Research On Influencing Mechanism Of The Mixtures Of Surfactants In The Flooding Process

In the process of oil displacement,surfactant can reduce the interfacial tension of oil/water interface,improve the capillaries number,thereby increasing the flow rate of crude oil through the hole.At present,most of the anionic surfactants for enhanced oil recovery are poor in salt tolerance,which makes it hardly to be applied in high salt reservoirs.Therefore their application range is limited to a certain extent.The related researches show that the mixtures of anionic and cationic surfactant have great potential for enhanced oil recovery,because of the strongest synergistic effect,and the superior interface properties over a single surfactant.However,only a small amount of studies have been conducted on mixtures of anionic and cationic surfactant.In view of these,to reveal,enrich and improve the mechanism of surfactants flooding,this paper studied the microscopic properties of anionic surfactants and anions/cationic complex surfactants in the oil/water interface layer from the molecular level by molecular dynamics simulation.The main works are as follows:In the second section,the effect of calcium salt on the properties of monolayer for the anionic surfactants OPCNa,Meta-SOPCNa and Ortho-SOPCNa at the oil/water interface have been investigated via molecular dynamics simulation.The results show that Ca2+ can enter and destroy the hydration layer around headgroup-COO-,resulting in the reduction of the hydrophilicity of surfactants,thus affecting the performance of surfactants.The binding energy barrier for headgroup(-COO-)-Ca2+-pair is relatively larger in the systems of Meta-SOPCNa,which means that it is the most difficult for Ca2+ to enter into the first water shell of headgroup to form the ion-pair.Thereby,Meta-SOPCNa can maintain better hydrophilicity,and has a stronger ability to resist calcium salt.In the third section,molecular dynamics simulation have been performed to investigate the effects of different molar ratios on the properties of monolayer for the mixtures of sodium nonyl-phenol-ethoxylated-carboxylate surfactant(PE2CNa)and octadecyl trimethyl ammonium chloride(OTAC)at the oil/water interface.The results show that the surfactants at the oil/water interface are more closely packed,and can form a denser surfactant adsorption monolayer,because of the electrostatic interaction among the head groups and hydrophobic interaction among the tails.Compared with the systems of PE2CNa or OTAC,in the complex system,with the increase in the molar ratio of another surfactant,the interactions between the hydrophilic headgroup of the previous surfactan and water weakens gradually,the number of hydrated water surrounding the headgroups decreases gradually,resulting in the reduced hydrophility.Because the hydrophilic of PE2CNa is different from OTAC,a relatively good hydrophilic-lipophilic balance value can be obtained by adjusting the molar ratios of PE2CNa and OTAC.Thus obtaining a lower interfacial tension of the oil/water interface,and improving the efficiency of washing oil.Based on the work of the third section,the fourth section further explored the effect of calcium salt on the properties of monolayer for the mixtures of PE2CNa and OTAC with different molar ratios at the oil/water interface.The results show that Ca2+ prefers adsorbing near the surfactants monolayer,neutralizes the partial charge of the monolayer.At the same time,Ca2+ causes the electrostatic repulsion to Na+,which makes Na+ stay in aqueous solution.Ca2+ has little effect on the hydration layer around the headgroup of OTAC,while it has a great influence on the hydration layer around the headgroup of PE2CNa.When the molar ratio of PE2CNa/OTAC is 2:8,the excess OTAC has the strongest electrostatic shielding effect on PE2CNa,thus weakening the electrostatic attraction of headgroup of PE2CNa on Ca2+.So PE2CNa can maintain better hydrophilicity,and the system has better resistance to calcium salt.