Geothermal energy currently uses hot deep brines. However, their exploitation can raise issues specifically due to their high salinity and the variations of temperature and pressure during their pumping. Main disorders are the potential degassing of fluids, including CO2, but also the precipitation of minerals in the facilities (production/injection wells, heat exchangers…) involving expensive maintenance works. The chemical and physical properties of the geothermal fluids are key control factors, determining the heatcarrying potential for energy transfer over the lifetime of the well. An improved understanding of the properties of these fluids is therefore necessary to avoid exploitation issues as mentioned above, and even optimize site developments and operations. Additionally, the future for exploration, prediction and utilization of novel geothermal technologies – namely enhanced geothermal systems and supercritical resources – is intimately tied to the understanding of the physical and chemical properties of the reservoir fluids. The REFLECT project aims to improve the accuracy and consistency of key thermodynamic and kinetic input data in order to optimize sustainable geothermal reservoir management, power and heat production and reinjection strategies. Geochemical modelling, which relies on the measurement of high quality data, is one of the numerical approaches needed to reach this goal. In a first step, published laboratory measurements were collected to estimate the properties of saline chloride fluids (NaCl and CaCl2) containing dissolved CO2. A focus was made on properties such as CO2 solubility for medium temperature brines up to 473.15 K. In parallel to data collection, a modelling work was setup to calculate these fluid properties using the specific tool named PhreeSCALE. Relying on the Pitzer equations, it is able to compute thermal and volumetric properties of aqueous solutions such as heat capacity or density of the geothermal fluids. The methodology relies on successive steps that include the estimation of Pitzer’s interaction parameters for the CaCl2-H2O, CaCl2-NaCl-H2O, and CaCl2-NaCl-CO2H2O systems to reproduce literature data. The model applies from 298.15 up to 473.15 K, and to elevated pressures.