Water injection into hydrocarbon reservoirs has been studied in great detail both from the subsurface and from surface perspectives, usually aiming at maximizing the production of low-cost oil. Here, the exergy concept is used to examine the potential life-cycle impact of injecting water into hydrocarbon reservoirs by considering the energy requirements of the process. It is found that the exergy recovery factor, being the ratio between the produced exergy corrected for material and process exergy requirements for its extraction and the gross exergy of the source decreases with time. Usually the process exergy requirements to produce the exergy increases with time. In the case of water injection the main contributors to the process exergy are due to treatment of water and the pumping of reservoir fluids. The method presented in this paper can also quantify the amount of CO2 per unit volume of the produced oil. It is contended that the volume of water required to produce the oil is an important indicator of the efficiency of water drive recovery of oil. Moreover, the amount of carbon dioxide produced for the extraction of one barrel of oil depends strongly on the water cut fw.in the producers. Below fw = 80% little CO2 is produced; however, when fw> 90% a small increase in the water cut leads to a large increase of carbon dioxide production. This emphasizes the importance of water management in water drive recovery of oil.