Monitoring injected CO2 migration using time-lapse crosshole seismic testing with downhole DAS receivers and an electric sparker source

• A crosshole seismic survey was conducted using a high-frequency source and distributed acoustic sensing (DAS) technology to detect CO₂ volumes in a shallow formation during and after injection • Straight-ray tomography results indicate CO₂ presence in the anticipated cross-section area during injection, with migration towards the surface at the end of injection and thereafter • The study demonstrated the applicability of DAS for CO₂ leakage monitoring and its potential for detecting ore bodies Time-lapse surface and borehole seismic methods are widely used for conformance and containment monitoring of CO 2 injected into the subsurface due to superior spatial resolution of these methods compared to other geophysical techniques. When closely spaced wells are available, still higher resolution can be achieved by using crosshole seismic monitoring with sources located in one well and receivers in another. The resolution can be enhanced even further by using high-frequency sources such as an electric sparker in combination with downhole distributed acoustic sensors (DAS). This technology was tested during the injection of 16 tonnes of CO₂ gas into a shallow well at the CO2CRC’s Otway International Test Centre in the Australian State of Victoria. After the start of the injection, a velocity anomaly was located in the vicinity of the injection point and grew as the injection progressed. Another velocity anomaly closer to the surface was detected close to the end of the injection, indicating upward CO₂ migration. These results confirm that using DAS with a high-frequency downhole source is a viable approach for CO₂ monitoring in shallow formations.