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Methane hydrate nonstoichiometry and phase diagram

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Abstract

A previous study discovered that, when formed from different ethylene oxide (EO) feed solutions, hydrates had different small-cage occupancies with essentially total occupation of the large cavities of sI hydrate. An EO + H2O isobaric phase diagram was proposed with a solid solution range to explain this phenomenon. Since methane and CO2 also occupy both small cages and large cages of sI hydrate, new phase diagrams were proposed which allowed the methane or CO2 hydrate composition to vary as a function of feed composition at fixed p-T conditions. Raman spectroscopy experiments now show that H2O + CH4 hydrates formed at the vapor–liquid interface have a different composition than dendrites growing into the water below the interface. Interfacial hydrates were typical laboratory samples formed with excess gas, while dendrites coexisted with excess water and were similar to seafloor hydrate. Integrated peak intensities were used to calculate relative cage occupancies in Raman data analysis. Interfacial hydrates had an average of 95.4% small cages occupied at 30 MPa and 275.15 K, while dendrite hydrates had only 82.8% small-cage occupancy at the same conditions. This result suggests that hydrates in the ocean floor may have lower methane concentrations than hydrates formed under normal laboratory conditions at the same pressure and temperature.

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