Using abandoned wells from oil and gas industry or dry holes, geothermal single well applications enable the prevention of drilling costs and prospecting risks which are crucial inhibitions in geothermal development. Recently, deep geothermal single wells were numerically investigated for their thermal and hydraulic performance. For the first time, we now include the chemistry of the produced brine in a fully coupled THC model for studies of scaling formation. As a case study, the 2011 circulation test of the GeneSys well Gt1 Gro(sci)-Buchholz (Hanover, Germany) was modeled successfully using the fully coupled THC code TOUGHREACT. High salinity requires application of the Pitzer ion interaction model which was verified for this particular chemical system using literature data. Modeled wellhead temperature is in very good accordance with measurements. Also simulated depth of scaling formation and total scaling volume fit to onsite observations. Evaluation of initial reservoir brine composition reveals its large impact on the range of depth where scaling occurs. Furthermore, it is shown that the presence of CaCl2 reduces halite solubility considerably and favors scaling formation. Results show that our modeling concept is capable of quantifying the complex coupled THC processes in single wells. (C) 2016 Elsevier Ltd. All rights reserved.