Molecular Dynamics Simulation of CO₂ Molecular Behaviors in Silica Nanopores: Effect of Nanoscale Surface Roughness

Nanoscale roughness of reservoir skeleton surfaces inevitably affects the CO₂ geo-sequestration, and its exact microscopic mechanism remains elusive. Here, nanosecond molecular dynamics (MD) simulations were performed to investigate this effect with silica nanopore models. We classified the surface into "nano-valleys" and "nano-peaks" by the median z-coordinate of surface atoms and further divided nanovalleys into shallow and deep types. The results demonstrate that the nanovalleys can trap CO₂ molecules, resulting in lower CO₂ diffusivity and higher local concentration compared to nanopeaks. Generally, the total CO₂ quantity on nanovalleys and nanopeaks is increasing as surface roughness increases. A further exploration shows that the CO₂ concentration of the deep valley is always higher than that of the shallow valley under the same degree of roughness and exhibits an increasing trend as surface roughness increases. Furthermore, CO₂ molecules enter nanovalleys vertically and adsorb parallel to the surface, while water molecules orient randomly. In a high CO₂ concentration system, CO₂ nanobubbles are observed in nanovalleys. The nanobubbles are smaller but more numerous as the surface roughness increases. In a dual-phase system, the boundary between CO₂ and liquid phases connects the nanopeaks of top and bottom layers, embedding the CO₂ phase in concaves, which indicates the restrictive effect of nanopores on the CO₂ phase. These molecular insights confirm the accumulation and retention of prestored CO₂ due to nanoscale roughness on the reservoir surface.