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Abstract

This study quantifies the initiation and progressive development of borehole breakouts near pre-existing fractures through numerical modeling. Focusing on a 1000-m-deep borehole (BS34) at the Xinchang site, a candidate for China's high-level radioactive waste repository, we conduct an integrated geomechanical analysis to understand borehole breakout behavior. Despite high rock strength and low in-situ stress levels, breakouts have been observed near pre-existing fractures. These breakouts exhibit strong azimuthal correlations with the fracture dip directions, highlighting the fracture influences on breakout development. Using a finite element model based on the fracture geometry and far-field stresses at a depth of 146 m in BS34, we find that a pre-existing fracture can significantly perturb the local stress field via slip-induced stress drop. After drilling, the locally-disturbed stresses are further concentrated around the major axis tips of the resultant elliptical hole. Combined with the fracture- and drilling-induced rock mass damage, the stress concentrations explain breakout initiation near the fracture dip direction, even under low stress conditions. As breakouts propagate downward or upward, they show rotational realignments back to the far-field Shmin orientation, with tapering width and depth as stress concentrations gradually diminish. The results reveal the role of borehole-fracture intersection in controlling local breakout rotations, and emphasize the importance of accounting for fracture influences when using breakout data to estimate far-field stresses. This study underscores the need for detailed fracture and borehole information to improve the interpretation of stress heterogeneity in the Earth's crust.