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No / A crystalline solid called a gas hydrate has gas molecules surrounded by water molecules. There are several gases with suitable structures for the production of hydrates, but methane-rich gas hydrates are more common and form in seas and on the ocean. The place of hydrates formation is usually the sediment of the ocean floor and the polar regions, which largely covered with ice. It is also found in large quantities in combination with ambient ice in the ever-frozen polar regions. The importance of gas hydrates is due to the great ability of gas hydrates in natural gas storage, which makes it attractive to use them for the purposes of storing and transporting natural gas and other gases as a competitor to liquefaction and condensing methods. Due to the significance potential of these reserves as the world's future energy supplier and their direct impact on changes in climate conditions due to the greenhouse effect of methane, as well as their geological risks during water hydrocarbon discoveries, marine science researchers have been studying them over the past few years. Acoustic and seismic methods are helpful instruments for measuring subterranean hydrated reserves because there is not the technology to measure hydrated reserves directly.

The aim of this thesis is to develop a set of intermolecular potentials that enable the study of nucleation or decomposition of methane hydrates. The potentials are developed for water, methane, and the water-methane pair by fitting to SAPT(DFT) reference energies. The first set of potentials developed differ from recent polarisable models in that they have rank 4 ISA multipoles, rank 3 anisotropic polarisabilities, rank 3 isotropic dispersion, and anisotropic exchange-repulsion terms. These potentials are validated based on the structures and energies of small clusters and second virial coefficients. The potentials are then significantly simplified for use in MD simulations with DL_POLY4. Simplifying the methane potential makes it difficult to fit simultaneously the global minimum dimer and a set of randomly generated dimers used as reference energies. Several methods are tested to account for polarisation in water within the limitations of DL_POLY and it is found that for MD simulations good results can be attained by increasing the charge values to match the multipole moments of a water molecule in a dielectric. Simulations are carried out for liquid water, ice Ih, and methane gas to validate the new models. The models developed are compared in MD simulations with TIP4P/Ice and the United Atom Methane (UAM) model in simulations of sI methane clathrate; both under stable conditions and while undergoing decomposition at different temperatures. It is found that the melting behaviour differs according the methane- and water-methane interactions; the behaviour of methane under clathrate decomposition using either methane model is discussed.

Natural gas hydrate, a type of inclusion compound or clathrate, are composed of gas molecules trapped within a cage of water molecules. The presence of gas hydrate has been confirmed by core samples recovered from boreholes. Interests in the distribution of natural gas hydrate stem from its potential as a future energy source, geohazard to drilling activities and their possible impact on climate change. However the current geophysical investigations of gas hydrate reservoirs are still too limited to fully resolve the location and the total amount of gas hydrate due to its complex nature of distribution. The goal of this thesis is twofold, i.e., to model (1) the heterogeneous gas hydrate reservoirs and (2) seismic wave propagation in the presence of heterogeneities in order to address the fundamental questions: where are the location and occurrence of gas hydrate and how much is stored in the sediments. Seismic scattering studies predict that certain heterogeneity scales and velocity contrasts will generate strong scattering and wave mode conversion. Vertical Seismic Profile (VSP) techniques can be used to calibrate seismic characterization of gas hydrate expressions on surface seismograms. To further explore the potential of VSP in detecting the heterogeneities, a wave equation based approach for P- and S-wave separation is developed. Tests on synthetic data as well as applications to field data suggest alternative acquisition geometries for VSP to enable wave mode separation. A new reservoir modeling technique based on random medium theory is developed to construct heterogeneous multi-variable models that mimic heterogeneities of hydrate-bearing sediments at the level of detail provided by borehole logging data. Using this new technique, I modeled the density, and P- and S-wave velocities in combination with a modified Biot-Gassmann theory and provided a first order estimate of the in situ volume of gas hydrate near the Mallik 5L-38 borehole. Our results suggest a range of 528 to 768×10^6 m^3/km^2 of natural gas trapped within hydrate, nearly an order of magnitude lower than earlier estimates which excluded effects of small-scale heterogeneities. Further, the petrophysical models are combined with a 3-D Finite Difference method to study seismic attenuation. Thus a framework is built to further tune the models of gas hydrate reservoirs with constraints from well logs other disciplinary data.

Natural gas hydrate, a type of inclusion compound or clathrate, are composed of gas molecules trapped within a cage of water molecules. The presence of gas hydrate has been confirmed by core samples recovered from boreholes. Interests in the distribution of natural gas hydrate stem from its potential as a future energy source, geohazard to drilling activities and their possible impact on climate change. However the current geophysical investigations of gas hydrate reservoirs are still too limited to fully resolve the location and the total amount of gas hydrate due to its complex nature of distribution. The goal of this thesis is twofold, i.e., to model (1) the heterogeneous gas hydrate reservoirs and (2) seismic wave propagation in the presence of heterogeneities in order to address the fundamental questions: where are the location and occurrence of gas hydrate and how much is stored in the sediments. Seismic scattering studies predict that certain heterogeneity scales and velocity contrasts will generate strong scattering and wave mode conversion. Vertical Seismic Profile (VSP) techniques can be used to calibrate seismic characterization of gas hydrate expressions on surface seismograms. To further explore the potential of VSP in detecting the heterogeneities, a wave equation based approach for P- and S-wave separation is developed. Tests on synthetic data as well as applications to field data suggest alternative acquisition geometries for VSP to enable wave mode separation. A new reservoir modeling technique based on random medium theory is developed to construct heterogeneous multi-variable models that mimic heterogeneities of hydrate-bearing sediments at the level of detail provided by borehole logging data. Using this new technique, I modeled the density, and P- and S-wave velocities in combination with a modified Biot-Gassmann theory and provided a first order estimate of the in situ volume of gas hydrate near the Mallik 5L-38 borehole. Our results suggest a range of 528 to 768×10^6 m^3/km^2 of natural gas trapped within hydrate, nearly an order of magnitude lower than earlier estimates which excluded effects of small-scale heterogeneities. Further, the petrophysical models are combined with a 3-D Finite Difference method to study seismic attenuation. Thus a framework is built to further tune the models of gas hydrate reservoirs with constraints from well logs other disciplinary data.

Gas hydrates are a well-known problem in the oil and gas industry and cost millions of dollars in production and transmission pipelines. To prevent this problem, it is important to predict the temperature and pressure under which gas hydrates will form. Of the thermodynamic models in the literature, only a couple can predict the hydrate-formation temperature or pressure for complex systems including inhibitors. I developed two simple correlations for calculating the hydrate-formation pressure or temperature for single components or gas mixtures. These correlations are based on over 1,100 published data points of gas-hydrate formation temperatures and pressures with and without inhibitors. The data include samples ranging from pure-hydrate formers such as methane, ethane, propane, carbon dioxide and hydrogen sulfide to binary, ternary, and natural gas mixtures. I used the Statistical Analysis Software (SAS) to find the best correlations among variables such as specific gravity and pseudoreduced pressure and temperature of gas mixtures, vapor pressure and liquid viscosity of water, and concentrations of electrolytes and thermodynamic inhibitors. These correlations are applicable to temperatures up to 90ºF and pressures up to 12,000 psi. I tested the capability of the correlations for aqueous solutions containing electrolytes such as sodium, potassium, and calcium chlorides less than 20 wt% and inhibitors such as methanol less than 20 wt%, ethylene glycol, triethylene glycol, and glycerol less than 40 wt%. The results show an average absolute percentage deviation of 15.93 in pressure and an average absolute temperature difference of 2.97ºF. Portability and simplicity are other advantages of these correlations since they are applicable even with a simple calculator. The results are in excellent agreement with the experimental data in most cases and even better than the results from commercial simulators in some cases. These correlations provide guidelines to help users forecast gas-hydrate forming conditions for most systems of hydrate formers with and without inhibitors and to design remediation schemes such as: · Increasing the operating temperature by insulating the pipelines or applying heat. · Decreasing the operating pressure when possible. · Adding a required amount of appropriate inhibitor to reduce the hydrateformation temperature and/or increase the hydrate-formation pressure.

Univ., Diss.--Würzburg, 1998.

Bacillus subtilis capable of producing surfactin was cultured to evaluate effects of microbial cell mass on natural gas hydrate formation, dissociation, and stability characteristics. The direct molecular influences of microbial cell wall polymers inhibited gas hydrate formation significantly, decreased hydrate formation rates, and increased dissociation rates. Upon the introduction of bentonite, significant synergy was observed in the system in the form of a catalytic effect. Microbes cultured from seafloor seawater-saturated sediments collected from Mississippi Canyon 118 (MC-118) produced similar effects and generalized the observed trends. MC-118 cultures also produced biosurfactant in several culture media, which was shown to catalyze natural gas hydrate formation in porous media. Microorganisms inhabit gas hydrate macrostructures and consume hydrocarbons and other substrates from within. Sulfate reduction and anaerobic hydrocarbon oxidation occurred within gas hydrate during incubations with MC-118 indigenous consortia. A mathematical model was developed to explore the diffusion-reaction implications in massive seafloor gas hydrates.

Cette étude doctorale a consisté à décrire la dynamique géochimique de deux pockmarks à hydrates de gaz de la marge africaine en considérant deux approches différentes. La première zone d’étude, appelée Preowei, est située au large du Nigéria. Elle est caractérisée par un grand nombre de pockmarks de tailles différentes, plus ou moins proche les uns des autres. Les analyses géochimiques des échantillons de fluides interstitiels, combinées aux données géophysiques (séismiques) ont permis de mieux comprendre le schéma de migration des hydrocarbures pour un ensemble composé de quatre pockmarks très rapprochés. L’utilisation de ces données géochimiques dans un modèle de transport- réaction a conduit à une datation de plusieurs séquences de libération de gaz au sein de ces structures. Un schéma conceptuel décrivant les processus de formation et d’évolution temporelle des pockmarks a été proposé pour synthétiser les conclusions obtenues. Finalement, cette étude a montré que l’ensemble des pockmarks étudiés sont actifs depuis 2700 ans, qu’ils sont en phase de formation d’hydrates pour certains, et de carbonates pour d’autres. La deuxième structure étudiée est le pockmark Regab. Il est situé au large du Gabon, au nord du canyon sous-marin alimenté par le fleuve Congo. Il est caractérisé par la présence d’hydrates affleurant et une faune abondante et très variée sur toute sa surface. L’originalité de ce travail a été d’étudier la distribution de la mégafaune présente sur ce pockmark en fonction de la nature des fluides qui migrent dans le sédiment superficiel, et qui est libérés dans la colonne d’eau. Une attention particulière a été portée au méthane car c’est un élément central dans le cycle énergétique des microorganismes qui vivent en symbiose avec cette mégafaune. Trois nouveaux habitats ont été étudiés. Les données obtenues, associées à celle de la littérature ouverte, renforcent les conclusions des travaux antérieurs. Les Mytilidés ont besoin de très fortes concentrations de méthane pour se développer. Elles colonisent les zones de sortie de bulles et celle caractérisées par des hydrates affleurants. Les tapis bactériens sont associés à des zones où l’oxydation anaérobique du méthane se déroule dans le sédiment superficiel, avec une méthanogenèse dans la couche sous-jacente. Les Vésicomydé polychètes vivent dans des zones pauvres en méthane et sont très sensibles à sa variation de concentration. / The present work describes the dynamics of two pockmark areas, off West Africa. The intention is to propose two different approaches to study the relationships between fluid migration and pockmarks. The first investigated area corresponds to a pockmark cluster called Preowei, located off Nigeria. Geochemical analyses and modeling were combined with seismic data to detail the hydrocarbon migration pattern at this area, with implication on both the pockmark formation and the evolution of their morphology. The proposed interpretation seeks to identify the conceptual bases of pockmark evolution over time at this area. It is argued that the cluster has been active for at least 2700 years, and it is still at the stage of hydrate formation for some pockmarks and carbonate formation for other. The second investigated pockmark, called Regab, is located off Gabon. It is a giant pockmark of 800-m diameter, characterized by an ecosystem rich in fauna, with a large variety of living species. The main core of the work done on this pockmark was focused on finding a link between the fluid chemistry and the spatial distribution of the living communities which populate it. This was achieved by combining new geochemical and bathymetric results with a well-compiled dataset from the literature.

Methane derived authigenic carbonate (MDAC) precipitation occurs within marine sediments as a byproduct of the microbial anaerobic oxidation of methane (AOM). While these carbonates form in chemical and isotopic equilibrium with the fluids from which they precipitate, burial diagenesis and recrystallization can overprint these signals. Plane polarized light (PPL) and cathodoluminescent (CL) petrography have allowed for detailed characterization of carbonate phases and their subsequent alteration. Modern MDACs sampled offshore in northern Cascadia (n =33) are compared with paleoseep carbonates (n =13) uplifted on the Olympic Peninsula in order to elucidate primary vs. secondary signals, with relevance to interpretations of the carbonate record. The modern offshore environment (S. Hydrate Ridge and Barkley Canyon) is dominated by metastable acicular and microcrystalline aragonite and hi-Mg calcite (HMC) that with time will recrystallize to low-Mg calcite (LMC). The diagenetic progression is accompanied by a decrease in Mg/Ca and Sr/Ca ratios while variation in Ba/Ca depends upon the Ba-concentration of fluids that spur recrystallization. CL images discern primary carbonates with high Mn/Ca from secondary phases that reflect the Mn- enrichment that characterizes deep sourced fluids venting at Barkley Canyon. Methane along the Cascadia continental margin is mainly of biogenic origin, where reported strontium isotopic values reflect a mixture of seawater with fluids modified by reactions with the incoming Juan de Fuca plate. In contrast, the Sr-isotopic composition of carbonates and fluids from Integrated Ocean Drilling Program (IODP) Site U1329 and nearby Barkley Canyon point to a distinct endmember (lowest ⁸⁷Sr/⁸⁶Sr = 0.70539). These carbonates also show elevated Mn/Ca and δ¹⁸O values as low as -12‰, consistent with a deep-source of fluids feeding thermogenic hydrocarbons to the Barkley Canyon seeps. Two paleoseep carbonates sampled from the uplifted Pysht/Sooke Fm. have ⁸⁷Sr/⁸⁶Sr values similar to those of the anomalous Site U1329 and Barkley Canyon carbonates (⁸⁷Sr/⁸⁶Sr = 0.70494 and 0.70511). We postulate that the ⁸⁷Sr-depleted carbonates and pore fluids found at Barkley Canyon represent migration by the same type of deep, exotic fluid as is found in high permeability conglomerate layers down to 190 mbsf at Site U1329, and which fed paleoseeps in the Pysht/Sooke Fm. These exotic fluids likely reflect interaction with the 52-57 Ma igneous Crescent Terrane, which is located down-dip from both Barkley Canyon and Site U1329. This previously unidentified endmember fluid in northern Cascadia may have sourced cold seeps in this margin since at least the late Oligocene. / Graduation date: 2012