Thermal and fluid processes in a closed-loop geothermal system using CO2 as a working fluid

A co-axial closed-loop geothermal system is a new method to extract geothermal energy. It uses a continuously closed wellbore and the closed system can avoid many problems of traditional geothermal development methods, such as reservoir blockage and heat transmission fluid leakage. Generally, water as a working fluid will cause a series of problems which are not conducive to the operation and maintenance of the geothermal system. Recently, using CO2 as a working fluid has attracted considerable attention. CO2 has a larger mobility and buoyancy that result from its lower density and viscosity. However, a large flow rate will lead to incomplete heating of the working fluid, which will reduce the thermal performance of the geothermal system. In addition, CO2 has a smaller specific enthalpy, which is not beneficial to the heat extraction of the geothermal system. Therefore, it is necessary to study the advantages and disadvantages of CO2 and water as working fluid. In this work, a wellbore reservoir coupled model is built to simulate the operation fluid flow and thermal processes in a co-axial closed-loop geothermal system. The difference in productivity between water and CO2 as working fluids is investigated. The fluid flow and thermal processes of CO2 and water along the wellbore and the heat-extracting mechanism are analysed. The results show that although CO2 is more efficient, its output temperature is lower than water. Therefore water is still suitable for geothermal reservoirs with lower temperature. The modelling and analysis methods presented here may provide a theoretical reference for the selection of a working fluid in geothermal engineering developments.