Carbon capture and storage (CCS) is considered to be an effective method to mitigate anthropogenic carbon emissions that have been the major cause of global warming. One of the possible sites to store CO(2)is in geothermal reservoirs. In this study, an attempt to simulate CO2-brine reservoir rock interaction inside a geothermal reservoir is carried out using the PHREEQC program. The study utilizes published rock mineralogy of the assumed reservoir lithology and chemistry of the hot water in the Ungaran geothermal field, Java, Indonesia. The simulation is based on equilibrium and kinetic modeling and assumes a single stage CO(2)injection kept at a constant temperature and pressure. The amount of injected CO(2)is determined by solubility modeling of CO(2)in hot water under estimated reservoir conditions. The modeling predicted (i) the effect of solubility trapping at early stages of CO2-brine rock interaction, (ii) dissolution of Ca-bearing silicates (plagioclases) coupled with calcite precipitation as a potential chemical processes relevant to a possible CO(2)mineralization, (iii) progressive transition from solubility to mineral trapping becoming significant after 30 days following injection, (iv) minor porosity increase (similar to 0.5%), and (v) achievement of equilibrium between CO2-brine-rock in 10 years after injection. Sensitivity analysis associated with the uncertainties for altering mineral proportion and rock porosity reveal no significant change in the ability of the modeled reservoir to trap injected CO(2)into mineral phases. Concerning the CCS studies so far carried out in geothermal fields in volcanic reservoirs, this modeling comprises one of the first performed for fields with intermediate volcanics. The result from this study can be utilized as foreknowledge for possible future CCS operations in Indonesian geothermal fields. (c) 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.