A clean solution? - The potential and limitations of the CH4-CO2 exchange in gas hydrates

Natural gas hydrates occur at all active and passive continental margins and are supposed to contain tremendous amounts of methane, which makes them attractive as a potential source of energy. During the last two decades several methods have been developed and tested to produce methane from gas hydrates in lab experiments and field trials. In principle, three different approaches, namely depressurization, thermal stimulation and chemical stimulation have been tested. The injection of CO2 into hydrate-bearing sediments as a variant of a chemical stimulation appears to be particularly favorable because it combines the production of methane from the hydrate phase with the storage of CO2 as a solid clathrate. Despite the fact that numerous experimental and modelling studies have been conducted worldwide in order to understand the process on different scales and to evaluate its efficiency, some issues remain pending. We obtained laboratory data from micro-scale experiments using analytical methods such as Raman spectroscopy and X-ray diffraction providing information about the structural changes of the hydrate phase and the gas exchange processes on a molecular scale. The results indicate that the processes related to the exchange of the guest molecules are quite complex. It is generally accepted that the driving force for the release of methane from the hydrate lattice and the incorporation of CO2 into the hydrate structure is the chemical potential gradient between the hydrocarbon hydrate phase and the injected CO2 phase. Raman spectroscopic and X-ray diffraction measurements indicate that this process correlates with a (partial) decomposition or opening of the hydrate cavities. In case of mixed hydrocarbon hydrates with structure II the exchange of the guest molecules with CO2 also induces a change of the hydrate structure (sII to sI). In general, the exchange of the guest molecules results in the formation of a secondary mixed hydrate phase containing CO2 besides methane and other hydrocarbons depending on the composition of the surrounding gas phase. We also obtained laboratory data from large-scale experiments providing information about the methane recovery and fluid migration, hydrate dissociation rates and the formation of a secondary hydrate phase, heat and mass transfer. The experimental results show a wide variation indicating that the recovery rate of methane strongly depends on the experimental conditions such as the volume of the sample, hydrate saturation and thus permeability of the hydrate bearing sediment, hydrate morphology as well as pressure and temperature of the injected CO2 phase. To avoid clogging due to the formation of a CO2 hydrate in the vicinity of the injection area we used gas mixtures containing N2 besides CO2 which also has an impact on the recovery rate of methane and the potential formation of a secondary hydrate phase. In any case, the injection of CO2 results in the production of a gas mixture rather than the extraction of pure methane because the injected CO2 forms a mixed hydrate with a certain amount of methane remaining in the hydrate phase. In addition, the long-term stability of the secondary formed CO2-rich hydrate phase also depends on the potential changes of the chemical environment which may result in the release of CO2 due to a re-exchange of the hydrate bonded gas molecules or dissociation of the hydrate phase. Maybe these results make the approach of methane production from natural gas hydrate deposits via CO2 injection less attractive with regard to efficiency and safe CO2 sequestration than originally expected. This contribution will present the experimental results of our investigations on various scales using different analytical methods and the overarching interpretation and conclusions based on these data.