Repeatability Analysis of Continuous Downhole DAS Data from Permanent Seismic Sources Over 2.5 Years of Operation

Time-lapse seismic is an essential technique for monitoring the subsurface. The procedure is widely applied in oil and gas production and will be particularly important in CO2 geological carbon storage projects. In many cases, it is desirable to be able to detect small changes in the subsurface caused by alterations of the fluid in pore space. This ability requires good data repeatability. Unfortunately, repeatability cannot be totally controlled during data acquisition because seismic data is affected by many factors, especially seasonal variations of near-surface properties caused by annual weather changes.

At the CO2CRC Otway International Test Centre, the Curtin University geophysics team acquired about two and a half years of continuous, downhole, seismic-monitoring data using distributed acoustic sensors and permanent sources to monitor a small CO2 injection. Analysis of these data shows the seasonal effect on seismic data repeatability. Seasonal variations are particularly pronounced in changes in the arrival times and amplitudes of direct P and S waves. These changes are different for different P and S wave types. In addition, the pattern of surface-related multiples is also variable. In the presentation, we will explore long-term repeatability and several methods that can help improve it.