CO2 capture and storage (CCS) is one of expected methods to reduce its emissions into the atmosphere. The Japan consortium to proceed the CO2 sequestration into coal seams has carried out a project on CO2 injection at Yuhbari City, Hokkaido. A targeted coal seam at Yuhbari is located about 1,000m below the surface. However, supercritical condition of CO2 has not been set up due to heat loss along the deep injection tubing. The absolute pressure at the bottom hole is approximately 15.5MPa and the CO2 temperature is about 28°C before the injecting into coal seam. Therefore, it can be assumed that CO2 is injected in liquid phase to the coal seams. Replacements of usual tubing with thermal insulated tubing were carried out, however, the temperature at the bottom hole was still lower than the CO2 critical temperature. The liquid CO2 causes decreasing injectivity into the coal seam due to high viscosity and swelling of the coal matrix.
This study has provided a numerical procedure to predict the CO2 characteristics includes a phase change (supercritical or liquid) in considering heat transfer from the injector to surrounding casings and strata. Present study has focused on the feasibility of supercritical CO2 injection because viscosity of supercritical CO2 is 40% smaller than that of liquid CO2. Present predictions on CO2 temperature at the bottom hole have been matched with measurement results by the wire line test. We have successfully predicted CO2 temperature in order to keep CO2 in supercritical conditions from the surface to the bottom for various CO2 injection rate and electric heater power to keep CO2 temperature. Finally, the minimum CO2 injection rates have been presented as 12 ton/day without a heater and 11 ton/day with heating by 1.43kW heater respectively for a model using the thermal insulated tubing.