A condition in which the [[drill stem]] becomes stuck against the wall of the [[well bore|wellbore]] because part of the drill stem (usually the [[drill collar|drill collars]]) has become embedded in the [[mud cake]]. Necessary conditions for differential-pressure sticking (or wall sticking) are a permeable [[permeability|permeable]] formation, a [[pressure]] differential across a nearly impermeable mud cake, and drill stem.