Simulation models may be used to predict not only where the mixing of incompatible brines occurs in water-flooded reservoirs. but also tile impact of in situ deposition Oil scaling ion concentrations in and around the production wellbore. The location of maximum sulphate scale deposition and the resulting brine compositions at the production well are calculated for a range of sensitivities. including reservoir properties (Layered, homogenous, aquifer), well geometry (location and orientation within field and with respect to other wells and the aquifer). and various production scenarios (desulphation and squeeze treatment). In conventional systems with no aquifer, it is demonstrated that maximum scale deposition occurs within or in the immediate vicinity of the production wellbore, and therefore low produced cation concentrations would indicate that squeeze treatments have not been successful. In systems where water injection is into the aquifer, low cation concentrations may result from deposition deeper within the reservoir. Maximum scale dropout still occurs as the fluids approach the production well. but may be sufficiently far from the wellbore not to have any major impact on productivity, or to be affected by squeeze treatments. However, sufficient concentrations of scaling ions are still delivered to the production well to necessitate squeeze treatments. although using lower volumes of inhibitor. Once cation concentrations have been reduced by deposition deep within the reservoir, it is predicted that they will never pick up again. This paper also assesses the impact of desulphation and produced water re-injection on scale deposition in the reservoir, and the need to protect injection and/or production wells is discussed.