In practice, geothermal fluids are sampled under surface conditions for geochemical interpretations. However, the physical and chemical properties of geothermal waters change as the waters flow from a reservoir to the surface along the well due to processes such as mineral scaling, degassing, cooling and boiling. The objective of this study is to estimate the geochemical characteristics of water-dominated geothermal reservoirs and to model the geochemical evolution of fluids in geothermal wellbores to provide information on the parameters that contribute to sustainable energy production. A workflow was developed for commonly used non-commercial software PHREEQC and geochemical modeling approach was demonstrated for fluid samples of four geothermal wells located in the Kizildere geothermal field (SW Turkey). According to the modeled reservoir chemistry, the reservoir type in the field is water-dominated, and the sum of partial pressures of dissolved CO2 and steam in the reservoir varies from 135-160 atm. First gas bubble depths, where an inhibitor should be injected below, were calculated for relevant flow rates of the wells as 1597, 1751, 1884 and 1579 meters. The calculated initial total calcium concentrations in the reservoir are 3.67, 5.93, 5.04 and 6.01 mg/kg. Besides calcite, 16 minerals including silica polymorphs, amphibole, serpentine, pyroxene, carbonate and phyllosilicate groups, are gained precipitation tendency under the separator conditions. To provide sustainable energy production by preventing mineral scaling in reservoirs and wells, the required parameters such as; appropriate depth for inhibitor injection in wellbore, type and initial concentration of scaling minerals, total dissolved gas and steam pressures in reservoir to limit maximum flowrate can be determined for any water-dominated geothermal field by using the proposed workflow in the PHREEQC software.