Illinois Basin – Decatur Project pre-injection microseismic analysis

• The Illinois Basin – Decatur Project included subsurface microseismic monitoring.
• Microseismic events are detected through permanent downhole geophone arrays.
• Microseismicity during the pre-injection phase relates to drilling and well activity.
• Local ambient microseismicity was low; regional seismicity was also observed.
• Events associated with known site operations helped validate monitoring system.

The Illinois Basin – Decatur Project (IBDP) is a large-scale carbon dioxide (CO2) injection and storage demonstration project. Over a three-year period one million metric tons of CO2 were injected deep into the Mt. Simon Sandstone, a deep saline reservoir in the Illinois Basin. This study examines the microseismic data gathered for the period from May 2010 to November 2011, which preceded the CO2 injection.Microseismic events are detected through permanent geophone arrays installed in two wells. A CO2 injection well drilled to the depth of 7236 feet (2205 m) is outfitted with a 4-component geophone array. Situated approximately 185 ft away (56 m), the 3502 feet (1067 m) deep geophysical monitoring well contains a 3-component geophone array.During the pre-injection period, a total of 7894 microseismic events were detected and 99% of these correlate with drilling and other well-related operations. Eight local microseismic events were identified that appear unrelated to well activity and appear representative of the background level of microseismicity.Regional seismicity was also examined by changing triggering parameters and producing a second dataset. Under this configuration, the system detected twelve regional seismic events that correlate to the United States Geological Survey (USGS) earthquake record, and approximately 1100 distant events are believed associated with quarry-related blasting operations.