Impact of fracture networks on borehole breakout heterogeneities in crystalline rock

This study describes a systematic analysis of breakout occurrence, variation of breakout orientation and fracture characteristics. We infer the impact of fracture networks on the development of breakouts from detailed analysis of 1221 borehole elongation pairs in the vicinity of 1871 natural fractures observed in the crystalline section of the GPK4 well of the Soultz-sous-Forêts geothermal field (France). Breakout orientation anomalies are found to concentrate in the immediate vicinity of fault cores and to decrease with distance to the fault core. Patterns of breakout orientation in the vicinity of natural fractures suggest that the breakout rotation, relative to the mean Shmin direction, is strongly influenced by the fracture orientation. Even a direct relationship between fracture and breakout orientations is found in some depth intervals. In highly fractured zones, with different fracture families present, breakout orientations are especially heterogeneous, resulting from the overlapping effects of the fracture network. Additionally, breakouts are typically found to be asymmetrical in zones with high fracture density. Borehole breakout heterogeneities do not seem to be related to the principal stress heterogeneity only, but also to the effect of mechanical heterogeneities like weak zones with different elastic moduli, rock strength and fracture patterns. Consequently, care has to be taken when inferring the principal stress orientation from borehole breakout data observed in fractured rock.