Application of the Debye-Hückel theory for chemical reaction modeling of geothermal brines does not yield sufficiently accurate results. Thus, for the development of a new chemical reaction module for the numerical simulation model SHEMAT (Clauser and Villinger, 1990), the Pitzer formalism (Pitzer, 1991) is used to calculate aqueous speciation and mineral solubilities. It is based on an extended code of PHRQPITZ (Plummer et al., 1988). Using temperature dependent Pitzer coefficients, the system Na- K-Mg-Ca-Ba-Sr-Si-H-Cl-SO4-OH-(HCO3-CO3-CO2)-H2O can be modeled with sufficient accuracy for temperatures from 0° to 150°C. The incorporated carbonic acid system (set in parentheses in the list above) is valid for temperatures from 0 to 90°C, only. Flow, heat transfer, species transport, and geochemical reactions are mutually coupled for modeling reactive flow. Changes in porosity and permeability influence the flow and transport properties of the reservoir. These changes are taken into account by a relation derived from a fractal model of the pore space structure (Pape et al., 1999). A conceptual case study of the injection behavior of a geothermal installation focuses on the immediate vicinity of the well. The injection of cold water has a great influence firstly on the hydraulic conductivity of the aquifer indicated by continuous head pressure increase at the well and secondly on the equilibria between the minerals of the formation and the geothermal fluid. Reservoir changes are studied for the two cases of temperature dependence of solubility, prograde (i.e. barite) and retrograde (i.e. anhydrite). Dissolution of anhydrite induced by cooling down increases the permeability of the formation in a growing region around the borehole and precipitation at the temperature front decreases it. During the initial period of reinjection considered in this study, the negative effect on the injectivity by the colder water is partially compensated by the dissolution of anhydrite. Precipitation of barite around the borehole does not alter the permeability of the formation significantly because the volume of relocated mineral is too small.