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/(N2 + CO2) and sH (isopentane + CH4) gas hydrate. From high-power decoupling 13C NMR and Raman spectra, we observed the structural transition of sH to sI hydrate. It was found that N2 molecules considerably promoted this structural transition during swapping, because N2 molecules prefer to attack CH4 molecules imprisoned in small cages. Due to this favorable structural transition and N2-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.

From Microscale (400 μl) to Macroscale (425 L): Experimental Investigations of the CO2/N2‐CH4 Exchange in Gas Hydrates Simulating the Iġnik Sikumi Field Trial

“…Regarding the interactions between a CO 2 ‐N 2 gas mixture and an initial CH 4 hydrate phase, Park et al () assumed an idealized cage‐specific pattern of CO 2 and N 2 , whereby N 2 molecules “attack” CH 4 molecules occupying 5 12 cavities and CO 2 molecules replace most of the CH 4 molecules in 5 12 6 2 . This specific exchange of the guest molecules in the hydrate phase results in mixed hydrate containing 23% N 2 , 62% CO 2 , and 15% CH 4 (Park et al, ; Shin et al, ).…”

Replacement micro-mechanism of CH4 hydrate by N2/CO2 mixture revealed by ab initio studies

“…Recently, nitrogen has been explored as a replacement promoter that can greatly improve the CH 4 recovery rate [25][26][27][28]. Park and coworkers [27] have experimentally estimated an 85% CH 4 recovery rate when exposing the methane hydrate to the N 2 /CO 2 gas mixture.…”

Enhanced CH4 Recovery Induced via Structural Transformation in the CH4/CO2 Replacement That Occurs in sH Hydrates

“…25−39 However, little attention has been paid to the CH 4 /CO 2 replacement mechanism involved in sH hydrates. 42 The cage-specific guest distributions, preferential partitioning of guest molecules, structural transition, hydrate dissociation and reformation, and complex hydrate behavior involved in the injection of external CO 2 molecules into the existing sH hydrates have not been fully understood. Therefore, this study was extended to an exploration of the CH 4 /CO 2 replacement in sH hydrates that have been confirmed to exist in deep sea sediments.…”