Grain Boundary-Facilitated CO₂/CH₄ Replacement in Gas Hydrates

Area of Science:

• Geochemistry
• Materials Science
• Energy Science

Background:

• Natural gas hydrates are a significant potential clean energy source.
• The CO2/CH4 replacement technique is a promising method for hydrate exploitation and CO2 sequestration.
• The precise mechanism of CO2 entering hydrate interiors during replacement is not well understood.

Purpose of the Study:

• To investigate the CO2/CH4 replacement mechanism in gas hydrates.
• To understand the role of microstructural features, specifically grain boundaries, in the replacement process.
• To elucidate the pathways and driving forces behind CO2-CH4 replacement.

Main Methods:

• Utilized microcanonical ensemble molecular dynamics simulations.
• Simulations conserved the exothermic heat of replacement, unlike previous methods.
• Examined replacement in both monocrystalline and polycrystalline CH4 hydrates.
• Performed cage transition analysis to map guest molecule transitions.

Main Results:

• Grain boundaries (GBs) significantly facilitate CO2/CH4 replacement by acting as permanent active pathways.
• CO2 penetrates deeply into hydrate interiors through GBs, sustaining the replacement process.
• Non-standard cages within GBs, with transient lifetimes and higher diffusivity, drive the replacement.
• Identified cage transition pathways involving large, mixed guest cages as intermediates.

Conclusions:

• Established a "grain boundary diffusion and replacement" mechanism for CO2/CH4 replacement in gas hydrates.
• Demonstrated the critical role of microstructural defects (GBs) in enhancing hydrate reactivity and replacement efficiency.
• Provided crucial insights into optimizing CO2-based gas hydrate exploitation and carbon sequestration strategies.