Estimating future performance of waterflooded reservoirs is a challenging reservoir engineering problem in complex reservoirs that are poorly defined. Flow simulation models characterized by limited spatial data of a key heterogeneity do not always represent the reservoir dynamics for water movement. In this paper, an efficient and rapid workflow is presented to estimate the recovery performance of an existing vertical-well, pattern-based waterflood recovery design using knowledge management and reservoir engineering in a collaborative manner. The knowledge management tool is used to gather production data and calculate pattern-based recoveries and injection volumes by defining pattern boundaries and allocating annual well injection/production volumes in a systematic manner. Classical reservoir engineering forecasting methods, namely, a combination of oil cut versus cumulative recovery performance curves, and decline curve analyses are applied to forecast the performance of the waterflood pattern of interest. The methodology is first applied to a synthetic reservoir model, and then to data from a real field developed by an inverted 9-spot recovery design. Extrapolating established trends of oil cut vs. recovery for each pattern for both the synthetic and real cases quantified future performance assessments. Time can also be attached to the performance by introducing liquid rate constraints. Forecasts based on real data highlighted the optimistic nature of the dated full-field simulation history match and provided a more realistic forecast for business planning. Forecasting using both constant and declining liquid rates differentiated the impact of deteriorating reservoir pressure and oil-cut trends on individual pattern oil rate forecasts thus defining current efficiency of each pattern. The methodology as a result turned into a simple yet powerful forecasting workflow that can be used by any asset team. This study is a very good example of how knowledge management tools can be used to increase the capabilities of classical forecasting methods in reservoir engineering.