Experimental investigation on the natural convection heat transfer in the vertical annulus of a CO2 injection well under steady-state conditions

The wellbore dynamics of CO2 injectors is of great importance to all CO2 geological storage and utilization technologies. Natural convection heat transfer of the annular fluid was found to have a non-negligible influence on CO2 temperature distribution in the injection well, which determines CO2 properties when entering the reservoir and thus influences CO2 injectivity, storage and utilization efficiencies. An experimental system was set up in the study combining supercritical pressure CO2 flowing inside the inner tube and natural convection of the annular water. The uniform heat flux was given to the outer cylinder to represent the geothermal heat flux from the surrounding rock. The natural convective heat transfer coefficient and the CO2 temperature increase were acquired with different heat fluxes, CO2 inlet temperatures, inlet pressures and mass flow rates. Measurements were made after establishing steady-state conditions. The fitted correlations of natural convective heat transfer coefficient and equivalent thermal conductivity of the annular water were obtained and compared with the direct simulation of the natural convection in Ordos and an offshore CCS projects, showing good consistency. The results indicated that obtained equivalent thermal conductivity of the annular water could be a useful parameter for simplification and validation of modeling CO2 injectors.