why does shale breakout occur along the min horizontal stress direction and frac at max hori stress direction?

Question

is it true that if the well is inclined to the principal stress then breakout direction may not always represent the min principa stress direction?

Answer 1

Because the minimum stress axis (sig-3) is, by definition, the least compressive for that tensor's orientation. Fracture depends upon clay content and whether or not you're taking about pure or simple shear.

Assume any reasonable failure envelop and go look a various shale's empirical Moore Circles. That should help answer your questions.

Answer 2

yes it is true