Fibre-optic seismic imaging at the geothermal research platform Groß Schönebeck/Germany

The geothermal research platform Groß Schönebeck is located ca. 50 km northeast of Berlin and was set up for the development and research of geothermal energy technology in the Northeast German Basin. The geothermal reservoir lies at approximately 4 km depth and counts as a low-permeability geothermal reservoir that requires the usage of Enhanced Geothermal Systems (EGS), which is typical for large parts of Northern Europe. Siliciclastic sediments and volcanic rocks of Lower Permian (Rotliegend) age, overlaid by Zechstein salt, represent the target reservoir zone. Seismic research activities at this site encompass high-resolution 3-D reflection seismics at surface, combined with an extended 3-D Vertical Seismic Profiling (VSP) experiment. The 3-D surface seismic survey covers an area of 8 km by 8 km and was designed with a focus on reservoir depths of 4000 to 4300 m, in order to improve the knowledge about the geological structures and spatial distribution of the fault systems.  The VSP experiment took place in two 4.3 km deep boreholes using the Distributed Acoustic Sensing (DAS) technology with a wireline cable, surrounded by 61 vibroseis points in a spiral pattern with offsets ranging from 200-2000 m.  The processing of the DAS data was exposed to the challenge of wireline acquisition and, thus, a considerable level of ringing noise.  After preprocessing (vertical stacking, correlation with pilot sweep), several approaches for noise reduction were tested, and a new, two-step approach was introduced: 1) denoising by matching pursuit decomposition with Gabor atoms, and 2) subtraction of the noise model from the data. This imaging improvement finally allows a high-resolution spatial image surrounding the two boreholes (Kirchhoff migration with restricted aperture) at the site, even resolving small-scale features in the reservoir. The combination of the VSP and the large-scale 3-D surface seismic model shows, in comparison to former 2-D seismic interpretations, that some exploration-guiding hypotheses have to be discarded: no significant crustal faults are imaged, the reservoir shows no fracture-dominated character, the gas-water contact/free gas is not visible, and there are no seismic compartments below the salt.  This has substantial implications for the development plan at this site.  Our workflow presented here for wireline DAS measurements can be used in the future to also re-survey already existing boreholes. In conclusion, such combinations of experiments and new methodologies are applicable to many other areas of interest, such as waste disposal, energy generation, or any type of reservoir management.