Report on addressing methods to establish baseline levels post operational activity. SECURe - Subsurface Evaluation of Carbon capture and storage and Unconventional risks. D3.1,

Establishing a baseline for natural seismicity after the start of subsurface operations is no simple task, and best practice methods have yet to be created. In the SECURe project (Subtask 2.1.4.) a microseismic monitoring network has been established around the active natural gas storage facility in the Gassum Formation near Stenlille, Denmark. The Gassum Formation is a sandstone of Upper Triassic – Lower Jurassic age. A similar monitoring network was deployed near Dybvad, Denmark, approximately 200km from Stenlille, in 2014-2015. The geology and tectonic setting are similar at the two sites, and the natural seismicity is therefore expected to be similar as well. The measured natural background noise level is slightly lower near Stenlille. However, injection and production at the main pump dominate the recorded data at high frequencies at periods of high activity out to distances of approximately 3km. Preliminary data analysis has not yet detected any earthquakes near Stenlille. The natural seismicity is very low, and the relatively soft geology makes it challenging to detect small induced/triggered earthquakes. Based on the experience from the Dybvad network we do expect to be able to detect earthquakes down to a magnitude of 1.0, smaller during times of low or no pumping activity. The data analysis is ongoing. The approach being developed in SECURe will consist of microseismic monitoring around an active site, while experimenting with instruments in a range of distances from the well/storage facility, between 10 and 100 km, will help to establish best practice. These instruments are being installed during summer 2019.