Time-lapse seismic properties of CO2-filled fault zones: Field observations

We investigate seismic waveform changes in time-lapse along supercritical CO2-bearing faults from field data by analysing 4D seismic data from the Illinois Basin – Decatur Project. 1 million tonnes of CO2 was injected into the Lower Mt. Simon continuously over a three year period from 2011-2014. It has been established that the injected CO2 migrated vertically along faults at this site to reach the Middle and Upper Mt. Simon formations (Bukar et al., 2024). Time-lapse 3D vertical seismic profiles were acquired each year of injection in addition to a pre-injection baseline and a final survey two months post-injection. We study the time-lapse seismic waveforms in zones around previously interpreted faults. In post-stack, we observe waveform distortions in the monitor traces that manifest as phase changes when compared to the baseline traces. Interestingly, these distortions magnify with increasing injected CO2 volume, and decrease post-injection. To further investigate potential causes of these phase changes, we study the data in pre-stack. We also attempt to discriminate the contribution of CO2 saturation effects and pressure effects. This is crucial as pressure increase also causes a slowdown effect on seismic waves in fractured media due to positive physical strain (expansion) and an accompanied decrease in the rock bulk modulus. However, while CO2 injection is typically accompanied by pressure increases, the pressure would typically decline more quickly than CO2 would dissolve in brine; multiple pressure gauges at this site show a rapid decline in pressure once injection ceased. Therefore, time-lapse seismic acquired soon after stopping injection could offer insights. We also observe these distortions where faults have not been mapped – these could be CO2-filled fault zones with small throws that are below seismic resolution. This could potentially be used to illuminate unseen faults after CO2 injection.ReferencesBukar, I., Bell, R., Muggeridge, A. H., & Krevor, S. (2024). Carbon dioxide migration along faults at the Illinois Basin – Decatur Project revealed using time shift analysis of seismic monitoring data. Geophysical Research Letters, 51, e2024GL110049. https://doi.org/10.1029/2024GL110049