Every foot of hole drilled sits inside a pressure range bounded by two limits. Below the lower limit, pore pressure, formation fluid pushes into the wellbore and the surrounding rock loses the support it needs to hold its shape. Above the upper limit, fracture gradient, wellbore pressure exceeds what the rock can withstand and the formation breaks down, taking drilling fluid with it. Between those two limits sits the wellbore stability window, and everything about safe, efficient formation pressure drilling comes down to staying inside it.
Industry reporting on narrow-margin wells describes this condition directly: a small difference between pore pressure and fracture gradient is what turns borehole instability, downhole mud losses, and pressure cycling from occasional risks into standing operational threats.
The tighter that window gets, the less room there is for a delayed or imprecise pressure correction. In mature fields and depleted reservoirs, that margin often shrinks further still, since pore pressure and fracture gradient can converge to within a fraction of a pound per gallon at exactly the depths where offset wells already show a history of instability.
