Absolute pressure-actuated trigger charge firing system
The absolute pressure-actuated trigger charge firing system TCF-P is designed to offer maximum flexibility and control over the firing operation because the firing head is run separately from the gun string. The downhole fill sub assembly and receptor booster are run with the gun string. After the gun string is positioned at shooting depth, the trigger charge firing head is run on a slickline string with weights and a jar.
The assembly is gently latched onto the downhole receptor booster cover inside the fill sub. Jarring down unlatches the slickline tools from the firing head, allowing the slickline tools to be removed from the well.
After the slickline assembly is removed, the tubing pressure is increased to the calculated pressure to shear the traveling piston pins in the absolute pressure firing head HDF. An adjustable time delay is initiated before firing the guns. During this delay, tubing pressure is reduced to the desired underbalance.
In case of a problem, the firing head may be unlatched and returned to the surface for examination. Then, a decision is made to rerun another firing head or to run an impression block. If the firing head cannot be latched, bailers may be run. If necessary, the unarmed gun string may be pulled safely to the surface.
A minimum of 150-psi hydrostatic pressure is required to fire the percussion detonator, thus preventing firing at the surface.
In the unlikely event that the jar down mechanism is accidentally actuated while spudding against debris, the shaped-charge jet develops along the axis of the tubing and cannot damage the string. The well fluid also dampens the shaped-charge jet before it reaches the bottom of the firing head housing.
Because the pressure-actuated trigger charge firing head is run separately, its triggering pressure may be set at a lower level because pressure tests have been performed, running overpressures are under control or are absent, and safety factors are less stringent.