Enhanced CH_4 Recovery Induced via Structural Transformation in the CH_4/CO_2 Replacement That Occurs in sH Hydrates

摘要

The CH_4/CO_2 replacement that occurs in sH hydrates is investigated, with a primary focus on the enhanced CH_4 recovery induced via structural transformation with a CO_2 injection. In this study, neohexane (NH) is used as a liquid hydrocarbon guest in the sH hydrates. Direct thermodynamic measurements and spectroscopic identification are investigated to reveal the replacement process for recovering CH_4 and simultaneously sequestering CO_2 in the sH (CH_4 + NH) hydrate. The hydrate phase behavior and the ~(13)C NMR and Raman spectroscopy results of the CH_4 + CO_2 + NH systems demonstrate that CO_2 functions as a coguest of sH hydrates in CH_4-rich conditions, and that the structural transition of sH to sI hydrates occurs in CO_2-rich conditions. CO_2 molecules are found to preferentially occupy the medium 4~35~66~3 cages of sH hydrates or the large 5~(12)6~2 cages of sI hydrates during the replacement. Due to the favorable structural transition and resulting re-establishment of guest distributions, approximately 88% of the CH_4 is recoverable from sH (CH_4 + NH) hydrates with a CO_2 injection. The hydrate dissociation and subsequent reformation caused by the structural transformation of sH to sI is also confirmed using a high-pressure microdifferential scanning calorimeter through the detection of the significant heat flows generated during the replacement.