Systematic Investigation of Natural Gas Hydrate Dissociation Processes with Regard to Global Warming

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.