High enthalpy geothermal fluid is becoming more desirable for energy production with advancing technology. In this study, a new equation-of-state module termed EOS1sc was developed for iTOUGH2, to provide forward and inverse Modeling capabilities at supercritical conditions. As a verification exercise, test cases of five-spot geothermal problems and of a cooling pluton were studied. The IAPWS-IF97 and IAPWS-95 thermodynamic formulations were examined, and results of EOS1sc were compared to other simulators. Advantages of EOS1sc over current geothermal simulators include higher operational range for pressure and temperature, better accuracy, higher computational speed, and/or inverse modeling capabilities. (C) 2015 Elsevier Ltd. All rights reserved.