The rapid secondary formation of gas hydrate is a potential cause of flowline blockage in deepwater oil and gas production systems, posing serious flow assurance challenges. However, its microscopic formation mechanism remains an area of active research. Recently, the residual structure hypothesis has gained significant attention in explaining the rapid secondary formation of hydrates. In this study, massive molecular dynamics simulations are conducted to investigate the secondary formation of methane hydrates in solutions containing hydrate residual structures of varying sizes. The results indicated that residual structures, owing to their hydrate-like characteristics, facilitate the adsorption and capture of methane molecules, leading to the formation of local gas supersaturation regions. Residual structures promote hydrate formation through two key mechanisms: acting as nucleation sites and supplementing methane concentrations. Particularly, a synergy between residual structures and gas concentration was identified: high gas concentrations stabilize small residual structures, allowing them to serve as nucleation sites, while large stable structures can enrich methane even under low gas concentration.

This work not only provided a detailed understanding of the mechanisms of hydrate secondary formation but also provided valuable insight for hydrate blockage prediction and control in subsea oil and gas pipelines, contributing to improved flow assurance strategies.

Janus nanoparticles (JNPs) possess great potential in recovering the residual oil from reservoirs, however, the fundamental interaction mechanisms among nanoparticles, the oil, and reservoir wall characteristics remain to be elucidated. In this work, models of oil trapping grooves with different geometric features are subjected to molecular dynamics simulations for investigating the influences of roughness parameters on oil displacement dynamics by JNPs. Four key surface geometry parameters and different degrees of surface hydrophobicity are considered. Our results indicate that JNPs hold an outstanding performance in displacing residual oil on weakly to moderately hydrophobic surfaces. Overall, smaller entry and exit angles, the larger aspect ratio of the oil trapping grooves, and a bigger tip length of the rough ridges lead to superior oil recovery. Among the key geometric parameters, the aspect ratio of the oil trapping grooves plays the dominant role. These insights about the interaction of surface properties and JNPs and the resulting trapped oil displacement could serve as a theoretical reference for the application of JNPs for targeted reservoir conditions.