pH effect on wettability of oil/brine/carbonate system: Implications for low salinity water flooding

We thereby measured contact angles at two different pH (pH = 3 and 8) in the presence of either 1 mol/L Na2SO4 or 1 mol/L CaCl2 using a crude oil with acid number of 1.7 and base number of 1.2 mg KOH/g. Moreover, we performed a geochemical modelling study in light of the diffuse double layer to understand how pH controls the number of surface species at interfaces of oil/brine and brine/carbonate. Our results show that pH scales with oil/brine/carbonate wettability, demonstrating that pH is one of the controlling factors to govern the system wettability. Further, our results suggest that pH (6.5-7.5) likely triggers an oil-wet system, which is favourable for low salinity water flooding, but pH < 5 usually exhibits a water-wet system, which explains why low salinity effect is not always observed in carbonate reservoirs. This also confirms that CO2 flooding, carbonated water flooding, and CO2 huff-and-puff EOR very likely renders a strongly water-wet system due to H+ adsorption on the interface of oil/brine and brine/carbonate as a result of CO2 dissolution.