The Japanese consortium to enhance CO2-ECBM carried out a pilot project on CO2 injection from 2002 to 2007 in the city of Yubari, Hokkaido, Japan. However, supercritical CO2 could not be obtained because of low CO2 injectivity and heat loss along the deep injection tubing. The absolute pressure and CO2 temperature at the bottomhole was approximately 15.5 MPa and 28°C, respectively. Therefore, it can be assumed that CO2 was injected into the coal seam in its liquid phase. Liquid CO2 is less permeable in the coal seam because of its high viscosity and the resultant swelling of the coal matrix to decrease permeability.
This study provides a numerical system to predict CO2 flow characteristics of pressure, temperature, supercritical or liquid by considering heat transfer from the injector into surrounding casings and strata. This study focused on keeping supercritical CO2 in the tubing because the viscosity of supercritical CO2 is 40% less than that of liquid CO2. The CO2 temperature required to keep CO2 in its supercritical condition from the surface to the bottom was successfully predicted for various CO2 injection rates and electric heating powers.
Finally, injected CO2 is expected to be supercritical at an injection rate of over 12 ton/d without any heating.