Microseismic monitoring of a CO2 injection: a case study from the In Salah (Krechba) field, Algeria

We analyse microseismic data from a pilot installation in the commercial sized CO2 injection project at Krechba, Algeria, operated by the InSalah JV. Microseismic data have been recorded with 3C downhole geophones that are deployed in a single vertical well. Waveform data from 6 levels are recorded continuously since fall 2009. Only two levels of the geophone array provided reliable data during the main CO2 injection period and only the uppermost geophone at 80 m depth could be oriented successfully. We performed a cross-correlation-based automatic event detection using one master event. Between August 2009 and June 2011, we detect over 5000 microseismic events. Since the correlation between the occurrence of microseismic events and injection rate at well KB502 is high, we conclude that the majority of the microseismic activity is related to the CO2 injection at KB502. Our interpretation is mainly based on S-P wave onset times and polarization angles of the P-wave. Several event clusters with similar arrival-time differences and backazimuth can be identified. The overall direction of events is oriented in the direction of the largest horizontal stress. A geomechanical analysis suggests that the fracture pressure was temporarily exceeded and reservoir fracturing did occur during two periods in 2010, accompanied by increased microseismic activity. We further calculate spectra, using a multitaper approach, of all events over 0.3 s long time windows around the P-wave/S-wave onsets in order to determine moment magnitudes Mw. All spectra have been corrected for geometrical spreading and intrinsic attenuation. Most Mw estimates range between -1 and 0, with a few larger events up to 1. The b-value determined from the slope of the frequency-magnitude distribution is similar for P-wave and S-wave magnitude estimates for each cluster, but we observe significant variations between clusters. The lowest b-value (close to one) is observed for the most distant event cluster (~950 ms S-P time) with an azimuth of about 300 degree. Events from this cluster show no temporal correlation with the CO2 injection at KB502. Larger b-values between 1.5 and 2 are observed for the other clusters.