LITMIR.BIZ

Figure 4.12 Event location accuracy versus the total number of geophones for optimal design of a borehole geophone array in Figure 4.10.

4.6 CONCLUSIONS

      We have developed a methodology for the optimal design of seismic networks for monitoring induced microseismic events during CO₂ injection and storage. Based on the location of the target monitoring region where induced microseismic events may occur, we design a regularly spaced surface seismic array within an area centered at the target monitoring region, or regularly spaced borehole geophone array above the target monitoring region. We vary the number of seismic stations/borehole geophones, and compute the event location accuracy using both absolute and differential travel times. The optimal seismic network is determined using the trade‐off curve between the mean location accuracy and the number of seismic stations. We have demonstrated our methodology using a synthetic model for the Kimberlina CO₂ storage demonstration site in California. Our results show that for the specifically defined monitoring regions in this study, it needs approximately 20 surface seismic stations or 50 geophones for cost‐effective monitoring. These demonstration examples assume that seismic events of interest can be recorded by all stations within the network. Similar approaches can be applied to different scales of microseismic monitoring problems depending on the detectability of the smallest seismic events needed to monitor. For the same total number of seismic stations, irregularly spaced seismic stations produce similar results to those obtained with regularly spaced seismic stations, as long as they have good enough azimuthal and distance coverage. Three‐component seismic stations are recommended over one‐component seismic stations to include more accurate readings of S‐wave arrival times to improve the locations of microseismic events.

ACKNOWLEDGMENTS

      This research was supported by the U.S. Department of Energy (DOE) through the Los Alamos National Laboratory (LANL), which is operated by Triad National Security, LLC, for the National Nuclear Security Administration (NNSA) of U.S. DOE under Contract No. 89233218CNA000001. The work was part of the National Risk Assessment Partnership of the Carbon Storage Program managed by the National Energy Technology Laboratory. We thank William Foxall for providing us with the initial velocity model for the Kimberlina site. We thank two anonymous reviewers for their valuable comments.

REFERENCES