Abstract: The derived process of the relationships for water cut (f_w)vs.recovery of recoverable reserves (R^*)corresponded with waterdrive curve is recommended.It's shown that the relationships of f_w vs.R^* are a law of water cut raising being representative of essence of displacement characteristics in waterflooding reservoi rs.The f_w —R_*relationships of previously-recommended 4 waterdrive curves can reflect the different laws of water cut raising, i.e.Nazarov curve, Maksimov-Tong curve, Sipachev curve and Sazonov curve.It' s shown that oil viscosity is the most important natural control on the law of water cut raising in waterflooding reservoi rs.The principle for choosing waterdrive curves suitable for the actual reservoir is the agreement between the choosed waterdrive curves and water cut raising trend actual reservoirs.The purpose is to look for a waterdrive curve which can make calculation and prediction as early and accurate as possible in a reservoir.It' s demonstrated that to choose waterdrive curves by oil viscosity is helpful to realize this purpose and can avoid blindness in use of waterdrive curves.Finally, the method for determining the limits of oil viscosity (3 and 30 mPa°s)by which the waterdrive curve is choosed by using the Ivanova's f_w —R^* chart and the relationship between cumulative water-oil ratio (W_p/N_p)at R^*=1.0 and μ_o used in China is recommended.

Key words: Water drive, Characteristics, Curve, Viscosity, Selection, Prediction