A molecular insight into the effect of key ions on the detachment of crude oil from calcite surface: Implication for low salinity water flooding in carbonate reservoirs

The detachment of crude oil from calcite surface plays an important role in oil recovery and wettability alteration during low salinity water flooding in carbonate reservoirs. The effects of brine salinities and key ions on the dynamic detachment process have been much of debate in the past decades in theories and experiments due to complex mechanisms behind the crude oil-calcite interactions. Herein, molecular dynamic simulation was used to study the influence of brine salinities and ion concentrations on the detachment mechanism of crude oil from calcite surface by establishing 20 brine-crude oil-calcite molecular models varying salinities and key ion concentrations in brine. Molecular configurations, concentration distribution profiles, interaction energies, H-bond numbers and center of mass (COM) distances were carefully analyzed to characterize the dynamic crude oil detachment performance. The results reveal that reducing the brine salinities and ion concentrations can significantly weaken the interaction intensity between oil and calcite and accelerate the detachment process. Therefore, the oil detachment is induced and the effect of low salinity on oil detachment process is significantly time-dependent. The concentration distribution profiles shows that the strong polar oil molecules, for example acetic acid molecules, can't be detached from calcite surface by tuning the brine salinity and key ions. The present of Mg2+ and Na+ is able to delay the detachment process of oil molecules and as a consequence, result in the impeded oil detachment process according to time-dependent dynamic molecular configurations, interaction energies and COM distances analysis. Furthermore, the present of Ca2+ is able to strengthen the interaction between oil and calcite and in consequence, hinder the detachment of oil molecules from the calcite surface according to the interaction energy, the molecular configuration and COM distances. This study should provide the molecular-scale insights into wettability alteration and enhanced oil recovery by low salinity water flooding in carbonate reservoirs.