Monitoring is essential to assess whether sequestered carbon dioxide (CO2) behaves as expected and a variety of monitoring techniques have been proposed and examined. Although those techniques provide useful information, such as CO2 migration and distribution, individual information yields only limited insight into CO2 behavior in geological reservoirs. To deepen our understanding of migration and containment of sequestered CO2, a monitoring framework must be established to comprehend the individual sources of information. This paper describes the monitoring framework that was applied to a pilot project of CO2 storage conducted at an onshore site in Nagaoka, Japan. The target aquifer was the early Pleistocene sandstone bed, around 60 m thick and 1100 m below the ground surface. During the 554-day injection period, around 10,400 tonnes of CO2 were sequestered. Three monitoring wells were completed around an injection well and several monitoring schemes, including continuous measurements of pressure and temperature, well logging, crosswell tomography, and in situ fluid sampling, were applied. Based on the individual monitoring results, meso-scale as well as macro-scale migration of sequestered CO2 was examined. These multi-scale observations on CO2 migration were not in contradiction with each other but were consistent as a whole. In particular, the compositional analysis of in situ fluid samples in conjunction with time-lapse well logging played an important role in understanding meso-scale migration of CO2, and flow simulation provided remarkable insights into the process of macro- and meso-scale migration. This study discusses monitoring and simulation studies for assessing multi-scale migration, with emphasis on the importance of comprehensive analyses, since they can bridge the gap between the individual monitoring outcomes.
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