The Japan consortium to enhance the CO2 sequestration into coal seams has carried out a project on CO2 injection at Yuhbari City, Hokkaido. However, supercritical condition of CO2 has not been satisfied due to heat loss along the deep injection tubing. The absolute pressure and CO2 temperature at the bottom hole are approximately 15.5MPa and 28 ° C. Therefore, it can be assumed that CO2 is injected in liquid phase to the coal seam. 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 flow characteristics of pressure, temperature, supercritical or liquid in considering heat transfer from the injector to surrounding casings and strata. Present study has focused on the keeping supercritical CO2 in the tubing because viscosity of supercritical CO2 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. Finally, Injected CO2 is expected to be supercritical over 12ton/day of injection rate without any heating.
CO2 capture and storage (CCS) is one of expected methods to reduce its emissions into the atmosphere. There are various underground reservoirs and layers for the storages. For example, aquifer, drained oil and gas reservoirs and unmined coal seams. The coal seams have feasibility because coal can adsorb CO2 gas volume which is almost double of methane1). However, there is a problem that coal matrix swells2) by adsorbing CO2 and reduces its permeability. In order to increase CO2 injection rate into coal seams, supercritical CO2 condition is better than liquid CO2 because its viscosity 40% smaller than that of liquid CO2.
The Japan consortium to enhance CO2 sequestrations into coal seams has carried out the project on CO2 sequestration into coal seams at Yuhbari City, Hokkaido3). A targeted coal seam at Yuhbari is located about 900m below the surface. However, liquid CO2 has been injected due to heat loss along the deep injection tubing. The absolute pressure and temperature at the bottom hole are approximately 15.5MPa and 28 ° C. Replacements of usual tubing with thermal insulated tubing including a argon gas layer were carried out, however, the temperature at the bottom hole was still lower than the CO2 critical temperature.
This study has provided a numerical procedure to predict the CO2 temperature and pressure includes a phase change (supercritical or liquid) in considering heat loss transferred from the injector to surrounding casing pipes and strata. Furthermore, this study has provided numerical simulation results of temperature distribution of the coal seam after injection of CO2.
Figure 1 shows a schematic figure in order to show heat transfer phenomena at an injection well. Four thermal phenomena to construct numerical modeling were considered in order to predict CO2 temperature and pressure at the bottom hole.