3D deep geothermal reservoir imaging with wireline distributed acoustic sensing in two boreholes

Abstract. Geothermal exploration will help moving towards a low-carbon economy and provide a basis for green and sustainable growth. The development of new practical, reliable methods for geophysical characterisation of a reservoir has the potential to facilitate a broader application of deep geothermal energy. At the Groß Schönebeck in-situ laboratory, a unique vertical seismic profiling (VSP) dataset was recorded in two 4.3 km deep geothermal boreholes using fibre optic cables in early 2017. The experiment set-up consisted of 61 vibrator points organised in a spiral pattern around the well site to ensure a proper azimuth distribution in the target reservoir section. Data were processed using a standard workflow for VSP. As a result, a detailed 3-dimensional 0.75 × 1 × 4.5 km size image around the existing boreholes was created using the Kirchhoff migration algorithm with restricted aperture. The imaging resolved small-scale features in the reservoir essential for the future exploration of the geothermal research site. Borehole data with vertical resolution up to 16 m revealed the existing depth variations of the Elbe basis sandstone horizon at 4.08–4.10 km depth and indications of an unconformity in the area where we expect volcanic rocks. In addition, in the borehole data a complex interlaying with numerous pinch outs in the Upper Rotliegend reservoir section (3.8 to 4 km depth) was discovered. Thereby, we demonstrate that wireline fibre optic data can significantly contribute to exploration by providing an efficient and reliable method for deep geothermal reservoir imaging.