Microscale Processes and Dynamics during CH4-CO2 Guest-Molecule Exchange in Gas Hydrates

The exchange of CH4 by CO2 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 mu m of a CH4 hydrate sample during its exposure to liquid and gaseous CO2. 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 CH4. During our examination period, about 50% of the CO2 was bound by exchange for CH4 molecules, while the other half was bound by new formation of CO2 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 CH4 production by guest-molecule exchange and to combine it with an additional term for gas-hydrate dissociation.

Automated summary

This study used confocal Raman spectroscopy to investigate the exchange of CH4 by CO2 in gas hydrates, identifying both gas molecule exchange and gas-hydrate dissociation processes in the sample. Diffusion coefficients obtained from the analysis ranged from 10(-13) to 10(-18) m(2)/s, proposing a simple modeling approach for CH4 production through guest-molecule exchange combined with gas-hydrate dissociation term.