Seawater has been injected into high temperature and natural fractured North Sea chalk reservoirs to improve oil recovery with great success. Previous studies have shown that seawater will improve the water wetness and cause enhanced compaction of the matrix. The composition of the produced water will be quite similar to initial formation water at the start of the water injection. The formation water contains various amounts of divalent cations like Mg2+, Ca2+, Sr2+, and Ba2+. Later on, the composition will change due to the interaction between seawater and the chalk formation and some mixing with formation water. The fluid rock interaction will involve dissolution of CaCO3, substitution of Ca2+ by Mg2+ at the chalk surface, precipitation of CaSO4, SrSO4 and BaSO4 depending on the reservoir temperature. Because of environmental reasons, it is desirable to re-inject produced water together with seawater. In the forthcoming research, we will study experimentally the effect of this re-injection on oil recovery and chalk compaction by using mixtures of produced water and seawater as injection fluid. Based on model studies using the OLI-software package, the compatibility of mixtures of produced water and seawater has been studied at different temperatures by looking at the precipitation of CaSO4, SrSO4 and BaSO4. Also the impact of changes in the concentrations of Ca2+ and Mg2+ due to surface substitution is modelled. The results are discussed in terms of possible scale formation in the producer and injector. In addition, actual chemical equilibrium reactions in the chalk matrix are discussed in relation to variation in temperature during continuous injection of seawater. Special focus is made on wettability modification, irreversible thermodynamics, and impact on the mechanical strength of the chalk matrix.