Summary Equinor in collaboration with Shell and Total is working on maturing the carbon storage project to store industrially produced CO2 into geological subsurface offshore Norway. However, the selected candidates for CO2 storage are part of a region with moderate natural seismicity. In order to assure a safe storage, the background seismicity should be monitored to understand both the nature of natural seismicity and to detect possible events induced by the imposed pressure changes due to CO2 injection. Current existing seismic network onshore is rather sparse and due to the limitation of recording mostly from the Norwegian side, event location uncertainty is rather high which complicates associating individual earthquakes to specific faults. Integration of onshore network with selected offshore PRM stations improved detectability. However, a more local monitoring system is required to improve the detectability and reduce the location uncertainty. We here present a proposal for a seismic monitoring array design for the offshore setting around the Smeaheia and Aurora sites that includes an upgrade of the onshore monitoring network as well as some ocean bottom nodes. We argue that such a network will provide an adequate baseline dataset, which is crucial for understanding the site prior to injection.

Summary We present a novel method to model microseismic event detection and location capabilities of a fiber optic cable. We show how different wrapping angles affect recorded amplitudes of P- and S-waves on fibers with different wrapping angle. We then introduce the Fiber Illumination as a proxy for how well events can be detected on sections of the fiber above a certain SNR. Furthermore, we investigate the location capabilites of different well paths if instrumented with a fiber

DAS continues to be a promising and cost-effective technology for carbon storage monitoring applications including systems that monitor geological changes using active seismics, and also for passive mode operations, e.g. the monitoring of microseismic activity during CO2 injection. The authors have developed a DAS interrogator research platform that has enabled a better understanding of the critical equipment architecture and experimental factors influencing the collection and analysis of DAS data. The authors plan to test this at different CCS pilot installations. In the future, the performance and functionality of the DAS interrogator research platform will be expanded, and techniques for applying it developed further in order to meet CCS specific needs determined from wider collaboration.

Summary We assume two DAS cable geometries of 300 recording channels that replicate the Oseberg PRM (seafloor array) and a deviated well (akin to that used in hydraulic fracturing). We calculate the longitudinal strain at each channel, subject to the amplitude relationships described by Kuvshinov (2016) , for P-waves and a coupled-cable to-medium case. The DAS cable’s sensitivity across the medium is investigated for differing wrapping angles of helical fibre. We then test how a 3D event location performs in these scenarios and create probability density estimates of the mislocations. These findings and the future implications for DAS as a viable microseismic monitoring technique are then discussed.

In outdoor field-tests where a single optical fibre cable is used to make DAS recordings of hammer shots, coherent seismic arrivals are observed at up to 32 m from the hammer points with frequencies up to 600 Hz. Recording at higher sampling rates and decimating is preferable to downsampled data. Time domain SNRs of acquired DAS data at a given distance are ~1/2 those of complimentary geophones while in the frequency domain, the highest ratios are produced between 20 and 200 Hz. An automated STA/LTA trigger algorithm is able to define reasonable seismic onsets but is unable to identify the first-arriving energy on all traces. Shorter pulse widths produce lower RMS amplitudes for the noise windows while longer pulse widths produce higher RMS amplitudes for the signal windows. This means that the selection of optimum pulse width with respect to maximising the SNR is not clear. This work presents the initial results of field-tests that will be used to refine the use of the DAS system in further field and borehole experiments. This will ultimately be used to monitor the seismicity at active CCS injection sites in the future.