Molecular Dynamics Simulation of CO<sub>2</sub> Molecular Behaviors in Silica Nanopores: Effect of Nanoscale Surface Roughness
Hongyi Xu, Yunfeng Liang, Naipeng Zhao, Jiangtao Pang, Fulong Ning
Abstract
Nanoscale roughness of reservoir skeleton surfaces inevitably affects the CO 2 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 CO 2 molecules, resulting in lower CO 2 diffusivity and higher local concentration compared to nanopeaks. Generally, the total CO 2 quantity on nanovalleys and nanopeaks is increasing as surface roughness increases. A further exploration shows that the CO 2 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, CO 2 molecules enter nanovalleys vertically and adsorb parallel to the surface, while water molecules orient randomly. In a high CO 2 concentration system, CO 2 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 CO 2 and liquid phases connects the nanopeaks of top and bottom layers, embedding the CO 2 phase in concaves, which indicates the restrictive effect of nanopores on the CO 2 phase. These molecular insights confirm the accumulation and retention of prestored CO 2 due to nanoscale roughness on the reservoir surface.