A Abstractoptimizing water injection strategy in a mature field: a reservoir simulation study of field \\\"x\\\", south sumatraareza badrilnim: 171012200008master of petroleum engineering study program, faculty of earth and energy technology, trisakti university, jakarta, indonesiafield \\\"x,\\\" located in south sumatra, is a mature oil field characterized by a multilayer reservoir system that has been in production since june 1957. after producing a cumulative 86.5 million barrels of oil (mmstb), the field is currently experiencing declining production performance, with a recovery factor (rf) of 41.8% from an original oil in place (ooip) of 206.5 mmstb. pressure depletion and increasing water saturation across multiple reservoir layers have emerged as key challenges in sustaining production and optimizing recovery.this research aims to evaluate enhanced oil recovery (eor) strategies through reservoir simulation based on a validated 3d static geological model. the geological model estimates ooip at 206.6 mmstb, closely matching the simulation-derived ooip of 206.5 mmstb, indicating strong model reliability with only a 0.1% deviation.the focus of this research is on an integrated field redevelopment plan utilizing water injection. key efforts include optimal reservoir layer selection for injection, injection pattern and rate design, and strategic conversion of existing unproductive wells into injectors. additionally, the development strategy involves workover operations on low-productivity wells with remaining oil potential and drilling of new infill wells to maximize production.simulation results demonstrate that the proposed integrated development approach increases the estimated ultimate recovery (eur) to 93.7 mmstb, thereby improving the recovery factor to 45.4%. the optimized water injection strategy contributes to a recovery factor improvement of 1.1% of ooip, equivalent to an incremental production of approximately 2.5 mmstb, when compared to a base case scenario with no further development (nfa).the outcome of this research provides a robust technical foundation for implementing a cost-effective and targeted water injection program. the proposed strategy has the potential to significantly revitalize the performance of field “x” and extend its economic life.keywords: mature oilfield, multilayer reservoir, waterflood, reservoir simulation, injection rate, recovery factor, pattern injection.