS) on the association/dissociation rate constants. The importance and necessity of micelle formation in these surfactants has been studied. The effect of foam generation on the rate of formation of these hydrates has also been measured. SDS was found to significantly decrease the induction time for hydrate formation. There was an added decrease in the induction time when a foamed mixture of water and SDS was used. On the other hand CTAB and AOS had an inhibiting effect. The contribution of micelles towards promoting hydrate formation was demonstrated with a series of experiments using SDS. The micelles formed by these surfactants appear to serve as nucleation sites for the association of hydrates. New experimental data is presented to show that some surfactants and the use of foam can significantly increase the rate of hydrate formation. Other surfactants are shown to act as inhibitors. A new experimental setup is presented that allows us to distinguish between surfactants that act as promoters and inhibitors for hydrate formation. / text