Overview of microseismic response to CO2 injection into the Mt. Simon saline reservoir at the Illinois Basin-Decatur Project

• Porous injection zone resulting in low formation pore pressure increases.
• Maximum pore pressure increases far below fracture pressures.
• Microseismicity developed along undetected planes of weakness.
• Initial microseismicity influenced by maximum horizontal stress direction.
• Magnitudes ranged from −2.12 to 1.17, ninety-four percent below 0, average of −0.9.

The Illinois Basin-Decatur Project safely and successfully injected, over three years, nearly 1.1 million tons (1 million tonnes) of supercritical carbon dioxide (CO2) into the base of a 1640 ft (500 m) thick saline sandstone reservoir at a depth of 7025 ft (2.14 km). The injection interval, with its high porosity and permeability, allowed for injection pressures to be far below fracture pressures during the daily 1102 tons (1000 tonnes) injection rate. Microseismicity was monitored 1.5 years before injection, through the 3 years of injection and now during permanent shut-in which began in November 2014. The overall average of locatable events per day, during injection, was a little over 4, and events appear to be related to development on previously undetected planes of weakness. Some of these planes and active areas may be related to features developed during diagenetic or compactional processes associated with the Precambrian surface topography. Microseismicity during transient shut-in did not show a rate of decrease, large changes in magnitude, distance from the injection well, or depth.