Coupled modelling of fluid flow and reactive transport in geothermal systems is challenging because of reservoir conditions such as high temperatures, elevated pressures and sometimes high salinities of the formation fluids. Thermal-hydrological-chemical (THC) codes, such as FRACHEM and TOUGHREACT, have been developed to evaluate the long-term hydrothermal and chemical evolution of exploited reservoirs. In this study, the two codes were applied to model the same geothermal reservoir, to forecast reservoir evolution using respective thermodynamic and kinetic input data. A recent (unreleased) TOUGHREACT version allows the use of either an extended Debye-H¸ckel or Pitzer activity model for calculating activity coefficients, while FRACHEM was designed to use the Pitzer formalism. Comparison of models results indicate that differences in thermodynamic equilibrium constants, activity coefficients and kinetics models can result in significant differences in predicted mineral precipitation behaviour and reservoir-porosity evolution. Differences in the calculation schemes typically produce less difference in model outputs than differences in input thermodynamic and kinetic data, with model results being particularly sensitive to differences in ion-interaction parameters for high-salinity systems.

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