Dynamic drainage volume is a useful measure in evaluating well completions, well spacing, and water-flood operations. It is usually approximated with a two-dimensional circle or a three-dimensional (3D) box that encloses a well using empirical correlations and production/injection volumes. While this approximation may be convenient, it certainly is not a good estimation for the effective and dynamic drainage volume, which is key for improved recovery. This paper proposes a new method to compute dynamic drainage volumes based on reservoir simulation results. A 3D fluid flow velocity field is first generated and then visualized as a function of time. Through velocity thresholding, one can delineate flow regions, and accurately and parsimoniously determine well drainage in water-flood operations. Our new method was proven to be more efficient and practical as demonstrated in a field-based synthetic model with nine injectors and 16 producers formed as an inverted five-spot water-flood pattern commonly used in the field, and a benchmark SPE 9 model. The novelty of the method lies in that a 3D fluid velocity field is generated to determine dynamic drainage volume. Our new method could be applied to optimize well placement and improve well operation, and finally increase the production in a heuristic, instructive, and cost-effective manner to maximize the estimated ultimate recovery.