| To enhance oil recovery over water flooding and exploitate low permeability reservior favourably are of strategic significance for oil industry except a critical commercial value.Based on research results of the world, both the mechanism and limitation of mobilizing residual oil are investigated and the keys on mobilizing residual oil using ultra low interfacial tension(IFT) flooding technology are also analyzed.It is argued in the paper that mobilizing residual oil is influenced by all kinds of macro forces and micro forces. Futhermore,the impact of capillary radius, speciation and distribution characteristics of residual oil,rock wettability ,oil boundary layer thickness and pressure gradient should be taken into account.Keys mentioned above are helpful to mobilizing oil drops, there is no need to lower the ultra low equilibrium (IFT) magnitude to 10 3mN/m to improve oil displacement efficiency greatly.Core flooding results indicates that the effcet of main slug on oil recovery is higher than that of post slug. Oil displacement test results on cores display that 20%OOIP of oil recovery over waterflood as instantaneous(or dynamic) minimum IFT being the magnitude of 10 2mNm is almost equal to that as equilibrium IFT being lowered to the ultralow magnitude of 10 3mN/m. The core test results of reservoir core and compacted core display the consistency recognition.Conclusions based on core flooding obviously tests show that the instantaneous(or dynamic) minimum IFT strongly affects on oil displacement efficiency, and the main factor which affects oil recovery is not the equilibrium IFT, but the instantaneous(or dynamic) minimum IFT. On condition that the permeability is above 50×10 3 m2,with the inrease of water wetting, the oil recovery of waterflood and ultra low IFT flooding inreases gradually during the process of tranforming oil wetting into water wetting. while the permeability is from 10×10 3 m2 to 50×10 3 m2,Oil recovery of ultra low IFT flooding inreases gradually in the process of tranforming oil wetting into water wetting.However,except for strong oil wetting,others wettabilities of pores do not obviously affect on oil recovery of waterflood as the permeability is from 10×10 3 m2 to 50×10 3 m2.Oil displacment efficiency of ultra low IFT brine displacement increases gradually with the increment of permeability at different wettabilities conditions.With the rise of permeability,oil recovery of ultra low IFT flooding inreases slowly.As permeability rises to a certain value,oil recovery increases sharply.And then oil recovery increases slowly or unchanges with the increment of permeability.The corresponding permeability is defined as the critical pereability.The reason of cricital permeability existing in porous media is briefly discussed from the capillary theory in the paper.The minimum pore thoat radium and permeability for mobilizing residual oil drops is much smaller with the increase of water wetting of core.On condition that the permeability is above 50×10-3m2, the effect of wettability of core on displacement efficiency of ultra low IFT flooding is not obvious.Furthermore, the ultra low IFT brine is favorable for diffreent wettability of core to enhance oil recovery.So,there is no critical permeability in high permeability reservior.This means that the oil displacement brine with ultra low IFT is favorable for a much lower range of permeability.Therefore, besides the wettability of rock,the adaptive permeability range also should be considered as using ultra low IFT system flooding technology. Finally, combined characteristics of ultra low IFT system and laboratory experiments,the advantages of ultra low IFT system flooding to improve oil displacement efficiency are analyzed in this paper.the flooding system can tranform oil wetting into water wetting and decrease capillary resisitence and injection pressure.So,the displacement system shows a good effect of decompression and augmented injection for low permeability reservior.
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