In this numerical study the influence of different fracture propagation criteria on the evolution of breakouts around an isostatically loaded borehole is discussed. The material properties are reflecting those of a shale material. The simulation results are compared to fracture patterns as generated in laboratory experiments on borehole breakout formation in Posidonia shale (Meier et al., 2012). Comparing the fracture patterns as observed in the laboratory experiments of borehole failure in Posidonia shale to the numerical simulation results shows general similarities of fracture pattern attributes in case of fracture propagation based on the maximum shear stress criterion. This criterion assumes the fractures to grow in the direction of local maximum shear stress. In contrast, the simulations based on a tangential stress criterion, which assumes extension fracture propagation, shows borehole parallel fracture growth; this is not observed in the shale material. From these observations it may be concluded that the fracture growth in the laboratory experiments in shales is shear dominated; this was also concluded from microstructural observations by Meier et al. (2012).


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