The following questions about the environmental impact of CO2 storage are being addressed in the project.
Can earthquakes be triggered during CO2 storage?
If the pressure in the storage formation increases too much, fault zones could be reactivated, causing movements in the subsurface that could potentially trigger earthquakes. When stored under the seafloor, these earthquakes are too weak to cause damage on land. However, they could affect offshore wind turbines and other offshore infrastructure (e.g., power lines). The project therefore investigates the conditions under which such earthquakes are triggered and whether the operation of plants and infrastructure in the North Sea is endangered by these quakes.
The work in the project is focused on the two areas A (West Schleswig Block) and B (Entenschnabel). For this purpose, detailed geological models are prepared, which provide information about the location and history of the fault zones and faults (geology). Based on these models, the pressure development in the subsurface after CO2 injection is then calculated in simulations (geology) and it is evaluated whether earthquakes can occur.
Furthermore, it is investigated how these earthquakes influence the mechanical properties of the shallow subsurface in which the foundations of wind turbines are anchored. It will be investigated whether these changes in the shallow subsurface can affect the operation of wind turbines.
These geotechnical studies will be used to evaluate whether it is possible to store CO2 in the near field of wind turbines or whether a greater safety distance must be maintained (spatial planning).
Even with a small pressure increase, the movement of fluids in the subsurface (CO2 and displaced formation water) can lead to weak microseismic events (“mini-earthquakes”), which do not pose a risk but could be used to monitor CO2 storage facilities. Therefore, these events will also be investigated and their spatial distribution predicted in order to plan a suitable monitoring network (monitoring).
Are harbor porpoises affected by noise generated during the exploration and monitoring of CO2 storage facilities?
Harbor porpoises live in the North Sea and are particularly sensitive to noise. Seismic exploration of storage facilities generates noise that can affect these marine mammals. Noise could also be generated during reservoir monitoring if monitoring is carried out using active seismic methods. In contrast, injection of CO2 into the subsurface does not generate significant noise.
The project will therefore investigate how harbor porpoises respond to seismic noise. To this end, the distribution of harbor porpoises will be recorded using hydrophones before, during and after seismic deployment. This will allow us to determine whether harbor porpoise populations are disturbed temporarily or for long periods of time by the use of seismic. These data will be used to evaluate whether it is feasible and justifiable to construct CO2 storage facilities near harbor porpoise sanctuaries (spatial planning).
In addition, measures that help minimize noise disturbance will be investigated. To this end, the project is developing a new passive seismic monitoring concept that does not require active seismic and noise generation (monitoring).
Can CO2 escape from storage?
CO2 has been stored on a large scale in the North Sea and the Barents Sea for many years. So far, no leaks have occurred at these storage facilities (www.eco2-project.eu). However, there are a number of weak points where leakage could occur. These are primarily natural faults and old wells that cut through barrier layers designed to prevent CO2 from rising. Methane leakage has already been observed at such weak points, which are common in the North Sea (Vielstädte et al. 2015; Vielstädte et al., 2017; Böttner et al., 2020). Thus, it is possible that CO2 leakage could occur along such structures if sequestered CO2 reaches them.
The environmental impact of such leakage has already been studied in previous projects. It was shown that especially calcifying animals (e.g. mussels) living on the seafloor are affected, because the leaking CO2 is rapidly dissolved in the water, causing the bottom water to acidify (www.eco2-project.eu). Already at a distance of about 10 meters from the leakage site, CO2 release experiments could no longer detect acidification (pH decrease) (Vielstädte et al. 2019). In these experiments, CO2 was released directly to the bottom of the North Sea at a rate of about 30 tons per year. Thus, the damaging effect on the ecosystem is limited to the immediate vicinity of the leak if the leakage rates are in the range observed for natural methane spills and methane leaks at wells (about 1 – 50 tons per year).
The project will use available seismic data to investigate the frequency of natural faults in the vicinity of the two reservoir sites under investigation and whether natural gas is rising on these faults. In addition, an outcrop will be used to investigate whether methane is leaking from old wells located near the sites. Based on these data and numerical simulation of CO2 storage (geology), we will evaluate whether leakage can occur.
The rise of CO2 along faults and wells is also studied experimentally in the pressure laboratory. Reactions with rock and CO2 ascent mechanisms will be studied to gain more information on potential leakage rates that may be generated by the density difference between CO2 and water and injection overpressure in the reservoir.
Finally, a new technical concept is being investigated to minimize CO2 leakage from old wells. Old wells in the North Sea are typically sealed with concrete, sawed off at a depth of about 5 meters below the seafloor, and then covered with sand. In the new concept, the sand covering is replaced by a reactive barrier (e.g., lime-sand mixtures). The lime is supposed to react with the escaping CO2, so that CO2 is converted into harmless bicarbonate. The efficiency of this new process is being tested experimentally in the pressure laboratory.
Pressure vessel for high-pressure flow experiments to investigate reactive transport in the North Sea subsurface.
Böttner C., Haeckel M., Schmidt M., Berndt C., Vielstädte L., Kutsch J.A., Karstens J., Weiß T. (2020)
Greenhouse gas emissions from marine decommissioned hydrocarbon wells: leakage detection,
monitoring and mitigation strategies. International Journal of Greenhouse Gas Control 100, 103119.
Vielstädte, L., Karstens, J., Haeckel, M., Schmidt, M., Linke, P., Reimann, S., Liebetrau, V., McGinnis,
D.F. and Wallmann, K. (2015) Quantification of methane emissions at abandoned gas wells in the
Central North Sea. Marine and Petroleum Geology 68, 848-860.
Vielstädte L., Haeckel M., Karstens J., Linke P., Schmidt M., Steinle L., Wallmann K. (2017) Shallow gas
migration along hydrocarbon wells – An unconsidered, anthropogenic source of biogenic methane in
the North Sea. Environmental Science & Technology 51(17), 10262-10268.
Vielstädte, L., Linke, P., Schmidt, M., Sommer, S., Haeckel, M., Braack, M. and Wallmann, K. (2019)
Footprint and detectability of a well leaking CO2 in the Central North Sea: Implications from a field
experiment and numerical modelling. International Journal of Greenhouse Gas Control 84, 190-203.