Natural gas hydrates occur on most continental margins in organic-rich sediments at water depths >450 m (in polar regions >150 m). Gas hydrate distribution and abundance, however, varies significantly from margin to margin and with tectonic environment. The National Gas Hydrate Program (NGHP) Expedition 01 cored 10 sites in the Krishna-Godawari (K-G) basin, located on the southeastern passive margin of India. The drilling at the K-G basin was comprehensive, providing an ideal location to address questions regarding processes that lead to variations in gas hydrate concentration and distribution in marine sediments. Pore fluids recovered from both pressurized and non-pressurized cores were analyzed for salinity, Cl-, SO4 2-, alkalinity, Ca2+, Mg2+, Sr2+, Ba2+, Na+, and Li+ concentrations, as well as 􀀂13C-DIC, 􀀂18O, and 87/86Sr isotope ratios. This comprehensive suite of pore fluid concentration and isotopic profiles places important constraints on the fluid/gas sources, transport pathways, and CH4 fluxes, and their impact on gas hydrate concentration and distribution. Based on the Cl- and 􀀂18􀀁 depth profiles, catwalk infrared images, pressure core CH4 concentrations, and direct gas hydrate sampling, we show that the occurrence and concentration of gas hydrate varies considerably between sites. Gas hydrate was detected at all 10 sites, and occurs between 50 mbsf and the base of the gas hydrate stability zone (BGHSZ). In all but three sites cored, gas hydrate is mainly disseminated within the pore space with typical pore space occupancies being 􀀁2%. Massive occurrences of gas hydrate are controlled by high-angle fractures in clay/silt sediments at three sites, and locally by lithology (sand/silt) at the more “diffuse” sites with a maximum pore space occupancy of ~67%. Though a majority of the sites cored contained sand/silt horizons, little gas hydrate was observed in most of these intervals. At two sites in the K-G basin, we observe higher than seawater Cl- concentrations between the sulfate-methane transition (SMT) and ~80 mbsf, suggesting active gas hydrate formation at rates faster than Cl- diffusion and pore fluid advection. The fluids sampled within this depth range are chemically distinct from the fluids sampled below, and likely have been advected from a different source depth. These geochemical results provide the framework for a regional gas hydrate reservoir model that links the geology, geochemistry, and subsurface hydrology of the basin, with implications for the lateral heterogeneity of gas hydrate occurrence in continental margins.

Gas hydrates occur naturally around the world in the shallow-marine geosphere, and have received diverse attention, crossing many disciplines, ranging from interest as a drilling hazard in the petroleum industry through to their role in the carbon cycle, and their possible contribution in past and present climate change. Carbon dioxide (CO2) hydrates also occur naturally on Earth in the Okinawa Trough, offshore Japan, and they could exist elsewhere in the solar system. Additionally, CO2 hydrates are being investigated for their potential to store large volumes of CO2 to reduce atmospheric emissions of greenhouse gases as a climate change mitigation strategy. Although research into hydrates has rapidly gained pace in more recent years their mineralogy and formation processes are still relatively poorly understood. Various imaging techniques have been used to study gas hydrates, such as Nuclear Magnetic Resonance; Magnetic Resonance Imaging; X-ray Computed Tomography and Scanning Electron Microscopy (SEM). We have investigated CO2 hydrates formed within the BGS laboratories, using a cryogenic-SEM. This investigation has produced various different hydrate morphologies resulting from different formation conditions. Morphologies range from well-defined euhedral crystals to acicular needles, and more complex, intricate forms. Cryogenic-SEM of these hydrates has yielded a wealth of information, and with further investigation of hydrate formed within different formation conditions we may begin to comprehend the complex growth mechanisms involved.

Hydrate-bearing sediments may contribute to the availability of energy resources, affect climate change, or cause seafloor instability. The comprehensive study of hydrate-bearing sediments documented in this manuscript includes physicochemical aspects of hydrate nucleation near mineral surfaces, the validity of THF as a substitute guest molecule for the study of hydrate-bearing sediments, and the effects of hydrate formation on the electromagnetic and the mechanical properties of various soils with a wide range of specific surface. Natural marine sediments are included as part of this investigation to explore the effects of inherent fabric, salts, organic matter, and stress history on the geophysical properties of hydrate-bearing sediments. Experiments are designed to reproduce the state of effective stress in the field at the time of hydrate formation. A comprehensive set of instruments is deployed in this study, and the unprecedented development of electrical resistivity tomography for the study of hydrate formation and dissociation is also documented in detail. Results from this research have important implications for geophysical field characterization and monitoring processes such as production.

Natural gas hydrates store a substantial portion of the Earth's organic carbon, although their occurrence is restricted by thermobaric boundaries and the availability of methane-rich fluids. The complexity of geological systems and the multiphase flow processes promoting hydrate formation can result in a mismatch between the predicted and the observed hydrate distribution. The purpose of this research is to achieve a better comprehension of the factors that influence the distribution of gas hydrates and the mechanism of fluid movements beneath and across the gas hydrate stability zone (GHSZ). Therefore, this study integrates seismic, petrophysical and geochemical data from different gas hydrate provinces. This work provides evidence that hydrates can occur below bottom-simulating reflectors, in the presence of sourcing thermogenic hydrocarbons. The relationship between fluid-escape pipes and gas hydrates is further explored, and pipe-like features are suggested to host a significant volume of hydrates. The host lithology also represents a critical factor influencing hydrate and free gas distribution and, in evaluating a natural gas hydrate system, needs to be considered in conjunction with the spatial variability in the methane supply. The three-dimensional distribution of gas hydrate deposits in coarse-grained sediments, representing the current target for hydrate exploration, is shown to be correlated with that of the underlying free gas zone, reflecting sourcing mechanisms dominated by a long-range advection. In such systems, the free gas invasion into the GHSZ appears controlled by the competition between overpressure and sealing capacity of the gas hydrate-bearing sediments. Globally, the thickness of the free gas zones is regulated by the methane supply and by different multi-phase flow processes, including fracturing, capillary invasion and possibly diffusion. In conclusion, this research indicates that geological, fluid flow and stability factors interweave at multiple scales in natural gas hydrate systems.

Afin de conditionner les déchets technologiques issus du démantèlement de l’Atelier de Vitrification de Marcoule au Commissariat à l’Energie Atomique et aux énergies alternatives (CEA), leur blocage dans une matrice cimentaire est envisagé. Dans ce contexte, l’effet des rayonnements ionisants issus des déchets nucléaires sur la matrice de confinement doit être examiné afin de garantir d’une part l’intégrité du colis, et d’autre part une production de gaz de radiolyse limitée. Ce travail de thèse s’intéresse au comportement sous irradiation gamma et électronique de trois types de matériaux cimentaires aux constituants différents et se focalise sur la production de gaz de radiolyse et sur l’évaluation de la résistance physique à travers l’observation des modifications structurales. Le sujet est traité par une double approche à la fois sur pâte de ciment et sur phases modèles, c’est-à-dire sur les hydrates constitutifs des pâtes de ciment synthétisés indépendamment. Il apparaît clairement que l’eau porale ainsi que les hydrates constitutifs contribuent à la production d’hydrogène radiolytique, avec une forte variation selon la nature des matériaux cimentaires. Ainsi, les ciments à base d’aluminates de calcium et les ciments phospho-magnésiens présentent un intérêt notable par rapport aux ciments silico-calciques usuels quant à la production d’hydrogène. Aux très fortes doses (plusieurs GGy) la résistance structurale sous irradiation électronique a été évaluée par diffraction de rayons X. Les hydrates constitutifs des trois familles de ciment étudiées présentent une bonne résistance structurale. Malgré la présence de variations dimensionnelles et microstructurales, ils ne s’amorphisent pas sous irradiation, ce qui s’avère positif en vue de l’application industrielle envisagée. / In order to treat the technological waste arising from the dismantling of the Marcoule Vitrification facility of the French Atomic Energy Commission (CEA), conditioning in a cement matrix is being put forward. Within this context, the impact of ionizing radiation produced by the nuclear waste on the confinement matrix ought to be investigated in order to ensure both the integrity of the package and the limitation of the radiolytic gas production. This thesis investigates the behavior of three types of cement compounds with distinct constituents under gamma and electronic radiation. This study deals with both the radiolytic gas production and the physical resistance of the materials using a structural modification examination. A double and complementary approach is used treating cement pastes and synthetic cement compounds together. It clearly appears that the pore water and the hydrates themselves both contribute to the radiolytic hydrogen production, with a significant variation depending on the nature of the materials. As far as radiolysis is concerned, calcium aluminate-based cements and magnesium phosphate cements are of considerable interest in comparison with the usual calcium silicate cements. At very high doses (GGy range), the structural resistance under electron irradiation was evaluated by X-ray diffraction. The constituent hydrates of the three cement types studied exhibit a good structural resistance. Despite the presence of dimensional variations at the unit cell scale as well as microstructural evolution, no amorphization is observed under irradiation, which is an interesting result with respect to the intended industrial application.

The work presented in this thesis deals with the employment of Schiff base ligands used to synthesise novel 3d-4f polynuclear coordination clusters (PCCs) and the investigation into their potential magnetic, luminescent and catalytic properties. Chapter one provides a general introduction to the chemistry described in the thesis. It includes a general overview of 3d-4f PCC chemistry and the applications of these materials and previous synthetic strategies for the preparation of Schiff base PCCs. A rationale is presented for the ligands employed in the thesis and a synthetic strategy is devised for the synthesis of specific materials. The initial chapters are focused on the synthesis of 3d-4f PCCs with novel core topologies and the study of their magnetic properties. Several novel series of 3d-4f PCCs are presented with unique core topologies which are previously unobserved in 3d-4f PCC chemistry. In addition, some of the presented PCCs display single-molecule magnet (SMM) properties or a significant magnetocaloric effect (MCE). Chapter five bridges synthetic aspects discussed in the previous chapters, with a synthetic study targeting 3d-4f PCCs with a defect dicubane core (2,3M4-1) and introduces the term “isoskeletal” to describe PCCs which possess the same topology or related organic structures with the same host framework but different guests. Chapters seven to nine are focused on the development of a well characterised isoskeletal family of 3d-4f PCCs with a defect dicubane core and the investigation of their potential catalytic properties in a range of organic reactions including Michael Addition, Friedel-Crafts alkylations and multicomponent reactions. Characterisation of the 3d-4f PCCs is emphasised and verifies the stability of the 2,3M4-1 core in solution. An attempt at understanding the catalytic system and mechanistic aspects is undertaken, which is not explored in previously reported 3d-4f PCC co-operative catalysis. Chapter ten provides an overall conclusion to the work presented in the thesis, whilst highlighting the contributions of this work to the reported literature.

The possibility of commercial natural gas production from gas hydrates has been tested by researchers and industry for more than ten years. Depressurisation of gas hydrates in porous and permeable sandstones has successfully produced water and natural gas. However long term sustainable production is still elusive. Catastrophic sand production into the wellbore has terminated at least three of the significant depressurisation trials including the 2013 trial at the Daini-Atsumi knoll, Nankai Trough, offshore Japan. Sand production is generally thought to be the result of mechanical and hydrodynamic instability, however it appears the failure mechanism is not the same for all reservoirs and the location of reservoir porosity and pressure on the normal compression line for sands could be a controlling factor. Sand production in reservoirs at shallow depths and low confining stresses (less than 10 MPa) are likely to be influenced by fluid flow effects like those described by the Shields (1936) diagram. The relative density of the formation may also affect the nature of the sand production in these reservoirs. The Daini-Atsumi knoll is a structural high on the outer ridge of the Kumano forearc basin, offshore Japan. Hydrate saturations of 50 to 80 % occur within three geological units of the Middle Pleistocene Ogasa group. This group is made up of deep water sediments including sediment gravity flow deposits distinguished by alternating silt and sand layers. The presence of these alternating layers could have influenced the sand production seen during the trial. This reservoir heterogeneity at the 2013 Daini-Atsumi knoll gas hydrate production trial site was characterised using the descriptions of geological units, analogues and statistical techniques. Scenarios of this heterogeneity were tested in a high pressure plane-strain sand production apparatus. The results of these tests suggest the boundary shear stress of the fluid on the grains is a significant control on sand production for the Daini-Atsumi Knoll reservoir and the layering and grainsize structure of the sediments encourages sand production. Relative density of the sediments appears to impact the nature of the sand production where denser sediments show more localised movement. These results indicate that even minor weaknesses in sand control devices will result in uncontrollable sand production rates from the Daini-Atsumi Knoll gas hydrate reservoir. Managing the fluid flow rate in the reservoir and selectively completing coarser grained zones at the base of sand layers could help limit sand production in future trials.

This thesis studies methane hydrate crystal formation and decomposition at 1.0, 4.0 and 7.0°C in a new apparatus. Hydrate was formed in the interstitial space of a variable volume bed of silica sand particles with an average diameter equal to 329μm (150 to 630μm range). The initial pressure inside the reactor was 8.0MPa for all the formation experiments. Three bed sizes were employed in order to observe the effects of the silica sand bed size on the rate of methane consumption (formation) and release (decomposition). The temperature at various locations inside the silica sand bed was measured with thermocouples during formation and decomposition experiments. For the decomposition experiments, two different methods were employed to dissociate the hydrate: thermal stimulation and depressurization. It was found that more than 74.0% of water conversion to hydrates was achieved in all hydrate formation experiments at 4.0°C and 1.0°C starting with a pressure of 8.0MPa. The dissociation of hydrate was found to occur in two stages when thermal stimulation was employed whereas three stages were found during depressurization. In both cases, the first stage was strongly affected by the changing bed size whereas it was not found to depend on the bed size afterwards.

Thesis (M.S. in E.A.S.)--School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 2004. Directed by Daniel Lizarralde. / Includes bibliographical references (leaves 69-73).

Preformulation studies of two naturally occurring compounds, sulforaphane and myricetin, are presented. Both compounds have shown promise as chemoprevention agents throughout the literature. Despite this evidence, minimal information is available to guide the progression of formulations designed for future drug development. The presented work describes solubility, stability, and solid-state characterization of these compounds. Additionally, a mathematical model based on the ideal solubility equation, which reasonably estimates the solubility of a hydrate is described. This model accounts for the dehydration energetics of the solute as it transforms from hydrate to anhydrous prior to melting and conversion to a hypothetical super-cooled liquid (HSL). This model will lend itself to the appreciation of the solubility differences that can exist between hydrate and anhydrous drug forms. By improving the accuracy of solubility estimation, drug development studies involving hydrates can be designed more accurately.