By the year 2009 microseismic monitoring was not listed as one of the important tools for CO2 storage surveillance. However, we were convinced that continuous monitoring of microseismicity can become a vital method for the detection of leakage and to improve the understanding of fluid flow and creation of new fractures during CO2 storage.
Within the course of the SafeCO2 project we have demonstrated that microseismic monitoring is a unique tool for CO2 storage sites as it directly provides information on the geomechanics in and around the subsurface storage. We have developed new methods adapted to low signal microseismic events that turn valuable to detected even smallest stress changes in and around a reservoir. We worked with real data from the In Salah, Algeria CO2 sequestration site where we could differentiate between fracture and matrix flow of the CO2 based on a joined analysis of microseismic events and injection data. We further showed evidence for tensile components (partial opening of fractures) in the microseismicity during a high-pressure fluid injection experiment in Basel, Switzerland, which fell back to pure shear components once the injection stopped. In addition we are actively involved in the Svalbard CO2lab project where we contribute with the design of the microseismic network and understanding of the velocity model that is complicated by permafrost.
Most important research tasks
A first general task was to implement a new, migration-based, location algorithm to data of low signal-to-noise ratios. This method proved valuable in cases where many sensors are available. However, in the In Salah case study we had to rely on a rather sparse network and had to design a new detection method, based on correlation and noise-cancelation. We have learned significant lessons on instrumentation and network design in all case studies, which are essential for all new projects where a network will need to be set up.
A breakthrough for the use of microseismic data came when we combined its occurrence and patterns of source mechanisms with production data like injection rate and pressure. In this task we collaborated strongly with NGI, Statoil and BP. We also collaborated very efficiently with UNIS and the GFZ on the CO2Lab data interpretation and on stress field modelling, respectively. Mutual learning in between institutes was highly appreciated and resulted also in successful joined proposals.
Further use of the results for various communities
The results from the SafeCO2 project will flow in directly into our recently startedSafeCO2 II project, where we add on and extend the methodology also to ambient seismic noise tomography and aseismic deformation. These topics were actually identified during the SafeCO2 project. We will also continue to collaborate with UNIS, not only on permafrost issues but also on a new injection experiment at the CO2Lab project. The SafeCO2 project also significantly contributed to new project initiatives, as we were invited to lead the
geomechanics workpackage in a joined proposal to the DoE coordinated by the Geological Survey and the Decatur project, Illinois. Another side-track of the SafeCO2 project is the FORNY project FrackTrack that focuses on a commercial use of the software that we used and further developed within the SafeCO2 project, in this case, however, aiming at the shalegas industry.