Enhanced oil recovery methods are used to recover the percents of oil, which are not naturally recoverable from reservoirs. Water injection, as a secondary recovery, is used to maintain the pressure in water-drive reservoirs. An important point for having a successful injection is the compatibility of injection and formation waters. This article presents the predicting of calcium carbonate precipitation in formation water, injection water, and mixing of injection water with formation water at different pressures. The experimentally measured chemical analyses of formation water and injection water were used by OLI ScaleChem software to determine the amount of CaCO3 scale that can be formed. This software has been applied to investigate the potential of calcium carbonate scale precipitation in Iranian oilfields, either in onshore or offshore fields where water injection is being performed for desalting units' water disposal purpose or as a method of secondary recovery or reservoir pressure maintenance. As pressure was increased, the amount of CaCO3 precipitation decreased, as was predicted by OLI ScaleChem software.