Image analysis of acoustic data and interpretation of rock stress orientations for geothermal exploration in Gothenburg borehole GE-1, Southwest Sweden

Abstract

Abstract This study contributes to geothermal exploration in 1660-1520 Ma old reworked bedrock in Sweden. Our primary objectives are to constrain the orientation of horizontal stresses, and to discuss implications for geothermal exploration. High-resolution acoustic televiewer image data reveal the downhole distribution of stress indicators (borehole breakouts, drilling induced fractures and petal centerline fractures), pre-existing structures (natural fractures, foliation). About 135 m of stress indicators are measured from 0.2-1.0 km. The results suggest a uniform NNW-SSE mean maximum horizontal stress orientation. A total of 1525 pre-existing structures (natural fractures, foliation) are mapped in borehole GE-1. The prevailing stress regime controls if natural fractures and foliation are well-oriented for stimulation. For strike-slip and normal faulting stress regimes, well-oriented fractures are steeply dipping towards WSW. For a reverse faulting stress regime, shallow dipping fractures are well-oriented for simulation. The downhole distribution of stress indicators and other stress measurements in the region and other parts of Fennoscandia tentatively suggest a strike-slip stress regime, but additional studies are needed to constrain the complete stress field at study depth and towards EGS reservoir target depth. Our secondary objective is to highlight that interpretation of high-resolution acoustic data particularly in metamorphic crystalline rocks are subjective, and that more guidelines for data interpretation are needed. The interactive interpretation of the images is based on visual analyses of complex pre-existing structures and stress indicators with highly variable shapes. The application of three methods for data analyses in the GE-1 borehole propose that drilling induced fractures are little influenced by the method applied. Interpretations on individual borehole breakout azimuths may, however result in over 10 differences in orientation. Supplementary material at https://doi.org/10.6084/m9.figshare.c.7082902