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.