Background Seismicity Monitoring Using Conventional Offshore Geophones: An Input to the Carbon Capture and Storage Project

Summary Equinor in partnership with Shell and Total is working on a concept selection for a CO2 storage site offshore Norway. This storage site is planned to be monitored by passive seismic, prior and in the life time span of injection to make sure CO2 injection will not impose any seismic risk. We analyzed the recorded data on a limited set of offshore 3C-geophones on Grane Permanent Reservoir Monitoring (PRM) system, from December 2018 to May 2019, to evaluate the potential of conventional geophones for background seismicity monitoring. During these six months, 278 events with a local magnitude ranging from -0.7 to 2.0, could be located. Our study shows that good azimuthal coverage of stations can have large impact on accuracy of location and magnitude of event. In addition, improved velocity model can reduce the location uncertainties. A seismic network consisting of pre-existing offshore geophones on Grane/Snorre PRM and Oseberg Seismic Waste Injection Monitoring (SWIM), combined with a limited number of broadband seismometers from the Norwegian National Seismic Network, could optimize a cost-effective setting for passive monitoring. An ongoing study is evaluating the benefits of dedicated or enhanced seismometer deployments (offshore/onshore) to give further improvements to detection and location accuracy.