Reactive transport simulators are used to investigate the impact of geochemical reactions on in situ brine compositions, and to predict the behaviour of produced brines. These calculations are then used to inform scale management decision making, and to calculate risk of damage and cost of treatment. The authors describe the various types of reservoir interactions that occur during reservoir displacement processes, and then use examples of reactive flow simulations to illustrate the impact of these reservoir processes on scale management decision making and cost, using examples with field data from sandstone and carbonate reservoirs. Barium stripping in sandstone reservoirs undergoing seawater flooding is shown to reduce scaling tendency at the producers, while sulphate stripping due to anhydrite precipitation in a chalk field has an even greater impact, delaying the onset of scaling and reducing chemical treatment costs. The field examples are preceded by a discussion of the velocity at which various saturation, component and thermal fronts propagate, and how mixing regimes are established.