Carbonate oil reservoirs are often fractured with moderate water-wet conditions, which prevent spontaneous imbibition of water into the matrix blocks. Average oil recovery from carbonates is usually less than 30%. Recent studies on chalk cores from the North Sea have shown that seawater is able to change the wettability towards a more water-wet condition at temperatures >100 °C. Seawater contains favorable concentrations of the potential determining ions Ca2+, Mg2+, and SO42- that are active in the displacement of strongly adsorbed carboxylic material from the chalk surface. During seawater injection the initial formation water mixes partly with seawater, and the amount and composition of the produced water varies with time. Due to environmental reasons, produced water should be re-injected together with seawater into the chalk formation. The question is: “Will mixtures of seawater and produced water displace oil in a similarly good manner as pure seawater?” This work showed that at T >100 °C the oil recovery by using PW:SSW-mixtures was significantly higher than by using pure PW in a spontaneous imbibition process. In a viscous flood, SSW appeared to be much more efficient than PW to displace the oil, and high recovery values were reached.