Using fluid-induced microseismicity for reservoir characterization at Decatur

Better characterization of induced microseismic events can reveal important geomechanical parameters and support the assessment of seal integrity for fluid injection operations. The spatio-temporal evolution of seismicity in conjunction with source parameter analysis can provide more detailed insight into reservoir behavior. We perform a detailed characterization of microseismic events at the Illinois Basin – Decatur Project (IBDP), in Decatur, Illinois. About 4800 microseismic events were located with deep borehole sensors during the injection of 1 Mio tons of CO2 during 3 years. Using a waveform cross-correlation method, we can distinguish between events occurring in the sandstone reservoir and events only some tens of meters deeper in the adjacent uppermost crystalline basement. Full-waveform modeling can be used to identify observations of different phase arrivals. Further analysis of source parameters such as Brune stress drop and b-value, as well as the general evolution of microseismic clusters reveal a fluid-driven behavior of seismicity within a cluster, and a punctual hydraulic connection between reservoir and basement. Focal mechanisms are estimated for selected events by combining recordings from surface and downhole sensors. In addition, temporal changes in attenuation as measured from microseismic waveforms may also carry information about the progression of the CO2 plume. We apply a multiple empirical Green’s function approach on repeating multiplets to compare spectral ratios. The method yields systematic spatial variations of Q. However, no temporal variations of Q can be resolved from the microseismic data alone. However, as part of seismic monitoring, we aim to establish a new data analysis workflow integrating active and passive seismic data for a more holistic 4D seismic monitoring system. The larger aperture and higher resolution achieved by combination of all available seismic data may allow for a more precise mapping of the injected fluid.