The novelty of this study is the testing of crude oils with different acid number, from the North Sea Danish sector (Halfdan field) and Valhal formation water as aging fluid, at 115C, applying Modified Flotation Test (MFT) technique in order to investigate the crude oil-brine-rock interactions. Different dilutions of produced water were performed in order to regulate Dan chalk’s wettability, by decreasing salinity, and ionic strength. Thus, different combinations of crude oils and brines, comprising different properties and hence displaying different trends, in terms of reactivity towards the chalk
surface, were tested. Both crude oil and brine properties were analyzed to obtain better insight of the mechanisms taking place during the wettability alteration.