New Insights on a µm-Scale into the Transformation Process of CH 4 Hydrates to CO 2 -Rich Mixed Hydrates

The global occurrences of natural gas hydrates lead to the conclusion that tremendous amounts of hydrocarbons are bonded in these hydrate-bearing sediments, serving as a potential energy resource. For the release of the hydrate-bonded CH 4 from these reservoirs, different production methods have been developed during the last decades. Among them, the chemical stimulation via injection of CO 2 is considered as carbon neutral on the basis of the assumption that the hydrate-bonded CH 4 is replaced by CO 2 . For the investigation of the replacement process of hydrate-bonded CH 4 with CO 2 on a µm-scale, we performed time-resolved in situ Raman spectroscopic measurements combined with microscopic observations, exposing the CH 4 hydrates to a CO 2 gas phase at 3.2 MPa and 274 K. Single-point Raman measurements, line scans and Raman maps were taken from the hydrate phase. Measurements were performed continuously at defined depths from the surface into the core of several hydrate crystals. Additionally, the changes in composition in the gas phase were recorded. The results clearly indicated the incorporation of CO 2 into the hydrate phase with a concentration gradient from the surface to the core of the hydrate particle, supporting the shrinking core model. Microscopic observations, however, indicated that all the crystals changed their surface morphology when exposed to the CO 2 gas. Some crystals of the initial CH 4 hydrate phase grew or were maintained while at the same time other crystals decreased in sizes and even disappeared over time. This observation suggested a reformation process similar to Ostwald ripening rather than an exchange of molecules in already existing hydrate structures. The experimental results from this work are presented and discussed in consideration of the existing models, providing new insights on a µm-scale into the transformation process of CH 4 hydrates to CO 2 -rich mixed hydrates.