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OS1C-6：DEEP SEA FIELD TEST OF THE CH4 HYDRATE TO CO2 HYDRATE SPONTANEOUS CONVERSION HYPOTHESIS
发布时间:2014-07-28
Peter G. BREWER1, Edward T. PELTZER1, Peter M. WALZ1, Stephen H. KIRBY2, Laura A. STERN2, and John PINKSTON2
1. Monterey Bay Aquarium Research Inst., USA; 2. U.S. Geological Survey, USA
We have carried out a deep-sea field test of the hypothesis that CH4 gas can be spontaneously produced from CH4 hydrate by injection of CO2, thereby inducing guest swapping of the encaged molecules and formation of a solid CO2 hydrate with no associated liquid water production. We transported a solid cylinder of pure CH4 hydrate, contained within a pressure vessel, to the sea floor at 688m depth off shore Monterey, California using the ROV Ventana. Upon opening the pressure vessel with the vehicle robotic arm we emplaced the hydrate specimen on a metal stand and covered this with a glass cylinder full of a 25% CO2: 75% N2 gas mixture, thereby fully displacing the surrounding sea water (T= 4.92°C). The initial volume of the hydrate was 87.5 cm3 and the glass chamber contained 2.132 L gas after correcting for the displacing volume of hydrate. The experiment was thus at constant T and P, and any volume changes were compensated for by seepage under the cylinder base. We followed the course of the reaction by laser Raman spectroscopy with a stand-off optic, focusing the beam alternately on the gas phase, and on the solid hydrate surface, and recorded the spectra. We quickly observed ingrowth of CH4 into the gas phase, and thus dilution of the initial CO2 + N2 condition. Visually we observed gradual wetting and progressive thinning of the CH4 hydrate, with slight downward flow of the surface water phase. The experiment was followed for 2.5 hours on Day 1, and then 20 hours later on returning to the site. The sublimation of CH4 into the gas phase created a complex changing ternary gas mixture with a significant decline of ~ 50% in the rate of ingrowth over a 2.5 hour period. The spectral response was calibrated by laboratory comparison with prepared ternary gas mixtures. The gas phase composition at the end of the experiment, with all solid CH4 sublimed, was 70.6% N2: 21.6% CO2: 7.8% CH4. We found no evidence of the formation of a solid CO2, or CO2/N2 solid hydrate phase. We did observe rapid sublimation of the CH4 hydrate with release of liquid water from cage breakdown. Furthermore the ~ 10x greater solubility of CO2 over N2 and CH4 in the produced water that wetted the surface of the hydrate, and the increase in density this creates, strongly suggests that downward flow of CO2 enriched water will occur in any large scale experiment, removing CO2 from the gas reaction space. Phase equilibrium calculations strongly suggest that even if the integrity of the initial N2:CO2 displacement gas is maintained then a new hydrate equilibrium condition will form with only about 20% CH4 in the gas phase under the PT conditions used in this test.
1. Monterey Bay Aquarium Research Inst., USA; 2. U.S. Geological Survey, USA
We have carried out a deep-sea field test of the hypothesis that CH4 gas can be spontaneously produced from CH4 hydrate by injection of CO2, thereby inducing guest swapping of the encaged molecules and formation of a solid CO2 hydrate with no associated liquid water production. We transported a solid cylinder of pure CH4 hydrate, contained within a pressure vessel, to the sea floor at 688m depth off shore Monterey, California using the ROV Ventana. Upon opening the pressure vessel with the vehicle robotic arm we emplaced the hydrate specimen on a metal stand and covered this with a glass cylinder full of a 25% CO2: 75% N2 gas mixture, thereby fully displacing the surrounding sea water (T= 4.92°C). The initial volume of the hydrate was 87.5 cm3 and the glass chamber contained 2.132 L gas after correcting for the displacing volume of hydrate. The experiment was thus at constant T and P, and any volume changes were compensated for by seepage under the cylinder base. We followed the course of the reaction by laser Raman spectroscopy with a stand-off optic, focusing the beam alternately on the gas phase, and on the solid hydrate surface, and recorded the spectra. We quickly observed ingrowth of CH4 into the gas phase, and thus dilution of the initial CO2 + N2 condition. Visually we observed gradual wetting and progressive thinning of the CH4 hydrate, with slight downward flow of the surface water phase. The experiment was followed for 2.5 hours on Day 1, and then 20 hours later on returning to the site. The sublimation of CH4 into the gas phase created a complex changing ternary gas mixture with a significant decline of ~ 50% in the rate of ingrowth over a 2.5 hour period. The spectral response was calibrated by laboratory comparison with prepared ternary gas mixtures. The gas phase composition at the end of the experiment, with all solid CH4 sublimed, was 70.6% N2: 21.6% CO2: 7.8% CH4. We found no evidence of the formation of a solid CO2, or CO2/N2 solid hydrate phase. We did observe rapid sublimation of the CH4 hydrate with release of liquid water from cage breakdown. Furthermore the ~ 10x greater solubility of CO2 over N2 and CH4 in the produced water that wetted the surface of the hydrate, and the increase in density this creates, strongly suggests that downward flow of CO2 enriched water will occur in any large scale experiment, removing CO2 from the gas reaction space. Phase equilibrium calculations strongly suggest that even if the integrity of the initial N2:CO2 displacement gas is maintained then a new hydrate equilibrium condition will form with only about 20% CH4 in the gas phase under the PT conditions used in this test.
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