Enhanced CH4–CO2 Replacement in Structure H Hydrates: Kinetics, Mechanisms, and Implications for CO2 Storage and CH4 Production

Abstract

Guest replacement technology in the natural gas hydrate layer represents a cutting-edge approach for achieving carbon neutrality. In this study, a thorough analysis of the kinetics and mechanisms involved in CH4-CO2 replacement in structure H (sH) hydrates was conducted. An sH hydrate containing CH4 and methylcyclopentane exhibited a greater extent of CH4-CO2 replacement than that case of structure I (sI) hydrate under identical CO2 injection pressures. Time-dependent measurements obtained via powder X-ray diffraction (PXRD) and 13C nuclear magnetic resonance (NMR) spectroscopy demonstrated that nearly half of the initial sH hydrate underwent a structural transformation to become an sI hydrate within the first hour after CO2 injection. This accelerated structural transition altered the guest distribution within the hydrate phase, resulting in improved CH4 recovery and CO2 storage relative to isostructural CH4-CO2 replacement in sI. The experimental results revealed a replacement mechanism by which the injected CO2 molecules initially preferred the medium cages of the sH hydrate to replace CH4 molecules. Upon exceeding a critical CO2 concentration, destabilization of the sH hydrate occurred, leading to the rapid formation of a new sI hydrate with gas molecules from the surrounding vapor phase. The study findings enrich our understanding of the guest exchange behaviors of sH hydrates, with potential implications for enhanced CO2 storage in natural gas hydrates.