S-wave experiments for the exploration of a deep geothermal carbonate reservoir in the German Molasse Basin

Britta Wawerzinek,Hartwig Von Hartmann,David C Tanner,Hermann Buness

doi:10.1186/s40517-021-00189-w

Abstract

There are many successful geothermal projects that exploit the Upper Jurassic aquifer at 2–3 km depth in the German Molasse Basin. However, up to now, only P-wave seismic exploration has been carried out. In an experiment in the Greater Munich area, we recorded S-waves that were generated by the conventional P-wave seismic survey, using 3C receivers. From this, we built a 3D volume of P- to S-converted (PS) waves using the asymptotic conversion point approach. By combining the P-volume and the resulting PS-seismic volume, we were able to derive the spatial distribution of the vp/vs ratio of both the Molasse overburden and the Upper Jurassic reservoir. We found that the vp/vs ratios for the Molasse units range from 2.0 to 2.3 with a median of 2.15, which is much higher than previously assumed. This raises the depth of hypocenters of induced earthquakes in surrounding geothermal wells. The vp/vs ratios found in the Upper Jurassic vary laterally between 1.5 and 2.2. Since no boreholes are available for verification, we test our results against an independently derived facies classification of the conventional 3D seismic volume and found it correlates well. Furthermore, we see that low vp/vs ratios correlate with high vp and vs velocities. We interpret the latter as dolomitized rocks, which are connected with enhanced permeability in the reservoir. We conclude that 3C registration of conventional P-wave surveys is worthwhile.

Highlights

Deep sedimentary basins can provide geothermal reservoirs, which are mostly explored using active seismic methods, carried out almost exclusively using compressional waves.This allows to image the geological and structural framework and, increase the chance of targeting successful geothermal reservoirs
shear waves (S-wave) velocities are valuable in characterizing the subsurface, because they reduce the ambiguity associated with only Compressional wave (P-wave) information, as they react in a different way to, e.g., lithology, porosity, pore fluids, and shape, and anisotropy with respect to P-waves and allow better discrimination (Robertson 1987; Tatham and McCormack 1991; Wawerzinek et al Geotherm Energy (2021) 9:6
The vertical component (Fig. 4a) shows a strong reflection at the top of the carbonate platform, i.e., Top Priabonian (~ 1600 m below sea level). This reflection is prominent on the radial component (Fig. 4b), where the reflection of the downgoing P-wave splits into an upgoing P-wave and an upgoing S-wave, i.e., a P-toS converted wave

Summary

Introduction

Deep sedimentary basins can provide geothermal reservoirs, which are mostly explored using active seismic methods, carried out almost exclusively using compressional waves. This allows to image the geological and structural framework and, increase the chance of targeting successful geothermal reservoirs. Combining compressional and shear waves can provide additional information about the reservoir. Johnston and Christensen 1992; Wang and Szata 1999). Recording both Pand S-waves in parallel enables the derivation of Poisson’s ratio. Pure S-wave surveys have not gained widespread popularity, but recording and processing of P-to-S converted waves are applied to a limited extent in exploration nowadays (Hardage et al 2011, 2014; DeAngelo and Hardage 2014; Donati et al 2016; Lu et al 2016), mostly for solving specific tasks, such as illumining beneath gas-rich sediments or inside coal seams, or to derive fracture orientations, among other things (Stewart et al 2003; Chopra and Stewart 2010; Wei et al 2014)

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Discussion

Conclusion