Mineral scaling, corrosion and chemical reactions between the re-injected fluid and the aquifer rock affect the long-term exploitation of deep geothermal systems. We use numerical models patterned after the geothermal system at Bad Blumau, Austria, to track the passage of the production fluid through the system. As model input we use pressure, temperature, and fluid composition data collected at the ground surface, as well as evidence for CO2 gas exsolution within the production well. From these constraints we infer the chemical conditions in the reservoir, assess the geochemical implications of the extraction of heat and CO2(g) at the surface, examine the consequences of fluid re-injection into the reservoir, and identify geochemical indicators of incipient corrosion. Exsolution and the subsequent extraction of CO2(g) from the fluid decreases the total CO2. increases the pH and, if not suppressed by adding chemical inhibitors, causes precipitation of carbonate minerals. Calculations show that without the inhibition of Ca-bearing carbonates, the production well could be clogged within a few days. The compositional changes caused by CO2 extraction and the lower temperature of the reinjected fluid trigger disequilibrium reactions at the base of the injection well, potentially affecting the injectivity of the system. Owing to the high redox buffering capacity of the fluid, indicators for incipient corrosion include an increase in the fraction of pyrite in the production well and increase in Fe-oxides/hydroxides near the surface under higher pH and lower temperature conditions. (C) 2012 Elsevier Ltd. All rights reserved.