Methods
Overcoming perforating damage with differential to the wellbore when shooting has been used for sometime, especially with through-tubing type perforators run on wireline. This approach is not always a good solution to the problems of compaction and plugging.
In consolidated formations and/or fluid sensitive, shaley zones, the through-tubing guns may not give sufficient penetration to get beyond the invaded zone. Then, too, the small perforation hole size and the limited differential pressure used for wireline operation can result in having a lowered permeability zone and debris clogged perforation. Flow area then is greatly reduced.
Reduced flow area can result in excessive velocities that may cause water coning and/or sand flow.
Perforation with a casing gun and deep penetrating charges run in the conventional manner on wireline. The differential pressure is into the formation, and mud or a completion fluid is in the well bore. The debris from the shaped charge, the cement, the formation displaced, and the well fluid often combine to form a crushed and compacted zone.
This crushed and compacted low-permeability zone can have as little as 10 to 20 percent of the permeability of the undisturbed rock.
Only the perforations at the most permeable spots will be open. The others will remain plugged without additional treatment with acids, fracturing, etc. The inherent problems of high flow velocities through the few holes open will exist for these cases the same as the through-tubing perforations.
Flowing the well at low flow rates minimizes the production problems, but does not yield maximum possible natural production rates.
In the tubing conveyed casing gun, pressure differentials from 1,000 to 5,000 psi often are needed to expel the mud, cement, shaped charge debris, and low-permeability compacted zone. Field results have proved these large differentials are needed to clean perforations properly even in high-porosity zones.
It is important to have large flow areas to yield maximum production with no formation flow problems. This is accomplished best by shooting at least four shots per foot with large size (0.5-in. diameter) over the entire produceable interval. You need maximum differential pressure to clean each hole as it is made.
This can be done safely and efficiently by running casing guns on the bottom of the tubing, preferably below a retrievable packer, and mechanically firing the guns after the Christmas tree is installed.
Equipment Arrangement
The necessary guns to cover the perforating interval are screwed together, a mechanical firing head (protected by a tubing sub) is placed on the top
gun, and the assembly is put in the hole and held by the slips. We next run a tubing joint, mechanical tubing release sub, another tubing joint and a retrievable packer with a vent assembly.
One joint of tubing is placed above the packer. For depth control, a tubing sub then is installed with a radioactive tag in the bottom collar.
Depth Control
Prior to going in the hole, all components have been measured. The strapped distance from the top shot to the radioactive marker collar is known.
The assembly is run into the well in a normal manner with the tubing dry or filled with only the amount of fluid needed to give the desired pressure differential for the zone to be perforated.
When the correct depth for perforating is reached by tubing measurements, the assembly is hung off on the slips. A gamma-ray or neutron depth-control logging tool then is run inside the tubing. A depth tie-in is made to the base log some distance above the depth of the marked collar. Then the logging tool is lowered through the tubing sub and marked collar.
A positioning log is recorded. The actual position of the assembly is determined. Necessary depth correction is made then by installing tubing subs below the slick joint. The precision of the depth control is as good as the log correlation.
Packer-Actuated Vent Assembly
Allowing for packer stroke and desired tubing compression or tension, the packer is set. This opens the packer-actuated vent assembly ports that will serve as production port.
The stem or mandrel with ports is screwed to the bottom of the packer mandrel. The sliding sleeve covers the ports that then are sealed from the well fluids. The outer hull is adapted to the packer body over the sleeve and mandrel. It is adjusted such that a shoulder in the bottom engages the shoulder on the sleeve and opens the ports in the final few inches of packer setting.
Opening the ports relieves the hydrostatic pressure below the packer and gives the desired differential pressure for perforating.
Well Perforation and Clean-up
The Christmas Tree is installed, the flowline staked, a flare bucket lit, the tree cap removed, and a detonating bar dropped to fire the guns.
If the well is a good natural producer, the fluids in the tubing (if any), the rat hole fluid, gun and perforation debris, mud filtrate, and other contaminates will be brought to the surface. After removal of these contaminates, good wells will quickly finish cleanup and be ready for production.
Mechanical Tubing Release Sub
If you do not wish to leave the guns in place as a blast joint, you may drop them off in the rat hole.
Threaded fingers are held in a threaded housing by a latching device. To release the lower assembly, you run an appropriate shifting tool on slick line through the latching device. Jarring up against the latching device removes it from the fingers and the assembly drops into the rat hole.
Hookups
A typical hookup for a single completion. A hookup for a parallel string - dual zone completion. A parallel string - dual zone completion with double tubing conveyed gun assemblies.
Advantages
The mechanical firing system is very safe. Since there are no electric blasting caps, you eliminate the hazards of radio transmission, electrical storms, wind storms, stray electric currents, and storage and handling of electric blasting caps. You are subject to Class C rather than Class A explosive handling regulations.
Because the firing pin is pressure balanced, only a sharp mechanical blow will detonate the initiator. The system has been pressure tested to 19,000 psi and then fired mechanically in the lab. Well fluid pressures have been as high as 13,800 psi.
Very long intervals can be safely perforated with complete well control even with large differential pressures.
The simplicity of the entire system helps keep it reliable. Use of new components in all critical areas of the system further enhances reliability. Three firing initiators are used, any one of which will fire the gun.