A natural hydrate dissolution experiment on complex multi-component hydrates on the sea floor

• Published in: Geochimica et cosmochimica acta Vol. 73; no. 22; pp. 6747 - 6756
• Main Authors: Hester, K.C., Peltzer, E.T., Walz, P.M., Dunk, R.M., Sloan, E.D., Brewer, P.G.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15-11-2009
• Subjects: Correlation; Dissolution; Fabrics; Fittings; Hydrates; Marine; Mass transfer; Mathematical analysis; Natural gas; Sea beds
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2009.08.007

Dissolution of natural hydrate cores was measured using time-lapse photography on the seafloor at Barkley Canyon (850
m depth and 4.17
°C). Two types of hydrate fabrics in close contact with one another were studied: a “yellow” hydrate stained with condensate oil and a “white” hydrate. From thermogenic origins, both fabrics contained methane as well as heavier hydrocarbons. These multi-component hydrates were calculated to be well within
p–
T stability conditions (<200
m water depth needed at 4.17
°C). While stable in pressure and temperature, the hydrates were bathed in under-saturated seawater, which promoted dissolution. The flux of gas from the shrinking yellow hydrate core was 0.15
±
0.01
mmol
gas/m
2
s, while the white hydrate dissolved faster at 0.25
±
0.02
mmol
gas/m
2
s. To determine the controlling mechanism for the observed changes in the hydrate cores, experimental results were compared with an engineering correlation for convective mass transfer. Using water velocity as a fitting parameter, the correlation agreed well with results from a previous dissolution experiment on well-characterized synthetic hydrates. Even with a number of other unknowns, when applied to the natural hydrate, the mass transfer correlation predicted the dissolution rate within 20%. This seafloor-based experiment, along with visual observations of seafloor hydrate dissolution over a 3-day period, were used to further understand the fate of natural seafloor hydrates exposed on the seafloor. By showing that mass transfer is the rate-controlling mechanism for dissolution of these natural hydrate outcrops, proper hydrodynamic calculations can be employed to give a refined estimate on hydrate dissolution rates.