Abstract: Lots of experiments are made based on oil-oil displacement under irreducible water saturation and on water-water displacement under residual oil saturation. With low-velocity non-Darcy flow method for low permeability reservoir, the corresponding critical Reynolds numbers under these two situations are obtained. The study shows the different Reynolds numbers mean that the different critical Reynolds numbers of low-velocity non-Darcy flow under these two situations are resulted from different media for displacing and being displaced. Furthermore, this paper presents the detail reasons for them and the case study for calculation of the critical flow velocity under low-velocity non-Darcy flow by using these critical Reynolds numbers.

Key words: irreducible water, residual oil, saturation, non-Darcy flow, Reynolds number