Swapping phenomena occurring in deep-sea gas hydrates

Abstract

On the basis of crystallographic analysis results, a recent study reported that structure H (sH) hydrate exists in the natural environment, providing direct evidence from hydrate samples recovered from Barkley canyon, on the northern Cascadia margin. It was further indicated that sH is more stable than sI and may thus potentially be found in a wider pressure-temperature regime than are methane hydrate deposits. Accordingly, it is worthwhile to examine whether a swapping process can spontaneously occur between gaseous CO2/(N-2 + CO2) and sH (isopentane + CH4) gas hydrate. From high-power decoupling C-13 NMR and Raman spectra, we observed the structural transition of sH to sI hydrate. It was found that N-2 molecules considerably promoted this structural transition during swapping, because N-2 molecules prefer to attack CH4 molecules imprisoned in small cages. Due to this favorable structural transition and N-2-induced guest exchange, more than 92% CH4 can be recovered from methane hydrate deposits. The microscopic and macroscopic phenomena together imply that the swapping process between carbon dioxide and methane can be effectively used in the recovery of energy resources that are widely deposited in deep ocean sediments as well as for the sequestration of carbon dioxide to the methane hydrate layer.