Microseismic Monitoring, Event Location, and Focal Mechanisms at the Illinois Basin–Decatur Project, Decatur, Illinois, USA

The Illinois Basin–Decatur Project safely injected, over 3 years, nearly 1.1 million tons (1 million tonnes) of supercritical carbon dioxide (scCO 2 ) into the base of a 1,640 ft (500 m) thick saline sandstone reservoir at a depth of 7,025 ft (2.14 km) during CCS1 injection. Baseline data collection started 1.5 years before injection, documenting air, soil, groundwater, and bedrock conditions. Reservoir monitoring included microseismicity, formation pore pressures, surface acquired 2D and 3D seismic data, and petrophysical bedrock properties through cores and downhole geophysical logging. Data guided early hydromechanical models to predict pressure and scCO 2 fronts through time. A 3D velocity model developed for the site was critical for accurate location of microseismicity, which during injection averaged a little over four events per day, and maximum magnitudes as other CO 2 injection sites, while only eight microseismic events were detected during the 1.5 years of preinjection monitoring. Events appear to be related to previously undetected planes of weakness, which developed during diagenetic or compactional processes associated with the Precambrian surface topography and faults in the highly fractured Precambrian basement. The orientation of the planes of weakness in relation to the in situ stress field match the calculated focal mechanism, and modeling shows them to be critically stressed at these low pressure increases and capable of inducing seismicity.