Time-lapse seismic profiles derived from passive seismic interferometry in fluid-injection experiments

To construct a reliable and cost-effective monitoring system for injected CO2 in Carbon Capture and Storage (CCS) projects, we have developed a seismic monitoring technique using ambient noise. The passive seismic interferometry can continuously monitor the injected CO2, allowing us to detect accidental incident associated with CO2 injection (e.g., CO2 leakage from reservoir). Here we used seismic interferometry approaches for construction of virtual seismic data. By applying these methods to the passive seismometer data acquired during fluid-injection experiment in Svalbard in the Norwegian Arctic, we estimated variations of reflection amplitude. The reflectors around the reservoir can be identified on the common mid-point (CMP) gather constructed via seismic interferometry, and they enable us to estimate seismic velocity. Therefore, the technique we have developed in this study can extract subsurface structures around the water-injection field and provide its time-lapse information. On the time-lapse seismic profiles derived from seismic interferometry, the injected water can be imaged as a number of bright reflections within the reservoir. The amplitude anomaly appeared just after fluid injection. The amplitude variation extracted from our analysis has clear relation with pressure. Therefore the variation of reflection amplitude may be induced by pore pressure variation due to fluid injection. This low-cost approach is particularly attractive for long-term, continuous monitoring of CCS projects. We further applied surface-wave analysis for the ambient noise data and estimated variation of S-wave velocity structure in the shallow formation. This information enables us to evaluate the influence of shallow formation on the monitoring results of deep reservoir.