CO2 leak detection threshold using vertical seismic profiles

Abstract

At the Containment and Monitoring Institute Field Research Station in Alberta, Canada, CO2 is being injected into a saline aquifer at 300 m depth to simulate a shallow leak from a deep carbon sequestration operation. The injection interval is a 7 m thick sandstone with a porosity of 11%. The gas-phase CO2 plume was detected with time-lapse vertical seismic profiles (VSP) after 32 tonnes of CO2 had been injected. The VSP surveys had high repeatability with permanent borehole geophones and carefully repeated source locations. A data processing workflow was developed to produce directly comparable baseline and monitor amplitudes while preserving subtle amplitude changes caused by partial CO2 saturation. Seasonal variations in the physical properties of the near-surface glacial tills introduced spectral differences between baseline and monitor datasets that were successfully mitigated by deterministic deconvolution and selected band-pass filters. The 32 t detection threshold represents a lower limit of CO2 detectability with borehole geophones in this geological setting. These results provide a rare field data example of CO2 monitoring at shallow to intermediate depths and demonstrate that VSP surveys are capable of identifying loss of containment in industrial-scale CO2 sequestration operations.