Prediction model of bottom hole temperature and pressure at deep injector for CO₂ sequestration to recover injection rate

The field test of CO₂-ECBM was carried during 2003 to 2007 in order to evaluate technical feasibility of injecting CO₂ into the coal seam while producing CH₄ at Yubari City, Hokkaido, Japan. It aims to resolve global warming and to develop a system for injection and sequestration of CO₂ into coal seams. The targeted coal seam is located about 900 m below the surface. The absolute pressure at the bottom hole is approximately 15.5 MPa and the CO₂ temperature is about 28 ^{{ring operator}}C before the injecting into the coal seam. Thus, CO₂ is injected in liquid phase to the coal seam and supercritical condition of CO₂ has not been satisfied due to heat loss along the deep injection well. Replacements of usual tubing pipes with thermal insulated tubings were applied, however the temperature at the bottom hole was still lower than the CO₂ critical temperature. The project had a problem about decreasing injection rate after starting injection CO₂. It was evaluated by analysing the field test data that the maximu m decreasing ratio of the coal seam permeability was 0.065 and permeability around injection well was became 6 times larger than the original one. We assumed that its reason is caused with swelling of the coal seam around the injection well by liquid CO₂ injection. Present study has focused on the keeping supercritical CO₂ in the tubing because viscosity of supercritical CO₂ that is 40% smaller than that of liquid CO₂. The CO₂ temperature has been successfully predicted in order to keep CO₂ in supercritical condition from the surface to the bottom for various CO₂ injection rates and electric heater power. The minimum injection rate has been presented in order to keep CO₂ in the supercritical condition at the bottom hole of the injection tubing. Injected CO₂ is expected to be super critical over 12 ton/day of injection rate without any heating in the tubing.