In order to implement an effective iron scale mitigation strategy one of the key questions operators need to answer is “where does the iron come from?”.The problem is that establishing the main source of Fe2+ in higher temperature sour gas/condensate wells where iron sulphide and iron carbonate are formed is not a straight forward process. Many fields do not have a reliable formation water composition and the analysis of produced water often does not allow us to draw any conclusions when scale formation occurs in the well.This work describes a method to predict the maximum concentration of dissolved iron potentially present at equilibrium in a carbonate reservoir and to identify if iron deposits are formed from naturally occurring Fe2+ (in formation fluids) or solely from corrosion processes.The method uses PVT modelling to calculate the reservoir gas H2S and CO2 mole fraction and associated water carbonate and sulphide concentrations. This information is then input in a scale prediction model where the condition of carbonate equilibrium is imposed (CaCO3 saturation ratio =1). The resulting pH carbonate and sulphide speciation represent the equilibrium conditions. Finally we increase the concentration of Fe2+ ions in water until we predict scale precipitation. This concentration represents the maximum dissolved iron concentration which can be stable in the reservoir before iron starts precipitating.In addition the modelling output provides accurate pH trends and three phase H2S and CO2 concentration changes from reservoir to the first stage of separation which can be used to predict general corrosion rates at the given local conditions throughout the production system. These combined results are then used to determine high risk zones in the wells subject to sour corrosion and subsequent iron scale precipitation.Results from high H2S and CO2 gas/condensate wells clearly show that dissolved iron cannot be stable at the reservoir conditions within a carbonate formation and that the iron present in scale precipitated along the well must therefore arise solely from corrosion mechanisms.We have also shown how the carbonate reservoir keeps formation water pH above a threshold level dictated by the carbonate equilibrium. When water enters the wellbore this constraint no longer applies and we see a significant pH drop which combined with the high downhole temperatures is the cause of high corrosion rates downhole in systems of this type.The information provided by this combination of PVT and scale prediction modelling allows operators to focus on the right cause of iron scale in the system (corrosion and/or formation fluids) and to implement effective and efficientmitigation strategies.

Key words: Iron sulphide, iron source, sour corrosion, formation iron, souring, scale prediction, carbonate reservoir.