Monitoring CO2 geological storage reservoirs with seismological broadband noise - the Ketzin experiment

 

Monitoring CO₂ storage reservoirs over long periods of time is mandatory to ensure the safety of the CO₂ storage process, as the European Parliament Directive states. To ensure this goal, several methods are already available; however, they either require expensive costs (time-lapse active seismic surveys) or wells that affect the reservoir integrity (downhole microseismic instrumentation). The development of long range correlation of seismic noise allows us to consider new monitoring methods : by inferring the Rayleigh wave dispersion and its variation over time from ambient seismic noise recordings, we may be able to measure S speed variations in different layers.In order to test this method, we set up a broadband array on the CO₂ injection pilot at Ketzin (Germany) and recorded 5 months of data. 14 day stacks were enough to build stable correlations and allowed us to study the Rayleigh wave dispersion between 0.8 and 6 Hz. Several Rayleigh branches were observed. We were able to invert these dispersion curves and obtained the S vertical speed profile for the first hundred meters; looking at different time periods, we looked for temporal variations of S wave speed induced by the CO₂ injection. We compared these observations to models of the influence of the CO₂ plume presence on Rayleigh wave dispersion.