Geophysical Methods for CO2 Leak Detection and Plume Monitoring at the Southeast Regional Carbon Sequestration (SECARB) Anthropogenic Test Site near Citronelle, Alabama

Abstract:

The SECARB project is the largest demonstration of CO₂ capture, transportation, injection and storage from a coal-fired power station in the US. In August 2012, SECARB began capturing CO₂ emitted by Unit 5 at Plant Barry north of Mobile, Alabama and injecting it into the Paluxy Formation at a depth of 9,400 ft above the Citronelle oilfield.

Vertical seismic profile (VSP), cross-well and distributed acoustic sensing using fiber optics are being used to check for CO₂ leakage out of the storage reservoir and track the CO₂ plume. The acquisition plan includes one pre- and post-CO₂ injection survey using an 80-level VSP array with a vibroseis source and cross-well using a piezoelectric source. “Snapshot” VSP surveys are performed every 6-12 months using a shorter 18-level geophone array installed on production tubing in the observation well.

Good quality results were produced for both the 80-level VSP and cross-well baseline surveys. Mixed results were obtained using the 18-level VSP array due to the smaller aperture, large depth to the target and thin sand layers receiving injected CO₂. Time-lapse differencing shows weak illumination at the CO₂ injection depth for only one far-offset source point. The lack of bright spots prompted SECARB to move the second cross-well survey up in the schedule. A second cross-well survey was conducted in June 2014. This time the hydrophones were deployed in the production tubing to avoid removing the 18-level array. The acquired data exhibited signal degradation compared to the baseline survey and tube waves interfered with the reflections. First arrivals were used to build a post-injection velocity tomogram. Differencing of the pre- and post-injection tomograms was performed, producing a time-lapse image of good quality. The resulting image (Figure) shows a significant velocity difference, indicating the CO₂ plume has moved roughly 400 ft in zone. More importantly, no velocity anomaly or leakage is evident above the storage reservoir.