Microscale Processes and Dynamics during CH 4 –CO 2 Guest-Molecule Exchange in Gas Hydrates

The exchange of CH 4 by CO 2 in gas hydrates is of interest for the production of natural gas from methane hydrate with net zero climate gas balance, and for managing risks that are related to sediment destabilization and mobilization after gas-hydrate dissociation. Several experimental studies on the dynamics and efficiency of the process exist, but the results seem to be partly inconsistent. We used confocal Raman spectroscopy to map an area of several tens to hundreds µm of a CH 4 hydrate sample during its exposure to liquid and gaseous CO 2 . On this scale, we could identify and follow different processes in the sample that occur in parallel. Next to guest-molecule exchange, gas-hydrate dissociation also contributes to the release of CH 4 . During our examination period, about 50% of the CO 2 was bound by exchange for CH 4 molecules, while the other half was bound by new formation of CO 2 hydrates. We evaluated single gas-hydrate grains with confirmed gas exchange and applied a diffusion equation to quantify the process. Obtained diffusion coefficients are in the range of 10 −13 –10 −18 m 2 /s. We propose to use this analytical diffusion equation for a simple and robust modeling of CH 4 production by guest-molecule exchange and to combine it with an additional term for gas-hydrate dissociation.