Breakthrough of alkaline water from one producing zone mixing with formation water and oil from a different zone could potentially give severe calcium naphthenate scaling. In this scenario pre-equilibrated formation water and oil mix with alkaline water that has previously not been in contact with CO2. During mixing alkaline water will increase the pH, while CO2 will partition into the water to lower the pH. It was not known a priori whether CO2 would partition fast enough to inhibit naphthenate formation or if precipitation would occur before CO2 equilibrium had been established. Naphthenate scale risk was therefore examined by mixing formation and alkaline water at down-hole temperature (80 degrees C) with toluene as the model hydrocarbon. It is shown that CO2 partitioning from the oil phase mitigates naphthenate scale both at high and low shear conditions. Calculation of naphthenate scale potential in such a mixing situation should therefore include CO2 equilibrium between the hydrocarbon and aqueous phase.