Formation waters extracted with crude oil and natural gas, due to their amount and chemical composition can be a problem for petroleum companies operating hydrocarbon deposits. On average, the world generates 2 to 3 times more water than oil. On average, the world generates 2 to 3 times more water than crude oil. The amount of extracted water increases with the time of exploitation of the deposit, in the case of deposits at the final stage of depletion, the amount of extracted water is 5 to 8 times bigger than petroleum. Formation waters from hydrocarbons deposits are usually the highly mineralized brines. Large quantities of highly mineralized waters extracted with crude oil and gas are disposed of in various ways or neutralized. The most common way of disposing of these waters is by injecting them into rock mass. As a result of injection of reservoir waters into hydrocarbon deposits, the waters interact with the storage formations. In these formations, there may be numerous reactions of mineral water with the rock environment. The injection of reservoir waters will also cause mixing of waters that can disturb the state of thermodynamic equilibrium and will alter the chemistry of these waters. It was analyzed by the geochemical modeling of the interaction of the reservoir waters of Przemyśl natural gas field. Using the PHREEQC program, the chemical reactions related to the mixing of reservoir waters of different chemical types have been studied. It has been found that is possible to precipitation appropriated minerals as a result of mixing water with different chemical composition.