Ensuring borehole stability is a critical consideration throughout the entire process of deep geothermal projects, encompassing both the drilling and operational phases. One effective strategy to enhance stability, thereby averting costly drilling delays, involves deviating the well in alignment with the most advantageous orientation relative to the in-situ stress conditions. However, this trajectory often differs from the one that intersects a sufficient number of pre-existing fractures, which are the primary targets for stimulation. Moreover, the fractures that are intersected may not be conducive to hydro-shearing stimulation. To mitigate the aforementioned risks, it is crucial to conduct a comprehensive assessment of the in-situ stress conditions and rock strength, as well as evaluate wellbore failure before and especially during the drilling process. In this regard, borehole failure including breakouts (BOs) and Drilling-Induced Tensile Fractures (DITFs), can be employed through interpretation of data from tools like acoustic televiewer logs. The analysis of failure observations, namely BOs and DITFs, offers valuable understanding of the strength of rock, the in-situ stress conditions, and their interactions in the immediate vicinity of the borehole. This approach, distinct from predominantly point-based methods like mini-frac tests, allows for the assessment of stress and strength along continuous 1D profiles which makes it possible to characterize statistically their variability along boreholes over relatively short distances. A workflow was initially developed by Dahrabou et al. (2022) for interpreting borehole failure information through analytical and empirical solutions for estimating wellbore stresses and failure parameters combined with the regularized pilot points method. We have advanced this method and applied it to different geothermal reservoirs such as BS-1 borehole in Basel geothermal project. The aforementioned logs were used and interpreted shortly after drilling of the boreholes to prepare datasets of borehole failures (i.e., breakout width, extent and orientation, and existence of DITFs). These variable datasets are utilized to estimate model parameters. By applying our methodology, we are able to generate borehole profiles including, amongst others, the heterogeneous distributions (i) of the principal components of the stress tensor with corresponding directions, and (ii) of rock strength properties such as cohesion and friction as the major outcomes aiding us in reaching to our goal of being able to develop sustainable geothermal reservoirs by better understanding stress and strength variabilities. The results of this study reveal plausible sets of stress and strength parameters that closely replicate the complex breakout distribution observed in the studied boreholes and reservoirs. Additionally, it sets the base for quantitative predictions of wellbore failure and risk analyses. These are essential for designing and implementing advanced borehole completions that facilitate zonal isolation, a crucial step in realizing the full potential of geothermal projects.

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