A reservoir simulation study of CO2 injection and N-2 flooding at the Ishikari coalfield CO2 storage pilot project, Japan

The Yubari pilot at the southern part of the Ishikari coalfield of Hokkaido represents Japan’s first CO2 storage field trial in coal seams. Following a micro-pilot and two multi-well CO2 injection tests (involving an injection and a production well) between May 2004 and October 2005, a three-stage N2 flooding test, including two separate CO2 injection periods prior to and after the flooding, was performed in 2006 to evaluate the effectiveness of N2 flooding on improving well injectivity. The test showed that the daily CO2 injection rate was boosted by up to four-fold by the N2 flooding, but only temporarily. This paper reports the latest field test results and a history-matching and reservoir simulation study of the field data, using the Imperial College in-house ECBM simulator METSIM2 with its dynamic permeability modelling capability. History-matching of 2004 and 2005 multi-well field tests has shown that CO2 injection caused approximately one-order of magnitude reduction in the near wellbore permeability and thus in well injectivity. This is consistent with the reported well test results. The calibrated reservoir and permeability model was then used to evaluate the performance of the N2 flooding test. The simulation results indicate that an increase in the wellbore permeability of over two-order of magnitude (one order over the in situ permeability) is responsible for the observed dramatic short-term increase in the post-flooding CO2 injectivity.