Combined use of Optical-fiber DAS and a Permanent Seismic Source for Vertical Seismic Profiling Demonstrated at the Aquistore CO2 Storage Site

Seimic reservoir monitoring is a method to monitor fluid displacement in the reservoir. Long-term monitoring to measure differences over several years has been successful recently. However, short-term monitoring to measure changes in nearly real-time is still challenging because the expected changes in such a short-term are small. Permanent reservoir monitoring might enable short-term monitoring because we can increase data repeatability since sources and receivers are permanently fixed at the same position. This method saves the acquisition cost once the equipment is deployed, but the number of sources and receivers is limited due to the high initial install cost. To address this challenge, we have demonstrated VSP monitoring with a combination of a permanent rotary source and DAS sensor. DAS can record a wavefield at very dense and extensive points along an optical fiber, but the quality is regarded as less than for conventional geophones. By comparing data recorded in 2015 and 2016, we investigated the improvement of the signal-to-noise ratio of DAS. Hourly repeatability was checked by arranging the waveforms by the acquisition time. The depth migrated image of the offset VSP extended the imaging are further away from the receiver well. Our study confirmed the complementary relationship between the permanent source and DAS acquisition. Combining these technologies might enable us to monitor small changes in the reservoir in the short-term.