The Mibale field in offsore of the DRC has been producing oil since 1976. This field is faced with the arrival of massive water and the depletion of its reservoir leading to the drop in its oil production, while the injection of water is effective for several decades. Understanding the behavior of the aquifer in this reservoir is a solution to the application of effective water flooding for oil drainage to this field. The objective pursued in this study is to evaluate the performance of the aquifer on the basis of the material balance equation, to understand its behavior in maintaining or not the pressure in this reservoir in order to identify the causes related to this depletion and the influx of water despite the application of water flooding techniques. To reach this goal, the data collection during the internship made it possible to analyze and process this data using professional software. The results show that the overall drainage index of the water drainage mechanism is 84% (due to 20% for the aquifer alone and 64% for the water flooding) and 10% of oil compressibility. (IDOI), 6% of dissolved gas segregation (IDS). Reserves in this reservoir are estimated at 4.5 million barrels. The aquifer is inactive, semi-radial linear with a constant (U) estimated at 595.5 barrels per psi (bbl / psi) and an initial volume (WI) of 347.1 million barrels (Mbbl). Cumulative contributions from this aquifer are estimated at 173,868,933 barrels for the last 42 years of operation. This aquifer alone has no influence on the inflow of water and the maintenance of pressure, but its influence increases with water from injection wells. In conclusion, this inactive aquifer is located in the carbonate Karst of Upper Pinda to the north of the deposit. Being inactive, this aquifer is not at the origin of breakthrough or coning water acting in this field. It is likely that this phenomenon is amplified by water flooding. Which allows us to classify water flooding technology among aquifer drainage mechanisms; since this significantly activates the behavior of the aquifer and has the same effects as the aquifer.