In order to maintain reservoir pressure and stability and to reduce reservoir s ubsidence, reinjection of produced fluids into the reservoir is common practice . Furthermore, studies by Karvounis and Jenny (2012 ; 2014), Buscheck et al. (2015), and Saar et al. (2015) found that preheating the working fluid in shallow reservoirs and then injecting the fluid into a deep reservoir can increase the reservoir life span, the heat extraction efficiency, and the economic gains of a geothermal power plant . We have modif ied the TOUGH2 simulator to enable the reinjection of produced fluids with the same chemical composition as the produced fluid and with either a prescribed or the production temperature . T he latter capability is useful, for example, for simulating injecti on of produced fluid into another (e .g., deeper) reservoir without energy extraction. Each component of the fluid mixture , produced from the production well , is reinjected into the reservoir as an individual source term. In the current study, we investigate a CO 2 - based geothermal system and focus on the effects of reinjecting small amounts of brine that are produced along with the CO 2 . Brine has a significantly smaller mobility (inverse kinematic viscosity) than supercritical CO 2 at a given temperature and thus accumulates near the injection well. Such brine accumulation reduces the relative permeability for the CO 2 phase, which in turn increases the pore - fluid pressure around the injection well and reduces the well in j ectivity index. For this reason, and as injection of two fluid phases is pr oblematic, we recommend removal of any brine from the produced fluid before the cooled CO 2 is reinjected into the reservoir. We also study the performance of a multi - level geothermal system (Karvounis and Jenny, 2012; 2014; Saar et al., 2015) by injection of preheated brine from a shallow reservoir (1.5 - 3 km) into a deep reservoir (5 km). We f i nd that preheating brine at the shallow reservoir extends the lifespan of the deep, hot reservoir, thereby increasing the total power production.