LABORATORY STUDY OF BOREHOLE BREAKOUTS IN CORDOVA CREAM - A CASE OF SHEAR FAILURE-MECHANISM

We subjected vertical boreholes drilled in cubical specimens of Cordov a Cream to a 3-dimensional stress field, and increased one or both of the horizontal principal stresses until breakout failure occurred. Aco ustic emission, borehole strain, and visual monitoring of the borehole wall during testing revealed four stages of breakout development: cra ck initiation, extension, breakout formation, and stabilization. Thin section analysis suggests that breakout formation in Cordova Cream is the result of shear failure mechanism. The relationships at breakout s tabilization between the two horizontal principal stresses and between the span at the borehole wall and the least horizontal stress suggest a potentially simple method of estimating the far-field state of stre ss if 'breakouted' borehole geometry is accurately mapped.