Insights into CO2-CH4 hydrate exchange in porous media using magnetic resonance

• A new five component model to fully quantify composition during CO 2 -CH 4 hydrate exchange. • NMR T 2 data suggest that the CH 4 hydrate generates preferentially in larger pores. • 1D MRI profiles show the phase saturations along the axis of the porous media. Clathrate hydrates sediments present significant potential as future clean energy resources; injection of CO 2 into these natural gas hydrate reservoirs to perform CO 2 -CH 4 hydrate exchange is an attractive approach for recovery of CH 4 in an approximately carbon–neutral manner. However, deployment of this approach is hampered partially due to the limitations of experimental techniques capable of providing in-situ, pore scale fluid and hydrate characterisation in these opaque sediments during this dynamic exchange process. Here we demonstrate a novel suite of NMR techniques to determine the in-situ composition during both CH 4 hydrate generation and subsequent CO 2 -CH 4 hydrate exchange in a model porous medium, predominately outside CH 4 hydrate stability zone but within the CO 2 hydrate stability zone. This was able to quantify the evolution in the amount of liquid water, CH 4 gas, CH 4 hydrate, liquid CO 2 , and CO 2 hydrate present in the porous medium respectively and hence directly determine the fractional recovery of CH 4 during exchange. These data were complemented by 1D magnetic resonance imaging (MRI) of the axial distribution of fluids in the sample and NMR relaxation measurements of fluid pore size occupation. This revealed preferable formation of CH 4 methane hydrate in comparatively larger pores; however subsequent CO 2 -CH 4 exchange was observed to preferentially occur via comparatively smaller pores containing transient liquid water. Hence we demonstrate the ability of MRI and NMR to provide both quantitative in-situ composition analysis as well as pore-scale occupation data which will enable future systematic studies of this complex exchange process.