Understanding the stress field and potential fault activity – a key issue to drilling and stimulation in man-made geothermal reservoirs

It has been recognized that in-situ stresses have significant impact, either positive or<br>negative, on the short and long term behaviour of fractured reservoirs. The knowledge of the<br>stress conditions are therefore important for planning and utilization of man-made<br>geothermal reservoirs. The geothermal field Groß Schönebeck belongs to the key sites in the<br>north eastern German Basin in Germany. We present a combined approach of stress field<br>determination and application of the new knowledge for drilling and stimulation design at this<br>key site, where 4100 m deep sandstones and volcanic rocks of Lower Permian are ongoing<br>to be explored. In our comprehensive study we use detailed 3D fault mapping, based on<br>available well and 2D seismic data, stress regime determination based on empirical and<br>analytical methods, and slip-tendency analysis to estimate reactivation and leakage potential<br>of any fault population within the stress field under initial and changing pore pressure<br>conditions. We discuss the importance of various fault sets related to the stress field in terms<br>of their potential for conducting geothermal fluids based on the tendency of the faults to dilate<br>and slip. In particular, we demonstrate how the well path trajectory and mud weights can be<br>defined on the basis of principle stress orientation and magnitude to minimize formation<br>damage under mechanically stable borehole conditions and to optimise stimulation designs<br>of multiple fracs in multilayered rocks. Finally, the results of slip-tendency can be used to<br>control seismicity induced by massive stimulation campaigns at geothermal sites. Our<br>approach can be adopted to any other geothermal site investigation.