Geomechanical Monitoring of CO₂ Storage Reservoirs with Microseismicity

We analyse microseismicity induced during the Decatur, Illinois, carbon capture and storage (CCS) demonstration project. More than 10,000 microseismic events were detected during the injection of 1 Mio metric tons of CO₂ over the course of 3 years. The seismicity occurs in distinct clusters and shows little to no correlation to the progressing CO₂ front. For geomechanical reservoir characterization and seal integrity assessment, we need very high depth resolution for the event locations, such that events can be unambiguously attributed to specific formations. We therefore compare event locations using different sensor distributions (borehole and surface sensors) and/or including additional phase arrival information besides direct phases. Analysis of Brune-type stress drop of induced microseismic events with two different estimation methods exhibits signs of pore pressure diffusion processes within individual clusters, although the overall seismicity across clusters does not correlate with pressure gradient or CO₂ front. An observed distance dependence of stress drop from the nucleation point within a cluster suggests a local pressure gradient over the extent of the cluster.