Use of CO2 as Heat Transmission Fluid to Extract Geothermal Energy: Advantages and Disadvantages in Comparison with Water

Use of CO2 as heat transmission fluid to extract geothermal energy is currently considered as a way to achieve CO2 resource utilization and geological sequestration. As a novel heat transmission fluid, the thermophysical property of CO2 is quite different from water. It has many advantages, such as larger mobility and buoyancy resulted from the lower density and viscosity. This will reduce the consumption of driving pressure of the circulation, and save the energy consumption of external equipment. The cycle even can be achieved by siphon phenomenon under a negative circulating pressure difference. However, there are still some disadvantages for CO2 as a kind of heat transmission fluid, such as small heat capacity, leading to carry less heat at the same mass flow rate. At the same time, if temperature and pressure change, it will cause a more complex flow and thermodynamic processes because of the lager expansion and compression coefficient for CO2. Lager compressibility makes it possible to get high temperature at the bottom of the injection well, but lager expansion coefficient makes the temperature drops rapidly during the extraction process. Therefore, how to scientifically control the production pressure to guarantee the temperature at the head of production well to be high enough and then improve the efficiency of heat extraction is the key problem to be further studied and solved. Here, a classic idealized “five-spot” model coupled with wellbores is set up according to the geological and geothermal conditions and parameters of the central depression of Songliao basin. Our purpose is to (1) explore the flow and thermodynamics process of supercritical CO2 as heat transmission fluid, analyze the heat recovery mechanism, (2) compare the heat extraction efficiency of CO2 with water, and evaluate the advantages and disadvantages using CO2, (3) optimize the temperature and pressure of injection and production and other parameters for CO2, and (4) determine the favorable range of temperature and pressure of geothermal reservoirs, and provide a theoretical basis for the selection of heat transmission fluid. Results from this work may be useful for future field design of a CO2-geothermal system.