Capture, Storage and Use of CO₂ (CCUS). Seismology in relation to safe storage of CO₂ (Part of Work package 7 in the CCUS project)

Denmark is located in a low-seismicity region. Although many earthquakes are detected every year, they rarely exceed a magnitude (M) of 3. The largest instrumentally recorded earthquake in Denmark measured M 4.7. In historical times at least one larger earthquake may have hit Denmark, but earthquakes lager than M 5.3 +/- 0.1 are not expected. Subsurface activities can lead to induced and/or triggered earthquakes. These are known particularly from hydrocarbon production, but also geothermal projects and CCUS around the world. While induced earthquakes occurring in the vicinity of a well seldom exceed M 2, triggered earthquakes up to M 5.7 have been observed in connection with shale gas fracturing in Oklahoma, USA. Triggered earthquakes happen when a fault is reactivated by either a propagating pressure front or leaking liquids. CO₂ is stored in porous rocks where the flow is through a matrix as opposed to shale gas production where hard rocks are fractured. The risk of larger earthquakes in connection with CCS is much smaller, something that is supported by observations from CCS-projects worldwide. The CCS induced earthquakes are typically between M -2 and M -0.5, several orders of magnitude smaller than anything that can be felt by the local population, and most likely posing no risk to the reservoir seal. Monitoring microseismicity is a useful mitigation and prevention tool. An increase in the number of microearthquakes or an increase in observed earthquake magnitude can be a sign that the injection is shifting from matrix flow to fracture flow. Changes in microseismicity is a first sign, and if observed, a temporary change in injection pressure can bring the seismicity down again. This can be regulated through a pre-defined Traffic Light System. Microseismic monitoring at the Stenlille Gas Storage Facility in Denmark for almost two years did not detect a single event within the reservoir using surface instruments with a sensitivity down to M 0. For microseismic monitoring, downhole instruments are more effective, but can be supplemented by a local surface monitoring network.