Natural vents in the Gulf of Mexico are actively expelling water and hydrocarbons. They are ubiquitous along continental margins, and I characterize a single vent in the Ursa Basin at leaseblocks MC852/853. Seismic data reveal that the vent is elevated ~75 meters above the seafloor and is roughly circular with a ~1.2 km diameter. A transparent zone centered underneath the vent extends to ~1500 meters below seafloor; this zone is commonly interpreted to record the presence of gas. There is a strong negative polarity seismic reflection that rises rapidly at the vent’s boundaries and is horizontal within a few meters of the seafloor beneath the vent edifice. I interpret that this reflection records a negative impedance contrast, marking the boundary between hydrate and water above and free gas and water below: it is the bottom-simulating reflector. Salinities beneath the vent increase from seawater concentrations to >4x seawater salinity one meter below seafloor. Temperature gradients within the vent are ~15x the background geothermal gradient. I model the coexistence of high salinity fluids, elevated temperature gradients, and an uplifted bottom-simulating reflector with two approaches. First, I assume that high salinity fluids are generated by dissolution of salt bodies at depth and that these hot, saline fluids are expelled vertically. Second, I model the solidification of gas hydrate during upward flow of gas and water. In this model, free gas combines with water to form hydrate: salt is excluded and heat is released, resulting in the generation of a warm, saline brine. The two models result in predictable differences of salinity and temperature. A better understanding of the hydrogeological processes at vent zones is important for quantifying the fluxes of heat and mass from submarine vents and is important for understanding the conditions under which deep-sea biological vent communities exist. / text

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Methane hydrates are naturally-occurring ice-like substances found in permafrost and in ocean sediments along continental shelves. These compounds are often the source of cold seeps—plumes which vent methane into aquatic environments, and may subsequently release the potent greenhouse gas into the atmosphere. Methane hydrates and methane gas seeps are of particular interest both for their potential as an energy source and for their possible contribution to climate change. In an effort to improve location of hydrates through the use of seismic surveys and echo-sounding technology, this work aims to describe the low-frequency (10 Hz to 10 kHz) acoustic behavior of methane gas bubbles and methane hydrates in water under simulated ocean-floor conditions of low temperatures and high pressures. Products of the experiments and analysis presented in this thesis include (a) passive acoustic techniques for measurement of gas flux from underwater seeps, (b) a modified form of Wood's model of low-frequency sound propagation through a bubbly liquid containing real gas, and (c) low-frequency measurements of bulk moduli and dissociation pressures of four natural samples of methane hydrates. Experimental procedures and results are presented, along with analytical and numerical models which support the findings. / text

It is well known that methane hydrate undergoes several phase transformations at high pressures. At room temperature and low to moderate pressure, methane and water form a stable cubic structure I (sI) hydrate that is also known as MH-I. The structure is transformed to a hexagonal phase (sH/MH-II) above 1.0GPa. Another phase transformation occurs above 1.9GPa where the filled ice structure (MH-III) is stable up to 40 GPa before a new high pressure phase transition occurs. Experiments at such high pressures have to be performed in a diamond anvil cell (DAC). Our main interest, though, is to form sH methane hydrate at a lower pressure than reported in previous studies but with some methane in the large cages consequently increasing the methane content. This can be accomplished by introducing the molecules of the large hydrate forming substance (tert-butyl methyl ether/TBME) at a concentration slightly below the stoichiometric amount as suggested by molecular dynamics simulations. In this study we have synthesized mixed methane hydrate of sI and sH and loaded the clathrate with methane into several DACs. Raman spectra were collected at room temperature and pressures in the range of 0.1 to 11.3 GPa. The existence of sH methane hydrate was observed down to 0.2 GPa. However, the existence of methane in the large cages was visible only at pressure higher than 1.0 GPa. The excess methane in the system apparently destabilizes the sH clathrate at pressure below 1.0 GPa as it transforms to sI clathrate.

Estimating the in situ mechanical properties of sediments containing gas hydrates from seismic or log data is essential for evaluating the risks posed by mechanical failure during drilling, completions, and producing operations. In this paper, a method is presented for constructing correlations between the mechanical properties of gas hydrate bearing sediments and geophysical data. A theory based on micromechanics models was used to guide the selection of parameters that govern the physical behavior of sediments. A set of nondimensionalized relations between elastoplastic properties and those that could be inferred from log or seismic data was derived. Using these relations, a correlation for the Young’s modulus was constructed for sands with methane and THF hydrate using data from a wide variety of sources. It was observed that the correlation did not fit data obtained from samples with high THF hydrate saturations, due possibly to the existence of cohesive mechanisms that operate in such regimes.

Recently, by performing molecular dynamics simulations in the methane-water system, we have measured the static lifetimes of cagelike water clusters (CLWC) immersed in bulk liquid water, during which the member-water molecules of CLWCs are not allowed to exchange with their surrounding water molecules [J. Phys. Chem. C, 2007, 111, 2595]. In this study, we measure the dynamic lifetimes of CLWCs with permitting such water exchanges. It is found that the dynamic lifetimes of CLWCs are not less than the static lifetimes previously obtained, and their ratio increases with the lifetime values. The results strengthen that CLWCs are metastable structures in liquid water and the occurrence probability of long-lived CLWCs will increase if one uses the dynamic lifetimes instead of the static lifetimes. The implications of this study for hydrate nucleation are discussed.

The use of Kinetic Hydrate Inhibitors (KHI) is one of the optimum methods employed to control gas hydrate formation issues and provide flow assurance in oil and gas production systems. The application of this technology has several advantages to operators, including significant cost savings and extended life of oil and gas systems. This paper will highlight a specific case where a Major operator in the North Sea (UK sector) significantly reduced the cost of well intervention operations by applying a KHI in a subsea gas lift line. Considerable cost savings were realized by reducing volume of chemical required and this enabled the application to be performed from the FPSO eliminating the need for a dedicated Diving Support Vessel (DSV). Furthermore, the application of KHI also reduced manual handling and chemical logistics usually associated with this particular treatment. In order to prevent mineral scale deposition occurring in downhole tubing and near well bore and in the formation; scale inhibitor squeeze applications are standard practice. For subsea wells the fluids can be pumped down in to the well via gas lift lines. However, upon completion of previous scale squeeze operations at this particular location, hydrate formation was observed when a mixture of MEG and water was used following interventions via the gas lift line. By applying 1% KHI with a mixture of MEG and Water, the well was brought back into production following scale squeeze operations without hydrate formation occurring.

Water column methane measurements have been used to understand both the global distribution of methane in the oceans and the local flux of methane from geologic sources on the continental margins, including methane vents and gas-hydrate-bearing sites. We have measured methane concentrations in 1607 water samples collected along the central California continental margin. Methane supersaturation of the surface mixed layer (0-50 msbsl) is widespread and above a well-defined subsurface particle maximum (~50 mbsl) that generally corresponds with the pycnocline. Local production of methane appears to be occurring in the surface mixed layer above the particle maximum and may not be particle-associated. Methane concentrations in water column CTD cast profiles and ROV-collected bottom waters obtained in Partington, Hueneme, Santa Monica, and Redondo submarine canyons increase towards the seafloor and are distinctly higher (up to 186 nM) compared to open-slope and shelf waters at similar depths. These values are in excess of measured surface water methane concentrations and could not be generated by mixing with surface water. Elevated methane concentrations in these submarine canyons and persistent mid-water methane anomalies in Ascension and Ano Nuevo Canyons could result from restricted circulation and/or proximity to gas vents, seafloor exposure of methane gas hydrates, recently-eroded methane-rich sediment, submarine discharge of methane-rich groundwater, or particle-associated methane production. On the Santa Barbara shelf water column methane profiles near known gas vents also increase in concentration with increasing depth. Thus, elevated bottom water methane concentrations observed in submarine canyons may not be diagnostic of proximity to methane vents and may be caused by other processes.

In this paper we report a low symmetry version of structure H hydrate that results from the hexagonal form on cooling below 167 K. Phase changes with temperature in the common clathrate hydrates structural families I, II and H have not been observed before, except in doped systems where ordering transitions take place or in the structure I hydrate of trimethylene oxide where the guest molecule dipoles are known to order. Since there is an inverse relationship between the effect of temperature and pressure on ices, it may well be that the low symmetry form reported at low temperature can also be reached by applying high pressure, and that in fact some of the observed high pressure phases are lower symmetry versions of hexagonal sH.

Previously we have suggested the concept of tuning hydrate compositions which makes it possible to increase the gas storage capacity of binary hydrates. Herein, we report for the first time the existence of a critical guest concentration (CGC) and establish the complete tuning pattern that appears to exist in binary hydrates, including the water-soluble hydrate formers (promoters) and water insoluble guests,. The first attempt to verify the new features of clathrate hydrate compositions is executed on the binary hydrate of CH4 + THF and involves a detailed examination of the guest distribution by spectroscopic methods. THF molecules by themselves form sII hydrate from a completely miscible aqueous solution, and in this structure, because of their size, THF molecules occupy only the large 51264 cages. The CGC value appears to depend largely on the chemical nature of the liquid guest component participating in the binary hydrate formation. The present experimental findings on the existence of critical guest concentration and the complete tuning phenomenon can be expected to make a meaningful contribution to both inclusion chemistry and a variety of hydrate-based fields.