Molecular Insights into Nanobubble and Guest Molecule Dynamics Following CH ₄ –CO ₂ Hydrate Dissociation in Sandy Aqueous Systems

In the context of natural gas hydrate production via CO2 replacement, the reformation and agglomeration of hydrates within sand-bearing production fluids pose a critical challenge to flow assurance. To elucidate the microscopic transport mechanisms of guest molecules during this process, this study employs molecular dynamics simulations, focusing on the liquid-phase environment following the complete dissociation of mixed CH4–CO2 hydrates. We systematically investigate the influence of sand particles on the diffusion, aggregation, and nanobubble evolution of CH4 and CO2 molecules. The results reveal that the sand particle plays a dual and competing role: its surface acts as a preferential nucleation site, promoting the formation of spatially stable and continuously growing nanobubbles via strong adsorption. Concurrently, this adsorption depletes guest molecules in the bulk liquid, thereby suppressing spontaneous nanobubble nucleation in regions that are distant from the sand interface. It is found that the resulting mixed nanobubbles exhibit a significant selective enrichment of the more hydrophobic CH4 molecules, a phenomenon that is further amplified by the presence of the sand particle. Regarding molecular diffusion, the sand particle hinders the free migration of guest molecules through adsorption, leading to reduced diffusion coefficients. Conversely, the coalescence of nanobubbles into a continuous gas phase provides a low-resistance pathway that significantly enhances molecular diffusivity, with this enhancement being particularly prominent in a CH4-rich environment. These microscopic insights reveal the key mechanisms of competitive aggregation and transport of CH4 and CO2 in complex multiphase systems, providing a crucial theoretical basis for understanding hydrate-related flow assurance risks.