The Differential Sub is a tool which may be run in a TCP string to maintain a high differential pressure in favour of the annulus (underbalance), while keeping fluid from entering the tubing prior to gun detonation. This tool features a convex ceramic dome which can withstand high differential pressures imposed by fluid in the tubing-casing annulus.
A Drop Bar is released from surface to strike the Mechanical (Percussion) Firing Head. It also impacts the Differential Sub causing the ceramic dome to shatter. The instant the dome fragments, a passage is created for reservoir fluids to surge into the dry tubing after the guns fire. Since the formation is under pressure and the tubing is essentially dry, the fluid rushes into the tubing and cleans the perforation tunnels. This fluid cleaning action is the basis of underbalanced perforating.
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