Monitoring CO2 Injection at the Illinois Basin -- Decatur Project with Time-lapse 3D VSPs

Time-lapse three-dimensional (3D) vertical seismic profiles (VSPs) are an important component of the monitoring, verification, and accounting plan for the Illinois Basin –- Decatur Project. The VSPs will be used to provide information on CO2 plume development, demonstrate containment of the CO2 in the storage formation, and provide data to verify and update models and simulations over the life of the project. VSPs are more economical to acquire and process than surface seismic data and cause less disruption to local landowners as they have smaller acquisition footprints. The 3D VSPs are being acquired with a permanent geophone array. Three 3D VSP surveys have been acquired at the site to date: two baseline surveys and one monitoring survey acquired after ~70,000 tonnes of CO2 had been injected. Baseline 2 and Monitoring Survey 1 data have high repeatability as evidenced by the NRMs repeatability metrics. The final difference displays do not provide conclusive results regarding CO2 movement in the Mt. Simon formation; however, the NRMS depth slice at the injection interval shows higher NRMS values that may be suggestive of the presence of CO2. Future surveys are expected to produce more conclusive results as the volume of injected CO2 increases.