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

Clathrate hydrates sediments present significant potential as future clean energy resources; injection of CO2 into these natural gas hydrate reservoirs to perform CO2-CH4 hydrate exchange is an attractive approach for recovery of CH4 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 CH4 hydrate generation and subsequent CO2-CH4 hydrate exchange in a model porous medium, predominately outside CH4 hydrate stability zone but within the CO2 hydrate stability zone. This was able to quantify the evolution in the amount of liquid water, CH4 gas, CH4 hydrate, liquid CO2, and CO2 hydrate present in the porous medium respectively and hence directly determine the fractional recovery of CH4 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 CH4 methane hydrate in comparatively larger pores; however subsequent CO2-CH4 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.

Automated summary

In this study, a novel suite of NMR techniques was used to determine the in-situ composition during both CH4 hydrate generation and subsequent CO2-CH4 hydrate exchange in a model porous medium. The data obtained from MRI and NMR provided quantitative in-situ composition analysis as well as pore-scale occupation data, which will enable future systematic studies of this complex exchange process.