Keep the Well Open
A borehole is a hole punched into a loaded rock, and whether it stays open is a stress problem. Read the stress state friction allows, watch the wellbore concentrate it, see the wall break out and fracture, and find the mud-weight window that keeps the hole round. Then invert those breakouts back to the stress that made them, on one calibrated well.
You can place a stress state in the frictional polygon and classify its regime, read the hoop-stress concentration the Kirsch equations predict around a borehole, tell when the wall will break out and how wide or fracture in tension, pick a mud weight inside the safe window between collapse and losses, correct for a deviated trajectory, invert an observed breakout back to the maximum horizontal stress, and read the assembled Ogbon-1 stress model that ties the whole wellbore story to one calibrated field.
The stress state
Which of the three stresses is vertical sorts the crust into normal, strike-slip, and reverse faulting, and that ordering decides how a well behaves and where a fracture goes.
Friction caps how different the greatest and least effective stresses can be; the crust sits near that limit almost everywhere, so a stress state is bounded before it is measured.
The polygon is every horizontal-stress pair friction permits at a depth; drop a point in and it is classified, and the polygon breathes inward as the pore pressure rises.
The wellbore under stress
Cutting a hole concentrates the stress on its wall; the hoop stress peaks at the minimum-stress azimuth and can reach three times the far-field difference, which is where the wall fails.
Where the concentrated hoop stress beats the rock strength the wall spalls into breakouts at the minimum-stress azimuth; their angular width is set by the stress, the mud, and the strength.
Too heavy a mud drops the minimum hoop stress below the tensile strength and cracks the wall in tension, aligned with the maximum horizontal stress, the mirror image of a breakout.
Too light and the wall collapses or the well kicks; too heavy and mud is lost to a fracture. The safe window is the band between, and it narrows as the field depletes.
The safe window depends on the trajectory: a well drilled at an angle to the stresses meets a different hoop stress, so the same mud is stable in one direction and not another.
The image log and the caliper are where the theory meets the rock: breakouts and tensile fractures on the wall read the stress directions and, with the width, its magnitudes.
Calibrate and close
The pressure a formation fractures at is the least stress, and the honest spread between the Hubbert-Willis estimate and the measured closure is the uncertainty the mud plan carries.
The wellbore decisions all read from one depth-continuous model of the stresses, pressure, strength, and moduli, built once and calibrated on the well's own data.
The mud window is only as good as the rock strength, and logs give it as a bracket, not a line; the honest report is the band the correlations allow, calibrated to core where it exists.
The one stress no test measures directly is the maximum horizontal stress; but the breakout width inverts straight back to it, given the other stresses and the strength.
The six components assemble into one calibrated stress model reading a mobilized friction of 0.58, a field one bad decision from the frictional edge, and every wellbore decision reads from it.