1. n. [Drilling]
The rubblized rock just below the tooth of a rock bit. Rock in the crushed zone fails due to the high compressive stress placed on it by the bit tooth (in the case of a roller-cone bit). The effective creation of and removal of crushed zone rock is important to the efficiency of the drill bit. If the rock is not broken and removed efficiently, the result is akin to effectively drilling the hole twice.
See: roller cone bit
2. n. [Perforating, Well Completions]
The rubblized or damaged zone surrounding a perforation tunnel where the action of the perforating charge or bullet has altered the formation structure and permeability. Although it is generally damaging to production, the severity or extent of the crushed zone depend greatly on the characteristics of the formation, the perforating charge and the underbalance or overbalance conditions at time of perforating. Measures to reduce the effect of the crushed zone include underbalanced perforating in which the crushed zone and perforating debris are flushed from the perforating tunnel by the reservoir fluid as soon as the perforation is created. Where overbalanced perforating techniques are used, it may be necessary to acidize the crushed zone to achieve maximum productivity from the perforated interval.