Numerical Model of Temperature at Injection Tubing and Bottom Hole for Supercritical CO₂ Injection into Deep Coal Seams

Abstract

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.