In-situ synchrotron X-ray computed microtomography with sub-micrometer voxel size was used to study the decomposition of gas hydrates in a sedimentary matrix. Xenon-hydrate was used instead of methane hydrate to enhance the absorption contrast. The microstructural features of the decomposition process were elucidated indicating that the decomposition starts at the hydrate-gas interface; it does not proceed at the contacts with quartz grains. Melt water accumulates at retreating hydrate surface. The decomposition is not homogeneous and the decomposition rates depend on the distance of the hydrate surface to the gas phase indicating a diffusion-limitation of the gas transport through the water phase. Gas is found to be metastably enriched in the water phase with a concentration decreasing away from the hydrate-water interface. The initial decomposition process facilitates redistribution of fluid phases in the pore space and local reformation of gas hydrates. The observations allow also rationalizing earlier conjectures from experiments with low spatial resolutions and suggest that the hydrate-sediment assemblies remain intact until the hydrate spacers between sediment grains finally collapse; possible effects on mechanical stability and permeability are discussed. The resulting time resolved characteristics of gas hydrate decomposition and the influence of melt water on the reaction rate are of importance for a suggested gas recovery from marine sediments by depressurization.
The
exchange process between CO2 and methane hydrate
has been observed in numerous laboratory experiments, computer simulations,
and recently confirmed in a field test. Yet, to date there is no kinetic
model capable of accurately predicting the swapping process at given
fluid composition and p-T conditions. Major obstacles on the way to
an adequate mathematical description are caused by the insufficient
characterization of experimental environments and a nearly complete
lack of information on the time-resolved composition of the two-phase
fluid at the gas hydrate interface. Here we show that all necessary
data can be provided by a combination of cryo-SEM, Raman, and neutron
diffraction measurements that deliver accurate space-averaged, time-resolved
in situ data on the CH4–CO2 exchange
reactions at conditions relevant to sedimentary matrixes of continental
margins. Results from diffraction are cross-correlated with ex situ
Raman spectroscopy to provide reliable information on the preferential
sites for CO2 and CH4 in the (partially) exchanged
hydrate. We also show a novel approach based on scattering of neutrons
to probe the fluid composition during the in situ replacement in a
time-resolved, noninvasive manner. The replacement is seen as a two-step
process including (1) a fast surface reaction parallel to a fast enrichment
of the surrounding fluid phase with CH4 followed by (2)
a much slower permeation-limited gas swapping between the gas hydrate
and mixed ambient CH4–CO2 fluid. The
main part of the replacement reaction takes place in the second stage.
Based on our earlier experimental studies and existing literature
we work toward a quantitative gas exchange model which elaborates
the hole-in-cage-wall diffusion mechanism to describe the two-component
gas replacement.
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