Dissertations / Theses on the topic 'Gas hydrate structures morphology'

Gas hydrates are ice–like compounds found in deep sea sediments and permafrosts. Concise detection and quantification of natural methane gas hydrate deposits, will allow for a more robust assessment of gas hydrate as a potential energy resource or natural geohazard. Current seismic methods, used to identify and quantify gas hydrates, have proved to be unreliable in providing accurate information on the extent of natural gas hydrate deposits, due to the lack of understanding on how gas hydrate affects the host sediment. Direct measurement of some hydrate bearing sediment properties has been made possible in recent years through advances in pressure coring techniques, but methods for dynamically testing these samples at in–situ pressures are still unavailable. Laboratory tests on synthetic hydrate bearing sediments have shown that factors such as formation technique, sediment type and use of hydrate former affects the form and structure of hydrate in the pore space and how it interacts with the sediment. The aim of this research was therefore to create methane hydrate in sediments under a variety of conditions, so that the influence of hydrate morphology could be investigated. A number of experiments were conducted using two distinct formation techniques. The first technique formed methane hydrate from the free gas phase in almost fully water saturated conditions. Five sand specimens, with a range of hydrate contents from 10% to 40% were formed and tested in the gas hydrate resonant column (GHRC). Results from these tests were compared with previous results from tests where methane hydrate had been formed from free gas in partially saturated conditions. It was found that formation method had a significant influence on the properties of the hydrate bearing sand, and therefore the morphology of the hydrate in the pore space. The second set of experiments formed methane hydrate from free gas within partially saturated sediments, but where the sediments were made up of coarse granular materials with a variety of particle size and shape. As it had been established that hydrate acts as a cement when formed under partially saturated conditions, the experiments aimed to observe the effect of particle size and shape on hydrate bonding mechanisms. The results showed that the influence of disseminated hydrate on the physical properties of the specimens was affected by both mean particle size and by particle shape, with the surface area of the sediment grains influencing the volume and distribution of hydrate throughout a material and therefore it’s bonding capabilities. In addition to the experiments on synthetic hydrate specimens, five core sections containing naturally occurring gas hydrate in fine grained sedimentsweremade available to the University of Southampton from the Indian National Gas Hydrate Program (NGHP) 01 expedition. High resolution CT imaging of the core sections observed large volumes of methane hydrate as a network of veins throughout the specimens. Due to sample disturbance caused during the depressurisation and subsequent freezing of the samples prior to delivery, dynamic testing in the gas hydrate resonant column apparatus was not feasible. Therefore, the hydrate was dissociated and a number of geotechnical tests were undertaken on the remaining host sediment. Results from these tests suggested that hydrate dissociation could affect host sediment properties, due to a change in water content, salinity and structure.

Considerable research has been done to improve hydrate formation rate. One of the ideas is to introduce mechanical mixing which later tend to complicate the design and operation of the hydrate formation processes. Another approach is to add surfactant (promoter) that will improve the hydrate formation rate and also its storage capacity to be closer to the maximum hydrate storage capacity. Surfactant is widely known as a substance that can lower the surface or interfacial tension of the water when it is dissolved in it. Surfactants are known to increase gas hydrate formation rate, increase storage capacity of hydrates and also decrease induction time. However, the role that surfactant plays in hydrate crystal formation is not well understood. Therefore, understanding of the mechanism through morphology studies is one of the important aspects to be studied so that optimal industrial processes can be designed. In the present study the effect of three commercially available anionic surfactants which differ in its alkyl chain length on the formation/dissociation of hydrate from a gas mixture of 90.5 % methane – 9.5% propane mixture was investigated. The surfactants used were sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate (STS), and sodium hexadecyl sulfate (SHS). Memory water was used and the experiments for SDS were carried out at three different degrees of under-cooling and three different surfactant concentrations. In addition, the effect of the surfactant on storage capacity of gas into hydrate was assessed. The morphology of the growing crystals and the gas consumption were observed during the experiments. The results show that branches of porous fibre-like crystals are formed instead of dendritic crystals in the absence of any additive. In addition, extensive hydrate crystal growth on the crystallizer walls is observed. Also a “mushy” hydrate instead of a thin crystal film appears at the gas/water interface. Finally, the addition of SDS with concentration range between 242ppm – 2200ppm (ΔT =13.10C) was found to increase the mole consumption for hydrate formation by 14.3 – 18.7 times. This increase is related to the change in hydrate morphology whereby a more porous hydrate forms with enhanced water/gas contacts.

Les hydrates de méthane (MHs), composés de gaz de méthane et d’eau, se forment naturellement à haute pression et faible température dans les sédiments marins ou pergélisols. Ils sont actuellement considérés comme une ressource énergétique (principalement MHs dans les sédiments sableux) mais aussi une source de géo-hasards et du changement climatique (MHs dans les sédiments grossiers et fins). La connaissance de leurs propriétés mécaniques/physiques, qui changent considérablement avec la morphologie et distribution des hydrates dans les pores, est très importante pour minimiser les impacts environnementaux liés aux futures exploitations du gaz de méthane à partir des sédiments sableux contenant des MHs (MHBS). La plupart des études expérimentales concernent MHBS synthétiques à cause des difficultés pour récupérer des échantillons intacts. Différentes méthodes ont été proposées pour former MHs dans les sédiments au laboratoire pour reconstituer des sédiments naturels, mais sans grand succès. Cette thèse a pour objectif d’évaluer la morphologie, la distribution des MHs dans les MHBS synthétiques à différentes échelles et d’étudier les effets des MHs (leur morphologie et teneur en hydrate) sur les propriétés mécaniques des MHBS. Deux méthodes de formation d’hydrates dans les sédiments sableux ont été proposées. Au niveau macroscopique, la distribution des hydrates au niveau des pores est évaluée en se basant sur la vitesse de propagation d’onde de compression (mesurée et calculée à partir des modèles existants). Des essais triaxiaux ont été utilisés pour étudier l’influence des MHs à différentes teneurs en hydrate sur les propriétés mécaniques des MHBS. Par ailleurs, l’Imagerie par Résonance Magnétique a été utilisée pour étudier la cinétique de formation/dissociation d’hydrates et aussi la distribution des hydrates sur l’ensemble de l’échantillon. Les résultats montrent qu’un cycle de température en conditions non drainées complète la redistribution des hydrates dans les pores après la saturation en eau de l’échantillon à haute teneur en hydrate. La distribution des hydrates sur l’ensemble de l’échantillon devient plus homogène avec la saturation en eau suivie par un cycle de température. En outre, les propriétés mécaniques des sédiments augmentent avec l’augmentation de la teneur en hydrate.A l’échelle du grain, la tomographie aux rayons X (XRCT) et celle au Synchrotron XRCT (SXRCT, Synchrotron SOLEIL) ont été utilisées pour observer la morphologie et la distribution des MHs au niveau des pores des sédiments sableux. Ce travail n’a pas été facile car il nécessitait des dispositifs expérimentaux compliqués (pour maintenir la haute pression et faible température) mais aussi en raison du faible contraste entre MHs et l’eau sur les images de XRCT, SXRCT. Des dispositifs spécifiques ont été développés pour étudier la formation d’hydrates, la morphologie et la distribution à l’échelle du grain des MHs en utilisant XRCT, SXRCT. De plus, une nouvelle méthode a été développée pour déterminer plus précisément les fractions volumiques d’un milieu triphasé à partir des images XRCT. Des observations au Microscope Optique (en coopération avec l’Université de Pau) ont également été faites pour confirmer diverses morphologies de MHs dans les sédiments sableux. Les morphologies et distributions d’hydrates observées sont comparées avec les modèles existants. Les observations montrent que la formation des MHs dans les sédiments sableux est un processus instable et compliqué. Différentes morphologies et distributions au niveau des pores des MHs peuvent coexister. Il parait indispensable de tenir compte des vraies morphologies et distributions au niveau des pores des MHs pour les études numériques utilisant des modèles simplifiés.Mots-clés: hydrate de méthane, sédiments sableux, formation, dissociation, morphologies, distribution, propriétés mécaniques, XRCT, SXRCT, microscope optique, essais triaxiaux, modèle de mécanique des roches<br>Methane hydrates (MHs), being solid ice-like compounds of methane gas and water, form naturally at high pressure and low temperature in marine or permafrost settings. They are being considered as an alternative energy resource (mainly methane hydrate-bearing sand, MHBS) but also a source of geo-hazards and climate change (MHs in both coarse and fine sediments). Knowledge of physical/mechanical properties of sediments containing MHs, depending considerably on hydrate morphologies and pore-habits, is of the importance to minimize the environmental impacts of future exploitations of methane gas from MHBS. Existing experimental works mainly focus on synthetic samples due to challenges to get cored intact methane hydrate-bearing sediment samples. Various methods have been proposed for MH formation in sandy sediments to mimic natural MHBS, but without much success. The main interests of this thesis are to investigate morphologies and pore-habits of MHs formed in synthetic MHBS at various scales and to study the effects of MHs (MH morphology and MH saturation) on the mechanical properties of MHBS.Two MH formation methods (modified from two methods existing in the literature) have been first proposed to create MHs in sandy sediments at different pore-habits. At the macroscopic scale, MH pore-habits have been predicted via comparisons between sonic wave velocities, measured and that calculated based on rock physic models. The effects of MHs formed following the two proposed methods (at different hydrate saturations) on the mechanical properties of MHBS were investigated by triaxial tests. Furthermore, Magnetic Resonance Imaging (MRI) has been used to investigate the kinetics of MH formation, MH distribution along with sample height and also MH dissociation following the depressurization method which has been considered as the most economical method for MH production from MHBS. A temperature cycle in undrained conditions was supposed to not only complete MH redistribution in pore space after the water saturation of the sample at high hydrate saturation but also make MHs distributed more homogeneously in the sample even at low hydrate saturation. Furthermore, the mechanical properties of sediments (e.g. stiffness, strength) were found higher at higher MH saturation.At the grain scale, the MH morphologies and pore habits in sandy sediments were observed by X-Ray Computed Tomography (XRCT, at Navier laboratory, Ecole des Ponts ParisTech) and Synchrotron XRCT (SXRCT, at Psiche beamline of Synchrotron SOLEIL). It has been really challenging due to not only the need of special experimental setups (needing both high pressure and low temperature controls) but also poor XRCT, SXRCT image contrast between methane hydrate and water. Specific experimental setups and scan conditions were then developed for pore-scale investigations of MH growth and MH morphologies in sandy sediments by using XRCT, SXRCT. Besides, a new method has been developed for accurate determination of volumetric fractions of a three-phase media from XRCT images. Observations (at better spatial and temporal resolution) via Optical Microscopy (in cooperation with the University of Pau) were finally used to confirm diverse MH morphologies in sandy sediments. Comparisons between observed MH morphologies, pore habits, and existing idealized models have been discussed. Methane hydrate formation in sandy sediments was supposed to be an unstable and complex process. Different types of MH morphologies and pore habits could exist in the sample. It seems vital that numerical studies on the mechanical behavior of gas hydrates in sediments, based on four idealized hydrate pore-habits, should take into account realistic hydrate morphologies and pore habits.Keywords:Methane hydrates, sandy sediments, formation, dissociation, morphologies, pore-habits, mechanical properties, XRCT, SXRCT, optical microscopy, triaxial tests, rock physic model

Lors de la production d’hydrocarbures, les conditions de pression et température dans les conduites peuvent être favorables à la formation d’hydrates de gaz (composés cristallins formés par l’association de molécules d’eau et de gaz). Leur formation peut entraîner le bouchage des conduites et mener à l’arrêt de la production, entraînant d’importantes pertes économiques. Pour remédier au risque « hydrate », les pétroliers disposent de diverses méthodes dont l’utilisation d’additifs antiagglomérants. Les antiagglomérants sont des tensioactifs capables de s’adsorber à la surface des cristaux d’hydrate et de les maintenir dispersés dans la phase hydrocarbonée, qui est généralement majoritaire. L’objectif de cette thèse est de progresser dans la compréhension des mécanismes d’action de tensioactifs ioniques pour la prévention de l’agglomération d’hydrates de gaz. Plusieurs tensioactifs cationiques ont été étudiés sur un hydrate de cyclopentane (CP) (qui se forme à pression atmosphérique) et sur un hydrate de méthane/propane (qui se forme sous pression).Pour les deux hydrates, l’effet des tensioactifs sur la morphologie des cristaux et sur leur mouillabilité a été étudié, et leur performance antiagglomérante (AA) a été évaluée en réacteur agité pour différentes conditions et compositions des systèmes. Les tensioactifs conduisant à la formation de cristaux individuels présentent les meilleures performances AA. Les observations montrent qu’il n’est pas indispensable que les tensioactifs rendent les cristaux mouillables à l’huile pour qu’ils procurent une bonne protection contre l’agglomération dans un système agité où l’huile est la phase majoritaire. Nous avons vu que la modification (par ajout de sel par exemple) de l’environnement physicochimique des molécules tensioactives peut jouer un rôle déterminant sur leurs propriétés AA. De même, la modification de la structure des molécules (nature du contre-ion, longueur des chaînes hydrocarbonées) impacte leur adsorption sur l’hydrate, la morphologie et la mouillabilité des cristaux, et par suite leur performance AA. Les principaux facteurs identifiés pour la bonne performance d’une molécule tensioactive sont sa capacité à se fixer efficacement et en quantité suffisante à la surface de l’hydrate, et à rendre les cristaux d’hydrate hydrophobes, ou dans le cas où il les rend hydrophiles d’abaisser fortement la tension interfaciale entre les phases aqueuse et huileuse de manière à réduire l’intensité des forces capillaires entre les particules. Enfin, nous avons pu établir une corrélation entre les observations faites à l’échelle microscopique et la performance AA des tensioactifs évaluée à l’échelle macroscopique. Ce travail confirme que l’hydrate de CP est globalement un bon modèle pour des évaluations simples de la performance de molécules tensioactives. L’utilisation de l’hydrate de CP présente néanmoins des limitations pour mener des études à forts sous-refroidissements et avec de grandes fractions volumiques d’eau<br>Pressure and temperature conditions encountered in the pipelines of hydrocarbons production may be favorable to the formation of gas hydrates (crystalline compounds formed by the association of molecules of gas and water). Their agglomeration in pipelines may form plugs and lead to production shutdowns and cause significant economic losses. To prevent it, oil and gas companies use various methods and more particularly anti-agglomerant additives. Anti-agglomerants are surfactants that can adsorb at the hydrate crystals surface and keep them dispersed in a hydrocarbon phase. The objective of this thesis is to progress in the understanding of mechanisms of action of ionic surfactant to prevent the gas hydrates agglomeration. Several cationic surfactants were studied on a cyclopentane (CP) hydrate (formed at atmospheric pressure) and on a methane/propane hydrate (formed under pressure). For both hydrates, the effect of surfactants on the crystals morphology and on their wettability was investigated, and their anti-agglomerant (AA) performance was evaluated in an agitated reactor for systems at different conditions and compositions. The surfactants leading to the formation of individual crystals had the best AA performances. In order to have a good protection against the agglomeration, it is not necessary that the surfactants make the crystals oil wettable in a system where the oil phase is in excess. We showed that the modification (by the addition of salt for example) of the physicochemical environment of surfactant molecules plays an important role on their AA properties. Similarly, the modification of the structure of molecules (counter-ion nature, length of the hydrocarbon chains) affects their adsorption on the hydrate, the morphology and wettability of crystals and consequently their AA performance. The main factors identified for a good performance of a surfactant molecule are its capacity to be efficiently fixed and in a sufficient amount on the hydrate surface in order to make the hydrate crystals hydrophobic. In the case where it makes the hydrate hydrophilic, the surfactant has to strongly reduce the interfacial tension between the aqueous and oil phases and then reduce the intensity of capillary forces between hydrate particles. Lastly, we set a correlation between the observations done at the microscopic scale and the AA performance of surfactants evaluated at the macroscopic scale. This work confirms that the CP-hydrate is overall a good model for a simple evaluation of the surfactant molecules performance. However, the use of the CP-hydrate has some limitations to conduct studies at high subcooling and watercut