Borehole breakouts often occur with complicated failure mechanisms, involving both tensile and shear failures. To facilitate studies on borehole breakout phenomena, a new fracture propagation code FRACOD2D is introduced in this paper. The code simulates fracture propagation in the tensile mode (mode I), shear mode (mode II) and the mixed mode (mode I + II). It is capable of predicting borehole breakouts caused by a range of failure mechanisms. The code has been applied to study borehole breakouts in a number of cases with different stress ratios and fluid pressures. It was found that in most cases, shear failures are involved in the final formation of a breakout. Tensile failures alone do not form breakouts. The numerical models predicted the typical dog-ear shaped breakouts. The shape and extents of the breakouts (width and depth) were found to depend upon the magnitude of applied stresses as well as the fluid pressure in the borehole. The results of modeling are consistent with laboratory test results and field observations. Factors affecting breakout mechanism are discussed.
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