Comparative analysis on the heat transfer efficiency of supercritical CO2 and H2O in the production well of enhanced geothermal system

Supercritical carbon dioxide has been considered as a promising working fluid for enhanced geothermal system (CO2-EGS) since it can more effectively extract thermal energy from formation rock than water in geothermal reservoir, however higher heat transmission efficiency would unfavourably induce larger temperature drop in the production well. The concern about whether CO2 could economically transfer the extracted thermal energy from reservoir to surface through production well remains unrevealed. This paper has proposed a mathematical model to quantitatively calculate the heat transfer efficiency along the whole production well of both CO2-EGS and H2O-EGS. The model considers the influence of the compressibility of CO2 on radial heat transmission and axial pressure transmission in the cylindrical wellbore, and then the heat transfer efficiency of both CO2 and water are presented and analysed comparatively through case studies. The results show that, the heat transfer efficiency of CO2 decreases significantly as the reservoir temperature increases, and meanwhile the heat transfer efficiency of water increases lightly. The pressure drop in production well of CO2-EGS is merely 31%–45% that of H2O-EGS. Compared to water, CO2 is more preferable for low-temperature EGS. The total transferred energy of carbon dioxide could be enlarged by increasing mass flow rate.