Enhanced oil recovery methods are used to recover the percents of oil that are not naturally recoverable from reservoirs. Water injection, as a secondary recovery, is used to maintain the pressure in water-driven reservoirs. An important point for having a successful injection is the compatibility of injection and formation waters. This article presents the predicting scale tendency and scale precipitation of iron carbonate at different mixing ratios of injection water with formation water in Iranian oilfields. The experimentally measured chemical analyses of formation water and injection water were used by OLI ScaleChem software to determine the formation of this scale. This software has been applied to investigate the potential of iron carbonate scale formation in Iranian oilfields, as a method of secondary recovery or reservoir pressure maintenance.