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Abstract:
Natural gas hydrates are crystalline compounds that are formed from hydrogen-bonded water
molecules and gas molecules. They mainly contain climate-active CH4, but also other light
hydrocarbons, CO2 or H2S They exhibit a high sensitivity to variations in temperature and pressure,
mainly driven by environmental changes. The oceanic or atmospheric warming resulting from
climate change may trigger the decompositions of hydrates, potentially releasing significant
amounts of CH4. To assess the potential risks associated with CH4 release from destabilized
hydrate deposits, a precise understanding of the dissociation behaviour of gas hydrates becomes
crucial.
In this study, a systematic investigation on the dissociation process of sI CH4 hydrates, sII CH4+C3H8
hydrates, and sII multi-component CH4+C2H6+C3H8+CO2 mixed hydrates was reported. We
employed a combination of experimental and molecular dynamics (MD) simulations to provide a
more nuanced understanding of the hydrate dissociation behaviours, which primarily shed light
on the molecular aspects. The dissociation was induced through thermal stimulation to mimic
climate warming. Both in situ and ex situ Raman spectroscopic measurements were performed
continuously to characterize the hydrate phase. Throughout the dissociation process, hydrate
composition, surface morphology, and the large-to-small cavity ratios were determined. MD
simulations were carried out under similar conditions, providing advanced insights and
perspectives that couldn't be readily extracted from experimental observations alone.
Both experimental and simulation outcomes indicate that intrinsic kinetics likely govern the early
stage of hydrate dissociation. A significant development in the dissociation process is the
hindrance caused by the formation of a quasi-liquid or amorphous phase at the surface of the
hydrate particles after the breakup of the outer layer of hydrate cavities. The unstable (partial)
hydrate cavities that form within this quasi-liquid phase are oversaturated with gas molecules.
Consequently, gas hydrates undergo a cycle of decomposition-reformation-continuing
decomposition until the crystal eventually disappears. With decomposition dominating the
process, both experimental and numerical simulation results demonstrate that the breakup of
large cavities (51262) is faster than that of small ones (512) in sI hydrates. Conversely, a faster
breakdown of small 512 cavities in sII hydrates is observed. Additionally, during the dissociation
process of sII CH4-C3H8 hydrate, the cavities occupied by CH4 preferentially collapse compared to those filled with C3H8. Similarly, over the dissociation of multi-component hydrate, cavities filled
with CH4 exhibit a preferential collapse compared to those filled with C3H8, C2H6, and CO2. These
findings show the complexity and differences in the dissociation behavior of natural gas hydrates
depending on their composition and structure and can therefore make an important contribution
to an accurate assessment of CH4 release from destabilized hydrate deposits in response to
climate change.