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The Differential Sub is used in a perforating system to provide an economical method of achieving an underbalanced condition between the tubing and wellbore formation. The tool incorporates a high strength ceramic dome which prevents communication between tubing and tubing/casing annulus until a detonating bar is dropped from surface. This tool can be used in other applications where the operator requires the formation to be surged after or during a stimulation.
This tool is not recommended for snubbing operations without secondary fail-over systems in place, such as in the occurence of a ceramic dome failure.
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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