abstract: mineral scaling in oil and gas production equipment is one of the most important problem that occurs while water injection and it has been recognized to be a major operational problem. the incompatibility between injected and formation waters may result in inorganic scale precipitation in the equipment and reservoir and then reduction of oil production rate and water injection rate. and as we know the oil field scales costs are high because of drastic oil and gas production decline, frequently pulling of downhole equipment for replacement, reperforation of the producing intervals, reaming redrilling of the plugged oil wells, stimulation of the plugged oil-bearing formation, and other remedial work overs. this thesis presents the mechanism of scale formation by water in oil fields and suggests an accurate model capable of predicting scaling phenomena in offshores carbonate oil field due to mixing of incompatible waters and its influence on reservoir performance and production rate is investigated. for this purpose, a series of experimental work has been conducted to study the compatibility of sea and formation waters. amount and type of scales formed as the result of mixing different portions of these waters have been applied and incorporated with simulation studies. the experiments show that the amount of mineral scale formation increase about 57% when the temperature increase to attend 80 ?c and the saturation index decreases about 52.17 % at the same temperature. a new and reliable scale prediction model which can predict scaling tendency of common oilfield water deposits in water disposal wells, water-flooding systems and in surface equipment and facilities is developed and present. the development of the model is based on experimental data and empirical correlation. key words: scale formation; salt precipitation; water injection; oil reservoirs; saturation index