In application of Alkali-surfactant-polymer flooding, people always follow after ultra low IFT and negate the effect of ASP solution viscoelasticity on displacement efficiency. For getting the ultra low IFT, ASP solutions need the high concentration of alkali, for example, the ASP flooding needs 1.2%wt NaOH in Daqing oilfield. It brings not only many new technological problems such as scale production in well bore, sticking of pump, reduction of injectivity etc, but also decrease of polymer viscoelasticity. In addition, the selectable range of surfactant for ASP flooding falls a definite limitation due to the restriction of ultra low IFT.
As to ASP flooding in the pilot area of Daqing oilfield, the analysis of ASP solution's rheology and physical simulation experiments have been carried out for studying the effect of alkali on viscoelasticity of displacing phase and the effect of viscoelasticities of ASP solution on displacement efficiency in this paper. Experiments include two types, macro-displacements and micro-displacements. Cores have artificial heterogeneous models, natural cores and micro-models. The results show that decreasing alkali concentration increases the viscoelasticity of ASP solution, and the displacement efficiencies of ASP solution with low alkali concentration aren't less than that with high alkali concentration and ultra low IFT. At same time, the effect of ASP solution's viscoelasticity on displacement of residual oil in different diameter pores of parallel connection has been analyzed theoretically. The limit of capillary number theory has been discussed in ASP flooding. The mechanism of ASP flooding has been recognized again. The mechanism that viscoelasticity is more important than ultra low IFT as a factor of affecting displacement efficiency has been confirmed. Therefore, Decreasing the amount of alkali and weakening the demand of low IFT not only enlarges the selectable range of surfactant and decreases the costs of ASP flooding, but also solves the problem of alkali scale, and are of great importance to the further development of ASP flooding and its application.
The theory of capillary number is one of important theories to guide the development of oil field. According to the theory, ultra low IFT and high viscosity have been considered as two necessary conditions to get better displacement efficiency in ASP flooding. At the present time of development of chemical flooding, the capillary number theory still has an important meaning.
As Harkins and Zollman who are surface chemists in America studied the effect of salinity on IFT in 1926, the phenomenon of low IFT was discovered first. L.C. Uren and E.H. Fahry presented that the displacement efficiency of water was an inverse proportion with IFT of displacing phase as oil was produced in 1927. O.R. Wagner etc., proved that the IFT must be less than 0.07mN/m for enhancing the displacement efficiency in 1966. The more IFT decrease, the more displacement efficiency can increase. In 1973, W.R. Foster studied the relationship of capillary number (µ ?/s) and oil recovery, and presented the residual oil saturation was near zero as the capillary number was 10-2. For getting this capillary number, the IFT must decrease to 0.001mN/m. R.L. Reed etc., testified that forming and keeping ultra low IFT on interface of surfactant solution and oil was a key factor to decide displacement efficiencies, and as the IFT decreased to 10-1mN/m, the displacement efficiency increased obviously; as the IFT decreased to 10-3mN/m, the residual oil saturation was near zero.
In fact, scientists have found some regulations to form low IFT after the crisis of oil in 1960:
appropriate carbon number in oil phase (Nmin);
appropriate equimultiple carbon atom number (NE);
the average molecular weight;
appropriate concentration of surfactant and salt. Getting ultra low IFT needs too many appropriate values of factors. If one of these factors changes, the IFT will have a great change. That is to say, it is very difficult to keep ultra low IFT during ASP flooding due to the deference of adsorption and diffusion and other reasons.