Water Content Effect on Borehole Breakout in Hard Rock and Its Interpretation: An Experimental Study.

Borehole breakout is an important geological phenomenon that provides a significant reference for the estimation of the direction and magnitude of in situ stress. To better understand the water content effect on borehole breakouts, a series of experiments on red sandstone was conducted. The standard cylindrical samples and cubic samples containing a circular hole were loaded under uniaxial compression in four water states including dry, unsaturated, short-term saturated, and long-term saturated. Then, the water effect was discussed in terms of rock strength and deformation, initiation stress of borehole breakout, failure evolution process, geometries of breakout, fragments, and acoustic emission (AE) signals. In the water-bearing state, rock properties significantly deteriorate with the decline of strength, the enhancement of plastic deformation, and the restrain of the brittle failure mode. For cubic samples, the axial stress required for borehole breakout increases under the short-term saturated condition but decreases after the long-term immersion. In addition, obvious extrusion deformation of borehole walls toward the borehole space is observed in saturated samples, resulting in vague V-shaped void notches. Before the overall failure, more obvious AE signal fluctuation occurs in the water-bearing state and AE locations reveal a larger fracture concentration region. Two mechanisms are proposed to explain the water content effects on the borehole breakout initiation stress, involving strength weakening effect and stress redistribution mechanism. The shrinkage of borehole breakout geometries is discussed further on the influence on the in situ stress estimation. Highlights: The influence of water content on sandstone properties and borehole breakout is comprehensively investigated. Short-term immersion results in a slight rise of borehole breakout initiation stress, while it declines again after long-term immersion. Double mechanisms are proposed to explain the water content effect on borehole breakout initiation. Water induces the shrink of both breakout depth and angular span, which may further affect the in situ stress estimation. [ABSTRACT FROM AUTHOR]