Enhanced oil recovery with CO2/N2 slug in low permeability reservoir: Molecular dynamics simulation

Slug injection of CO2/N2 has proven to be a promising technology for the enhancement of oil recovery in low/ultra-low permeability reservoirs. In spite of this, the detailed displacement process and interaction mechanisms remain unclear. Adopting molecular dynamic simulations, the displacement behavior of CO2/N2 flooding was investigated at the molecular and atomic level. It was found that CO2 and N2 play varying roles: the solubility of CO2 is strong, and the propel effect of N2 provides an obvious advantage. Furthermore, the synergistic effect of the CO2 slug and N2 slug was found to be better than single phase injection and even better than mixture gas (flue gas) flooding. Particularly, it shows the best displacement efficiency when CO2 is used as a front slug. This study unveils the underlying mechanism of gas flooding in low permeability reservoirs, and the results may be proved helpfully in providing theoretical support for the optimization of the injection mode.