Grain Boundary-Facilitated CO2/CH4 Replacement in Gas Hydrates

Natural gas hydrates represent a vast potential clean energy resource, and the CO2/CH4 replacement technique offers a promising strategy for their exploitation while enabling CO2 sequestration. However, the underlying replacement mechanism, particularly how CO2 enters the hydrate interior, remains unclear. Here, by employing microcanonical ensemble molecular dynamics simulations that inherently conserve the exothermic heat of replacement, a key physical aspect artificially dissipated in previous simulated ensembles, we investigated CO2/CH4 replacement behaviors in monocrystalline and polycrystalline CH4 hydrates. Our results demonstrate that grain boundaries (GBs) play a critical role in facilitating the CO2/CH4 replacement process. GBs act as permanent active pathways, allowing CO2 to penetrate deeply into the hydrate interior and sustaining the replacement. This process is driven by the abundance of non-standard cages within GBs, which exhibit transient lifetimes and higher molecular diffusivity. Furthermore, a cage transition analysis uncovers the cage transition pathways, indicating that large, mixed guest cages serve as intermediates during the conversion from CH4-filled to CO2-filled cages. These findings establish a “grain boundary diffusion and replacement” mechanism for CO2/CH4 replacement in gas hydrates, underscoring the crucial influence of microstructural defects on hydrate reactivity and replacement efficiency. Natural gas hydrates represent a potential clean energy resource, with the CO2/CH4 replacement technique offering a promising strategy for their exploitation that additionally enables CO2 sequestration, but the underlying mechanism remains unclear. Here, the authors use microcanonical ensemble molecular dynamics simulations to study CO2/CH4 replacement behavior in monocrystalline and polycrystalline CH4 hydrates, showing that grain boundaries act as active pathways, allowing CO2 to reach the hydrate’s interior and sustaining the CO2/CH4 replacement.