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

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.