Comparison of CO2 and water as working fluids for an enhanced geothermal system in the Gonghe Basin, northwest China

• A novel geothermal production system using vertical wells is designed. • The heterogeneous fractured geothermal reservoir is considered. • The coupled wellbore-reservoir flow and heat transfer processes are analyzed. • The heat extraction rate of the CO 2 -EGS is approximately two times higher than the water-EGS. Because of the additional benefit of geological storage and the self-sustaining thermosiphon effect, CO 2 instead of water as a working fluid for geothermal energy production has attracted much interest. In this study, based on the geological and geothermal conditions in the Gonghe Basin of China, a more realistic physical model is built by coupling the wellbore and the thermal reservoir. In our model, a novel dual-vertical well pattern and the heterogeneous fractured reservoir are considered. Based on the novel dual-vertical well pattern, the CO 2 – based enhanced geothermal system (EGS) can avoid premature thermal breakthrough. The results demonstrate that the CO 2 -EGS is more efficient than the water-EGS. The heat extraction rate of the CO 2 -EGS increased by 58% when compared to the water-EGS. However, the CO 2 -EGS results in drastic drops of production temperature in the wellbore (approximately 62 ℃) due to the strong Joule-Thomson effect. In addition, the sensitivity results suggest that the CO 2 -EGS is more suitable for developing in low permeability geothermal reservoirs. This work provides important references for the working fluid selection and well placement to develop the clean power generation in the Gonghe Basin geothermal field.