CO2 capture and storage (CCS) is one of expected methods to reduce CO2 emissions into the atmosphere. The Japan consortium to proceed the CO2 sequestration into coal seams carried out the project "Japan CO2 Geosequestration in Coal Seams Project (JCOP) " on CO2 injection and CH4 production during 2002 to 2007 at Yubari City, Hokkaido, which is hereinafter called as Yubari ECBMR pilot test. A targeted coal seam at the project was located about 890m below the surface. The project had a problem on CO2 injection with low injection rate of about 3ton/day. In the pilot-test data, it was observed as a common pattern that CO2 injection rate was decreasing during 3 to 10 days after starting CO2 injection, because of decreasing permeability around the injector. The maximum decreasing ratio of the permeability was evaluated as 1/50 of the initial one. The reason was assumed by swelling of the coal seam around the injector by injection liquid CO2.
In this study, an analytical model has been presented in consideration of permeability reduction by swelling. Present predictions on CO2 injection rate with the model have been matched with monitoring data measured at the Yubari ECBMR pilot test. The ratio of permeability reduction of coal seams by the swelling (swelling factor) has been evaluated as 1/50 to 1/16.
In this research, numerical simulations, which use double porosity model and Palmer & Mansoori model to express permeability reduction by shrinkage of micro-pores caused by CO2 adsorption, called as coal matrix swelling, has been carried out with the ECLIPS (CBM option) . A history matching study was conducted to estimate CH4 desorption time and production bottom-hole pressure (BHP). Other reservoir parameters such as pore compressibility and gas-water relative permeability curve, were set based on the report of Yubari pilot test and previous researches by Yamaguchi et al. (2007) and Pekot & Reeves (2002) .
The numerical simulations for sensitivity studies on CO2 injectivity and CH4 productivity have been carried out by giving CO2 and CH4 adsorption capacities and viscocities of CO2 and water as a function of the coal seam temperature (30 to 60 °C) . Finally, the numerical simulation results with five spot model have been presented to evaluate CO2 injectivity and CH4 productivity by changing the well spacing. The results show that CO2 injection is roughly proportional to number of injectors, however the time at the maximum CH4 production rate is delayed with area including four production wells. From view of economical evaluation, drilling cost of wells and accumulated present value of revenue of CH4 production will be important to decide the optimum well spacing.
