Borehole Breakouts as Tools for Determining the Orientation and Magnitude of In Situ Stresses

摘要

One method that has been investigated to estimate principal stress magnitudes and directions utilizes the phenomenon known as borehole breakouts. Stress-induced instabilities develop around underground openings (boreholes, shafts, or tunnels) along the diameter perpendicular to the maximum far-field stress springline, where the maximum tangential stress condition occurs. Laboratory experiments conducted in crystalline and some sedimentary rocks produced "V-shaped" breakouts comparable to those observed in the field. Researchers have since been successful in estimating stress regimes when incorporating V-shaped breakout dimensions. More recent, borehole breakout experiments conducted in high-porosity, quartz-rich sandstone produced breakouts very different in size and shape. Breakouts occurred as long, narrow tabular "fracture-like" features oriented counterintuitively perpendicular to the maximum stress direction suggesting an unrecognized failure mechanism. Although fracture-like breakout dimensions are related to far-field stresses, they can also be affected by altering common drilling variables. That has made completely understanding the failure mechanism and the process by which these breakouts stabilize difficult. The purpose of this paper is to briefly discuss the borehole breakout phenomenon as it has been observed in the field and the laboratory.