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

The CH4/CO2 replacement that occurs in sH hydrates is investigated, with a primary focus on the enhanced CH4 recovery induced via structural transformation with a CO2 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 CH4 and simultaneously sequestering CO2 in the sH (CH4 + NH) hydrate. The hydrate phase behavior and the (13)C NMR and Raman spectroscopy results of the CH4 + CO2 + NH systems demonstrate that CO2 functions as a coguest of sH hydrates in CH4-rich conditions, and that the structural transition of sH to sI hydrates occurs in CO2-rich conditions. CO2 molecules are found to preferentially occupy the medium 4(3)5(6)6(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 CH4 is recoverable from sH (CH4 + NH) hydrates with a CO2 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.