| Due to the nonexistence of alkali in polymer-surfactant flooding system,the influence of alkali on displacement effect and equipments on the surface decreases, but at the same time, the interfacial tension between oil and water increases so that ultra-low interfacial tension (<10-3mN/m order) is hard to achieve, which is of disadvantage to improve displacement efficiency; without the synergistic effect between alkali and surfactant, the adsorption capacity of surfactant will greatly increase, which is of disadvantage to economies. In order to get the same low-interfacial tension of polymer-surfactant flooding system as ASP flooding system, the quality of surfactant will be highly required. Based on a new theory of the achievement of ultra-low interfacial tension with the nonexistence of alkali, a new type of surfactant which can help to get the ultra-low (<10-3mN/m order) interfacial tension was synthesized in this paper. Through the experiments in the lab, the interfacial tension properties,interfacial tension stability and the adaptability and the absorption characteristics under different oil-water conditions of this surfactant were studied,moreover, the amount of surfactant adsorption of the polymer-surfactant flooding system and ASP flooding system were compared and analyzed. The result shows that the requirement could be completely satisfied, using this type of surfactant.In the Experiment, the viscoelasticity of the two-component combinational flooding system without alkali and the start-up and migration laws of the residual oil after water flooding were studied,and through the sudden expansion,sudden contraction and other model the impact of elasticity on the oil displacement efficiency was studied. The results showed that the concentration of the alkali-surfactant had little effect on the viscoelasticity of the two-component system no alkali,while the polymer concentration,divalent ion concentration,salinity,shear rate and other factors had greater effect on the viscoelasticity. The greater the elasticity of the flooding system is, the higher the contribution on the oil displacement efficiency.Through the laboratory artificial core physical simulation experiments, the flow characteristics of different molecular weight polymer as well as the influence law of the two-component surfactant-polymer flooding slug size on the flooding effect of the alkali-free two-component flooding system were studied, which were used to select the formulas of the alkali-free two-component flooding system. Studies have shown that the core whose gas log permeability is greater than 0.3μm2 can use all the polymers whose molecular weight lower is than 2.5×107 (including the polymer whose molecular weight is 2.5×107);with surfactant concentration and slug size increases,the amount of chemical agents were increased,and the EOR range of alkali-free two-component flooding system was increased,but after it was increased to a certain value,the increase will be slowed.In the homogeneous and two-dimensional vertical heterogeneity of different variation coefficients artificial cores, the experiments about the alkali-free two-component system flooding effects as well as the influence law of coefficient of variation,injection timing,permeability and other factors on the flooding effect of alkali-free two-component flooding were conducted, and a comparative analysis was made on the flooding effect of the two-component flooding system with polymer flooding system, alkali ASP flooding system,and weak base ASP flooding system.The results showed that in the homogeneous cores with the same polymer concentration,when the main slug polymer concentration increased from 500mg/L to 2500mg/L,alkali-free two-component system chemical flooding recovery efficiency had the trend of obvious improvement;in the same coefficient of variation conditions,with the permeability of the model increases,the oil recovery efficiency improvement range of alkali-free two-component system showed an upward trend;in the case of the same permeability, the coefficient of variation increased,the alkali-free two-component system recovery efficiency increased, while the total recovery rate decreased as the coefficient of variation decreased;injecting alkali-free two-component flooding system ahead could improve the recovery efficiency; when core permeability and the coefficient of variation are constant,the flooding effect of alkali-free sulfobetaine surfactants-polymer flooding system was better than the displacement characteristics of ASP flooding solution and the weak base ASP flooding solution.
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