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Hydrate formation in natural environment Zatsepina, Olga

Abstract

A quantitative understanding of hydrate formation and dissociation in the sea floor requires detailed knowledge of the phase equilibrium between hydrate and sea water. An important factor is the concentration of dissolved gas in the pore fluid. The gas concentration required to form hydrate is examined by evaluating equilibrium concentration of dissolved gas (or solubility) when hydrate is present. The theoretical calculations predict how the solubility varies as a function of pressure, temperature and salinity. The calculations of gas solubility in sea water are used to predict the rate and distribution of hydrate formation and dissociation in the sea floor. When hydrate forms in the natural environment kinetics of the phase transition is important. Since hydrate formation occurs by means of nucleation and growth, an excess energy is required to overcome a nucleation barrier. The energy needed to initiate nucleation is expected to be bigger at low gas concentrations. Consequently, the formation of hydrate in marine sediments in the absence of bulk gas is sometimes questioned. Our experiments are the first to show that hydrate can form in porous media under realistic conditions when free gas is absent. The experimental results provide good evidence that hydrate should nucleate and grow from dissolved methane in fluids that migrate toward the sea floor from below. The concentration of gas required to form hydrate in the sea floor is found to be significantly lower than the concentration needed to form gas bubbles. Our experimental evidence for lower gas concentrations during hydrate formation imposes less stringent requirements on the biogenic sources of methane. Nucleation characteristics recovered from our experimental data are used to extrapolate to conditions in the sea floor. The results show that the position where hydrate is expected to nucleate occurs tens to hundreds of meters above the base of the stability zone. In summary, this study proves on thermodynamic ground that hydrate can form from solution. Second, it shows that although nucleation is the rate limiting step in hydrate formation, it should not impede the process in the ocean floor. Nucleation is expected when methane migrates from depth because the hydrate stability zones is deep enough to ensure that a sufficient overcooling can be achieved in the sediments. The study also suggests that local equilibrium in the immediate vicinity of hydrate crystals is readily established and that the kinetics of growth and dissociation is mainly limited by diffusion of gas when hydrate forms from aqueous solution.

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