Heterogeneous and Coexisting Hydrate Phases─Formation under Natural and Laboratory Conditions

Abstract

Natural gas hydrates are non-stoichiometric, crystalline solids composed of water and gas molecules. Dependent upon the source of the hydrate-forming gas, the structure and composition of the occurring natural gas hydrates may vary. In nature, the existence of structure I, structure II, and structure H hydrates containing predominantly methane but also other hydrocarbons, H2S, or CO2 could be verified. Interestingly, the number of reports on coexisting hydrate phases with different structures and compositions in natural gas hydrate reservoirs has increased in recent years. However, it has not yet been clearly clarified what leads to the formation of these coexisting hydrate phases. In the present study, we analyzed natural gas hydrate samples spatially resolved using Raman spectroscopy to check whether these natural samples only show heterogeneity with regard to their cage occupancy and composition or whether they already show coexistent phases. The samples available to us from the Hikurangi margin and the Cascadian margin showed strong fluctuations in cage occupancy and composition within the individual crystals but no coexisting phases. With complementary experiments, we were able to show that gas hydrates with a heterogeneous composition formed from a complex feed gas mixture. Furthermore, we were able to verify experimentally that coexisting phases may form when an initial methane hydrate phase was exposed to a complex gas mixture.