A model to predict pH and mineral scaling is presented, We have included a complete PVT model to describe the hydrocarbon phases. This model gives the CO2 and H2S fugacities which are used to
determine CO2 and H2S concentrations in the water phase. The water phase including all the common equilibria is described using the Pitzer model to correct for non ideality. The model is applicable in the temperature and pressure ranges 0-200°C, 1-500 bar, respectively, and for a ionic strength up to =3 M. Model calculations have been compared with experimental pH and solubility data, The agreement is within +-0. 1 pH unit and within 10% for solubility data of the normal scale forming minerals. Keywords: pH, scale, carbon dioxide, calcium carbonate, equations of state, PVT-model