CO2-based enhanced geothermal systems (EGS) are being widely studied due to great heat extraction and additional CO2 sequestration. However, the existing studies ignored surrounding formation permeability and approximately calculated CO2 sequestration based on flow tests of water-based EGS. In this paper, heat extraction and CO2 sequestration in a CO2-based EGS were simulated simultaneously based on a developed 3D thermal-hydrologic model, and the CO2-based EGS was compared to the water-based EGS for different reservoir permeability parameters, geothermal gradients and surrounding formation permeabilities. The CO2-based EGS has greater heat extraction than the water-based EGS, and the advantage of CO2-based EGS is larger for higher surrounding formation permeability, lower average reservoir permeability and lower initial reservoir temperature. Compared with water-based EGS, the CO2-based EGS has higher fluid loss rate and much higher production flow rate, which results in lower fluid loss ratio especially at low average reservoir permeability and initial reservoir temperature, indicating that it is not accurate to evaluate CO2 sequestration based on experimental data of water-based EGS. For CO2-based EGS, as surrounding formation permeability increases, CO2 sequestration amount increases, and heat extraction rate decreases slowly, showing that surrounding formation permeability has important influences on CO2 sequestration and heat extraction. (C) 2017 Elsevier Ltd. All rights reserved.