CO2-ECBM field tests in the Ishikari Coal Basin of Japan

The feasibility of extracting gas from coal seam while storing carbon dioxide underground was evaluated in Japan. A CO2-ECBM project had begun near the town of Yubari on the island of Hokkaido in northern Japan. The primary coal seam of interest was a 5–6 m thick Yubari coal seam located at the depth of 900 m. A micro-pilot test with a single well and multi-well CO2 injection tests, involving an injection and production wells, were carried out in the period between May 2004 and October 2007. There were a variety of tests conducted in the injection well, including an initial water injection fall-off test and a series of CO2 injection and fall-off tests. Although gas production rate was obviously enhanced by CO2 injection, water production rate was not clearly affected by CO2 injection. Several injection tests suggested that injectivity of CO2 into the virgin coal seam saturated with water was eventually increased as the water saturation near the injector was decreased by the injected CO2. It was estimated that low injectivity of CO2 was caused by the reduction in permeability induced by coal swelling. N2 flooding test was performed in 2006 to evaluate the effectiveness of N2 injection on improving well injectivity. The N2 flooding test showed that daily CO2 injection rate was boosted, but only temporarily. Moreover, the permeability did not return to the initial value after CO2 and N2 were repeatedly injected. It was also indicated that the coal matrix swelling might create a high stress zone near to the injection well.