Coiled Tubing
Coiled Tubing Perforation
Perforating with deep penetrating shaped charges and immediately backsurging at maximum differential pressure — using the formation's own energy to leave clean, efficient flow channels.
The problem: damage from drilling, cementing and perforating
Perforating with a casing gun in the conventional manner puts the differential pressure into the formation, with mud or completion fluid in the wellbore. Debris from the shaped charge, the cement, the displaced formation and the well fluid combine to form a crushed, compacted zone that can retain as little as 10 to 20 percent of the permeability of undisturbed rock.
Only the perforations at the most permeable spots open up. The rest stay plugged without acidizing, fracturing or other treatment — and the few holes that do flow carry excessive velocities that can cause water coning or sand flow.
Through-tubing perforators run on wireline are a partial answer, but in consolidated or fluid-sensitive shaley zones they may not penetrate beyond the invaded zone, and the small hole size and limited differential available on wireline can still leave a lowered-permeability, debris-clogged perforation.
The technique
Shoot dry, backsurge hard, clean every hole.
Casing guns are run on the bottom of the tubing, preferably below a retrievable packer, and fired mechanically after the Christmas tree is installed — with the tubing practically dry so the natural formation pressure does the cleaning.
1,000–5,000 psi
Differential pressures typically needed to expel mud, cement, charge debris and the compacted zone — proven necessary even in high-porosity zones.
4+ shots per foot
Large-diameter charges (0.5 in.) across the entire producible interval give the flow area needed for maximum production without flow problems.
Any completion
Hookups for single completions, parallel-string dual zone, and parallel-string dual zone with double tubing-conveyed gun assemblies.
Mechanical firing
No electric blasting caps, so the hazards of radio transmission, electrical storms, stray currents and cap handling are eliminated.
How the job runs
Four stages, from rig-up to clean-up.
- 01
Equipment arrangement
The guns needed to cover the perforating interval are made up together, a mechanical firing head — protected by a tubing sub — is placed on the top gun, and the assembly is run and held by the slips. A tubing joint, mechanical tubing release sub, a second joint and a retrievable packer with a vent assembly follow, with one joint of tubing above the packer.
- 02
Depth control
All components are measured beforehand, so the strapped distance from the top shot to the radioactive marker collar is known. A gamma ray or neutron depth-control logging tool is run inside the tubing and tied in to the base log, a positioning log is recorded, and any correction is made by installing tubing subs below the slick joint. Precision is as good as the log correlation.
- 03
Packer-actuated vent assembly
Allowing for packer stroke and the desired tubing compression or tension, the packer is set. The final few inches of setting open the vent assembly ports, which serve as the production port — relieving hydrostatic pressure below the packer and establishing the differential pressure for perforating. Mechanically, a ported stem screws to the bottom of the packer mandrel, a sliding sleeve covers those ports to seal them from well fluids, and the outer hull is adapted to the packer body over sleeve and mandrel — set so a shoulder engages the sleeve and opens the ports in the final inches of packer setting.
- 04
Perforation and clean-up
The Christmas tree is installed, the flowline staked, the flare lit and the tree cap removed, then a detonating bar is dropped to fire the guns. Tubing and rat hole fluids, gun and perforation debris, mud filtrate and other contaminants are brought to surface, after which a good well finishes clean-up quickly and is ready for production. If the guns are not wanted in place as a blast joint they can be dropped into the rat hole: threaded fingers held in a threaded housing by a latching device are released by running a shifting tool on slick line and jarring up, which frees the fingers and drops the assembly.
Advantages
Safer handling, longer intervals, higher reliability.
Because the firing pin is pressure balanced, only a sharp mechanical blow detonates the initiator. The system has been pressure tested to 19,000 psi then fired mechanically in the lab, and has been used with well fluid pressures as high as 13,800 psi.
Why operators choose it
- No electric blasting caps — Class C rather than Class A explosive handling regulations apply
- Very long intervals can be perforated safely with complete well control, even at large differentials
- Three firing initiators are used, any one of which will fire the gun
- New components in all critical areas of the system further enhance reliability
- Guns can be left in place as a blast joint, or dropped into the rat hole via the mechanical tubing release sub
- Cleaner producing perforations, especially in fluid-sensitive formations
Next step
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