A Study on Seepage Flow Characteristic Equation for High Water-Cut Oilfields

The linear relationship between the logarithm of relative permeability ratio and water saturation on the rectangular coordinate in the middle and low water-cut stages is the theoretical basis of the traditional water flooding theory. However, the relationship between Ln (Kro/Krw) and Sw in the high water–cut stage deviates from the linear relationship, bending down, which can’t be explained by the traditional water flooding theory. On the basis of the frontal-drive equation and the average water saturation equation proposed respectively by Buckley-Leverett and Welge, a new relation between Ln(Kro/Krw) and Sw was derived by focusing on the production performance of the oilfield in this paper. The new relation can simulate the bending down shown on the conventional linear relationship accurately and predict the development performance of reservoir at high water cut stage accurately. The root reason of the bending-down shown on the traditional relationship at high water-cut stage is that the reservoir is becoming more and more water-wet after long water flooding; the pore’s wall is covered by water phase mostly, the oil phase is separated in the pores by water phase, and changes from continuous phase into discontinuous phase, leading to the sharp decrease of the seepage capacity of oil phase.