Induced Seismicity Monitoring at the Decatur, IL, CO2 Sequestration Demonstration Site

Abstract:

Industrial-scale carbon capture and storage (CCS) will likely require the injection of large volumes of CO₂ into extensive undisturbed brine aquifers. Large-volume injection has the potential to induce earthquake activity (Zoback and Gorelick, 2012). To assess the seismic hazard posed by one such operation, the USGS has been monitoring seismic activity at a CCS demonstration site in Decatur, IL, where supercritical carbon dioxide is injected at 2.1 km depth into the Mt. Simon Sandstone, a basal brine formation over granitic basement. Injection began in November 2011 at a rate of about 1000 metric tons/day, which, pending permit approval will increase to about 3000 metric tons/day in 2015. Our seismic network, about 8 km in aperture, consists of 12 stations, three equipped with boreholes. The nine surface stations have three-component (3C) broadband seismometers and 3C force-balance accelerometers. The borehole stations have 2 Hz 3C geophones at 150 m depth and accelerometers at the surface. We derived a one-dimensional velocity model from a VSP survey and well logs and have used this model to locate seismic events. We identify phase arrivals using standard waveform inspection, spectral analysis and waveform cross-correlation. We calculated seismic moments by integrating the pulses of P- and S-wave ground displacement, a procedure that yielded moment-magnitudes M_W ranging from -0.8 to 1.1. These events locate in two distinct clusters: 0.4 to 1.0 km NE and 1.8 to 2.6 km WNW from the injection well. Double-difference relocations reveal that the cluster closest to the injection well forms a lineament trending N-NE to S-SW. A preliminary strike-slip focal mechanism for an M_w 0.54 event within this cluster is consistent with the orientation of the lineament and regional horizontal principal stress orientations, suggesting reactivation of a pre-existing basement fault. Even with nearly a million tonnes of CO₂ injected, no felt events have been detected so far.