A convenient, practical method is applied for rapid analysis and interpretation of after-water-breakthrough history of oil wells producing under waterdrive for determination of effective fluid saturation, relative permeability, heterogeneity and displacement efficiency. Formulation and methodology are presented for the characterization of parameters of radial unsteady-state flow of oil and water through producing-well drainage regions in oil reservoirs. Validity and capabilities of this approach for diagnosis of conditions in production-well drainage zones in waterdrive reservoirs are confirmed by analysis of typical oil field case scenarios. The present approach is proven to yield significant information of practical importance that can be used for effective selection of potential enhanced oil recovery (EOR)/improved oil recovery (IOR) methods for oil reservoirs.