Anomalous preservation of pure methane hydrate at 1 atm

Journal of Physical Chemistry B

Volumn 105, Issue 9, 2001, Pages 1756-1762

  Stern, Laura A ^a   Circone, Susan ^a   Kirby, Stephen H ^a   Durham, William B ^b  

^a U S GEOLOGICAL SURVEY   (United States)

^b LAWRENCE LIVERMORE NATIONAL LABORATORY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

DECOMPOSITION; DISSOCIATION; METHANE; NATURAL GAS; POLYCRYSTALLINE MATERIALS; PRESSURE EFFECTS; THERMAL EFFECTS;

METHANE HYDRATES;

GAS HYDRATES;

EID: 0035263559     PISSN: 10895647     EISSN: None     Source Type: Journal    
DOI: 10.1021/jp003061s     Document Type: Article

References (30)

1. Kvenvolden, K. Rev. Geophys. 1993, 31, 173.
2. Sloan, E. Clathrate Hydrates of Natural Gases, 2nd ed; Marcel Dekker: New York, 1998.
3. Lunine, J.; Stevenson, D. Astrophys. J. Suppl. 1985, 58, 493.
4. Borg, I.; Stephens, D.; Bedford, R.; Hill, R. Evaluation of research opportunities in gas hydrates, Lawrence Livermore National Laboratory Report UCID-19755, 1983.
5. Kvenvolden, K. Global Biogeochem. Cycles 1988, 2, 221.
6. Kvenvolden, K. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 3420.
7. Paull, C.; Buelow, W.; Ussler, W.; Boroski, W. Geology 1996, 24, 143.
8. Kayen, R.; Lee, H. Mar. Geotech. 1991, 10, 125.
9. Kennett, K.; Cannariato, K.; Hendy, I.; Behl, R. Science 2000, 288, 128.
10. Stern, L.; Kirby, S.; Durham, W. Science 1996, 273, 1843.
11. Stern, L.; Kirby, S.; Durham, W. Energy Fuels 1998, 12, 201.
12. Stern, L.; Hogenboom, D.; Durham, W.; Kirby, S.; Chou, I.-M. J. Phys. Chem. B 1998, 102, 2627.
13. 2O, where n = 5.89 ± 0.01. This composition is slightly closer to ideal than that which we reported previously (6.1 ± 0.1, on samples that contained 0-3% unreacted ice; see refs 10 and 11), due to significantly improved analytical and measurement capabilities provided by our gas flow meter and collection apparatus (see ref 14) and by the internal (sample) thermocouples that now allow detection of very small amounts of unreacted ice. Samples are also now routinely held at the highest P-T conditions during synthesis for several hours longer than in previous syntheses, to ensure reaction of the last several percent of ice to hydrate. (The difference between n of 5.89 vs 6.1 in our samples corresponds to 3.2 vol % unreacted ice.) X-ray diffraction analyses of as-grown material, and reflection microscopy of sample surface replicas indicated a lack of any preferred grain orientation or grain size heterogeneity in the final as-synthesized product (see refs 10 and II).
14. 3/min at the slowest rates, and gas mass measurements reported here are accurate to better than 1%.
15. Circone, S.; Stern, L.; Kirby, S.; Pinkston, J.; Durham, W. In Gas Hydrates; Challenges for the Future; Holder, G., Bishnoi, P., Eds.; 2000; pp 544-556.
16. In early tests, release of trapped pore gas was occasionally detected by the flow meter during the isothermal hold at 190 K and 0.1 MPa prior to heating the sample for dissociation. This release was measured as small but discrete decreases in the load cell weight, usually accompanied by audible crackling of the sample within the pressure vessel. The presence of trapped gas within closed-off pores prior to stoichiometry measurement was eliminated from later experiments by either performing the initial depressurization step at a faster rate (Figure 1B, points 2→3), or by significantly reducing the compaction of the seed ice during sample preparation procedures to produce a more porous final product. Likewise, no evidence was observed to indicate that any measurable adsorbed methane on the surface of the hydrate grains interfered with the stoichiometry measurements and calculations; such methane should desorb during the low-pressure isothermal hold prior to measurement, and no indication of such methane was detected by the flow meter as a baseline drift.
17. The specific time interval of the rapid pressure release is varied with the magnitude of the initial pressure overstep of the equilibrium curve (see Figure 1B); smaller oversteps require more rapid pressure release, but venting in less than 5 s was found to be less optimal for successful preservation.
18. Typically, 100 ± 3% of the expected total gas yield (based on stoichiometry number n = 5.89) was collected by the finish of each rapid-depressurization test after heating through 273 K. In several instances the error margin increased to ±5%, due to excessive loss of some hydrate-forming gas during the initial venting procedure (resulting in a low yield). or due to small amounts of trapped pore gas being included in the measurement (giving a high yield). The general topology of the flow curves usually made it apparent when the latter was the case, as release of trapped gas typically evolves in pulses, rather than continuous flow.
19. 2O ice, indicating that (1) preserved material was not a new phase that is greatly dissimilar from si hydrate, (2) the ice product formed during the early dissociation event does not occur homogeneously throughout the sample (on a grain-by-grain basis), but rather on a sample-wide scale, probably initiating at the vented end of the sample, and (3) that no other crystalline or amorphous phases were present at a level exceeding a few volume percent.
20. Buffett and Zatsepina (Buffett, B.; Zatsepina, O. Geophys. Res. Lett. 1999, 26, 2981), for instance, suggest that gas hydrate in marine environments may persist metastably to several degrees above its nominal equilibrium P-T stability field, due to maintenance of a high fugacity of the hydrate-forming gas in the surrounding liquid phase. In their model, the barrier to complete dissociation is largely dependent on the free energy requirements for nucleation of small bubbles in such a supersaturated solution, to create a vapor phase.
21. These pressure-release tests were conducted on pure methane hydrate samples that contained 40-45% pore volume (compared to 29% of standard samples), and on methane hydrate + quartz sand aggregates that were synthesized either as discrete, alternating layers of hydrate or sand, or mixed homogeneously. These samples were all tested at 268.5 ± 1.5 K, but displayed dissociation behavior very similar to those tests shown in Figure 4 between 255-265 K.
22. Yakushev, V.; Istomin, V. In Physics and Chemistry of Ice; Maeno, N., Hondoh, T., Eds.; Hokkaido University Press: Sapporo, Japan, 1992; pp 136-139.
23. Handa, Y.; Stupin, D. J. Phys. Chem. 1992, 96, 8599.
24. Peters, D.; Selim, M.; Sloan, E. In Gas Hydrates; Challenges for the Future; Holder, G., Bishnoi, P., Eds.; pp 304-313.
25. Davidson, D.; Garg, S.; Gough, S.; Handa, Y.; Ratcliffe, C.; Ripmeester, J.; Tse, J. Geochim. Cosmochim. Acta 1986, 50, 619.
26. Handa, Y. J. Chem. Thermodyn. 1986, 18, 891.
27. Gudmundsson, J.; Parlaktuna, M.; Khokhar, A. SPE Prod. Facilities 1994, 69.
28. Dallimore, S.; Collett, T. Geology 1995, 23, 527.
29. Dash, J.; Fu, H.; Wettlaufer, W. Rep. Prog. Phys. 1995, 58, 115.
30. 20 further suggest that gas hydrate metastability may be implicated in such issues, due to sudden decomposition of the metastable material producing substantial pore pressure in the sediments. We note, however, that the mechanism and geochemical requirement for metastability discussed by Buffett and Zatsepina is markedly different from the anomalous preservation effect discussed here.