Experimental investigation of borehole breakout formation in Gosford sandstone

This study investigates the in-situ borehole breakout formation using true-triaxial laboratory experiments on cubic Gosford sandstone. Four series of tests have been conducted under varying stress conditions following two approaches: PD method (loading onto specimens with pre-drilled boreholes) and PS method (loading onto intact specimens and then conducting drilling). Borehole camera observations show that a large amount of rock debris remains within the breakout zone under the PD conditions, while clear V-shaped notches are observed in the PS tests. The breakout geometries are extracted based on point cloud data obtained from 3D scanning. Both breakout width and normalised depth under both testing approaches exhibit positive correlations with the maximum horizontal stress while decreasing with minimum horizontal stress. The PS tests tend to yield wider and deeper breakouts compared to the PD tests. The difference in breakout width is generally consistent across various stress conditions at approximately 10° – 15°, whereas the variation in the normalised breakout depth increases with the severity of the borehole failure. The discrepancies are believed to be attributed to the excavation unloading induced strength-weakening and removal of rock debris by circulating fluid and drilling vibration. The outcomes of this study demonstrate that the stress path has critical impacts on the rock failure behaviours around openings, and the PS tests are suggested for investigating the borehole breakout phenomenon.