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

Abstract

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 µm 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 m2/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.

Numerical Simulation on the Dissociation, Formation, and Recovery of Gas Hydrates on Microscale Approach

“…The replacement process occurred during CO 2 injection into the CH 4 hydrate reservoir, due to the difference in chemical potential, so the CH 4 molecule left the hydrate cage, and the empty cage was filled by the CO 2 molecule [ 4 ]. Kossel, Bigalke [ 137 ] confirmed that the exchange of guest molecules and gas hydrate dissociations also contributed to the dissociation of CH 4 . Approximately half of the amount of CO 2 was bound by the exchange of CH 4 molecules, while the rest was bound by the new CO 2 hydrates formation.…”

“…Other studies [ 127 , 133 , 135 , 137 , 138 ] investigated the dissociation and formation process of methane and carbon dioxide hydrates by MD simulation. Kondori, James [ 135 ] evaluated the stability and dissociation of gas hydrate structure 1 for CH 4 –CO 2 cases using MD simulation, by considering various properties for gas hydrates, such as radial distribution function (RDF), mean square displacement (MSD), lattice parameters, density, potential energy, and molecular diffusion coefficient.…”

Building a Cost-Efficient High-Pressure Cell for Online High-Field NMR and MRI Using Standard Static Probe Heads: An In Situ Demonstration on Clathrate Hydrate Formation

“…Alternative NMR-and MRcompatible flow-through sample cells at high pressure have also been designed to study hydrate formation in sediments and at subseafloor conditions, but their complexity comes with its own challenges. 20,21 To overcome the complexity and high costs associated with high-pressure NMR, a low-cost sample environment suitable for NMR measurements of up to 30 MPa was developed. The performance of this new high-pressure sample environment was evaluated in high-field in situ studies monitoring the formation of clathrate hydrates of methane and ethane with 13 C NMR and 1 H MR spectroscopy and imaging.…”