Derivation of water flooding characteristic curve for high water-cut oilfields

The linear relationship between relative permeability ratio (K-ro/K-rw) and water saturation (S-w) on the semi-log coordinate in the stage of middle water-cut is the theoretical basis for the derivation of traditional water flooding characteristic curve. However, the relationship of K-ro/K-rw versus Sw deviates from the straight line in the high water-cut stage, which results in the upwarping of water flooding characteristic curve. In order to accurately predict the production performance and recoverable reserves in the late development stage, the relative permeability curves of actual cores were averaged. Furthermore, using the core data in the reference, a new expression of Kro/Krw versus Sw was obtained by regression for the high water-cut stage. On the basis of the frontal-drive equation and the average water saturation equation proposed respectively by Buckley-Leverett and Welge, a new water flooding characteristic curve was derived which is more applicable for high water-cut oilfields. The calculation results indicate that the recoverable reserve calculated by the new approach is almost equal to the result of the production decline method, proving it is a practical tool in the prediction of production indexes in the late development stage of oilfields.