In order to maintain reservoir stability and reduce reservoir subsidence, reinjection of produced fluids into the reservoir is often proposed. Furthermore, studies by Karvounis and Jenny (2012, 2014), Buscheck et al. (2015), and Saar et al. (2015) found that the reservoir life span, heat extraction efficiency, and economics of a geothermal system can be increased by preheating the working fluid in shallow reservoirs and then injecting the fluid into a deep reservoir. We modified the TOUGH2 simulator to specify the reinjection of produced fluids with the same composition at a prescribed or at the production temperature – the latter for cases of injecting the produced fluid into another 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 CO2-geothermal system and focus on the effects of reinjecting small amounts of brine that are produced along with the CO2. Brine has a significantly smaller mobility than supercritical CO2 at a given temperature and thus accumulates near the injection well. Such brine accumulation reduces the relative permeability for the CO2 phase, which in turn increases the pore-fluid pressure around the injection well and reduces the well injectivity index. For this reason, and as injection of two fluid phases is problematic, we recommend removal of any brine from the produced fluid before the cooled CO2 is reinjected into the reservoir. We also study the performance of the multi-level geothermal systems (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). It is found that preheating brine at the shallow reservoir extends the lifespan of the deep, hot reservoir, thereby increasing the total power production from the deep, hot reservoir.

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