Hydrate formation from clay bound water for CO2 storage

Abstract

Geological storage of CO2 in form of hydrate in marine sediments is an alternative option to mitigate global warming. Clay is one of the most dominant lithology in marine sediments, which has higher water adsorption capacity than sand. In this work, the formation of CO2 hydrate from clay bound water was experimentally investigated by using low-field NMR. The effects of mineral, water content and pressure on the capacity of CO2 storage were analyzed. With the increase of water content, there was a transition of CO2 storage from adsorption on the montmorillonite surface to hydrate formation. The strongly bound water competed with CO2 gas for the adsorption sites on the surface of montmorillonite when the water content was below 15%. Afterwards, CO2 hydrate formation started due to the availability of weakly bound water. The storage of CO2 in hydrate in illite increased continuously with the increase of water content in the range of 5% to 20% due to the low level of strongly bound water. During hydrate formation, the peak position of T2 distribution spectra of the wet clay shifted to the left, indicating an ordered conversion of weakly bound water to hydrate from the outer layer to the inner layer of diffuse double layer. The final amount of CO2 hydrate formed from weakly bound water increased with the elevated pressure due to the variation of activity of the bound water in diffuse double layer. These results will not only broaden the understanding of gas hydrate in clay bound water systems but also promote a new evaluation of the safety and potential of CO2 hydrate storage in clayey sediments.