Rock wettability is one of the most important factors driving oil recovery mechanisms, as it controls the location, flow and distribution of reservoir fluids. Wettability affects most of the petrophysical properties of reservoir rocks including capillary pressure, relative permeability, waterflood behavior, electrical properties, and enhanced oil recovery (EOR). Most reservoir rocks are water-wet since they were originally formed in marine or lacustrine sedimentary environments. During hydrocarbons migration, reservoir rocks can be reversed to oil-wet conditions because of surface electrical charges of the grains that attract the oppositely charged components contained in the migrating hydrocarbon phase. Carbonatic reservoir rocks normally show a moderate to strong oil-wet condition. Alkaline substances and some surfactants have the capability to reverse the oil-wet reservoir rocks into a more favorable condition for enhancing oil production. In particular, such mechanism is normally deployed to increase the oil recovery factor of chemical flooding EOR projects. Laboratory testing at reservoir conditions is always required before implementing any field application and the wettability reversal measurement is a key factor for the success of the above EOR application. The present study was performed at the Petroleum Engineering laboratory of the University of Bologna (Italy). Here, it has been clarified that the displacement-by-oil ratio significantly affects the Amott-Harvey wettability index, which affects the final oil recovery expected by the application of alkali and alkali-surfactant flooding EOR technique. On the other hand, only a small change of displacement-by-water ratio can be observed throughout the experiments. This observation can be interpreted as the improvement of oil recovery factor induced by alkali and alkali-surfactant flooding is caused primarily by a lesser preference to oil more than by the complete reversal of previously oil-wet into full water-wet condition. In particular, the maximum relative oil recovery of the strong oil-wet carbonatic rock is obtained when the Amott-Harvey wettability index is close to zero or when the reservoir rock turns into a neutral-wet condition.

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