Integrated Active and Passive Seismic Monitoring at the Decatur CCS Project

For holistic seismic monitoring of the progressing CO2 plume within the Mt. Simon sandstone reservoir at the Illinois Basin - Decatur CCS project, we aim to integrate passive microseismic data with complementary surface- and VSP data from active seismic surveys. A detailed microseismic analysis allows for spatial mapping of attenuation Q around the injection area. We identify time periods of possible reservoir changes in between active seismic time-lapse monitors and we can increase the temporal resolution of the 4D signal. In addition, finite-difference FD modeling of microseismic events can help explain the details of the observed waveforms including different phase arrivals for up- and downgoing waves. This, in turn, is essential to improve the depth resolution of microseismic event location and determine whether seismic activity is confined to the injection zone or growing outward. The discrimination of ray paths from FD modelling also allows to select the most relevant waveform phases for 4D time-lapse studies and is an important pre-processing step. In combination with simultaneous tomographic inversion of the velocity- and attenuation structure, it has the potential to maximize the resolution of 4D changes in the medium.