At present the only relatively environmentally safe and economically feasible method is the utilisation of surplus produced water into the receiving horizon. The purpose of the presented study was to demonstrate the necessity of chemical compatibility control during mixing of formation and injected associated waters at the Zapolyarnoye oil and gas condensate field. Mixing takes place during injection of produced water into the receiving aquifer. The amount of associated water to be utilised is measured in hundreds of thousands of m³ per year. Discharge of such water into surface water bodies is prohibited due to its high salinity and sodium chloride ion salt composition. Mineralization of waters of the receiving horizon is 10.5 g/dm³. The source of associated waters are productive horizons of Jurassic age with mineralisation of more than 20 g/dm³. The following tasks were solved in the course of the study: hydrogeological features of the target horizon-receiver were analysed, composition and volumes of associated water injection at the field were studied, chemical compatibility of formation water of the receiving horizon and associated water was calculated by thermodynamic modelling under given formation conditions. The programme was developed on the basis of the normative document 39-229-89 «Water for oil reservoir flooding». We obtained that at formation pressure of 88.23 atm and formation temperature of 42^oC a sediment of 0.153 g/dm³ is formed in the receiving horizon when the share of associated water in the mixture reaches 40%. The formed sediment may cause colmatisation of the receiving horizon. Obtaining correct data on the chemical compatibility of two types of waters of different compositions mixed in reservoir conditions is one of the aspects of environmental protection under conditions of long-term anthropogenic impact on the geological environment of the region.