perforation (4)(Types of perforation techniques)

Types of perforation techniques

                2.5.1. According to the relation between
                          reservoir and hydrostatic pressures

2.5.1.1.  overbalanced perforation

           main features of overbalanced perforation

         1-    hydrostatic pressure of fluid in well bore greater than reservoir pressure

         2-    provide control over well while performing completion

          3-    perforation can be plugged with debris in well bore "difficult in cleaning process"

2.5.2.2.      underbalanced perforation

        main features of underbalanced perforation

       1-    Hydrostatic pressure of fluid in well bore is less than the reservoir pressure

       2-    Well is "Live" after perforation and must be controlled

       3-    Perforation will be clean from surge into well bore

       2.5.2.     According to where we do perforation

 2.5.2.1.Shop perforated casing are classified to

                  - Round perforated

                  - Slotted perforated

        In round perforated casing the diameter of slots are 1/8 to 3/8 in and it made
 by milling or by oxyacetylene torch. The diameter of hole depends on the casing diameter.

 In slotted perforation the slots are 0.05 to 0.3 inch wide & 1.5 to 3 inch long and it 
also made by milling or by oxyacetylene torch. And we should take care that the total 
area of slots equal 2% of casing area. We can also use screen to prevent sand to 
enter the well. And we prefer slotted casing than rounded one in sandy formations.

2.5.2.2. Gun perforated casing:

        The second type of perforation is gun perforated casing which is our main point
 in our study.

Optimum perforation

Perforating is a critical part of any well completion process .
 perforating is the only way to establish conductive tunnels that link oil and gas
 reservoirs to steel cased well bores which lead to surface  .
 however perforating also damage s formation permeability around perforation tunnels .
this damage and perforation parameters like formation penetration hole size ,number
 of shots and the angle between holes have a significant impact on pressure 
drop near a well ,therefore, on production .optimization the perforation parameter
 and mitigating induced damage are the vital aspects of perforating.
 Ongoing before perforating is less than the formation pressure is essential
 in removing damage and debris from perforations.

2.5.3.           According to how we do perforation

There are three basic techniques employed today to perforate a well.
 Although the variations are virtually endless, the following discussion
is limited to a basic description of the three techniques. Wells can be perforated
 using casing guns conveyed on wire line, through-tubing guns, and 
tubing-conveyed guns. Because each method has advantages and limitations,
 the completion engineer must choose the most appropriate technique for each well.

 2.5.2.1.  WIRELINE CASING GUN TECHNIQUES

Perforating with a casing gun conveyed on wire line has been a standard 
technique for many years. Before the tubing and wellhead are put in place, 
a hollow carrier casing gun is lowered into the well on wire line, positioned 
opposite the productive zone, and detonated. The main advantages of this 
system are as follows:

- The diameter of the gun is limited only by the ID of the casing;
 therefore, large, high performance shaped charges can be conveyed 
in a multiphase, high shot density carrier.

- The casing gun offers high reliability because the blasting cap 
detonating cord and shaped charges are protected from the wellbore
environment and the carrier is mechanically strong.

- Selective firing is available between guns.

- Guns are accurately positioned opposite the zone of interest using
a casing collar locator.

- No damage occurs to the casing and virtually no debris is left 
in the well.

There are two main limitations to this method:

- As a general practice, the well must be perforated with the wellbore pressure 
greater than the formation pressure. This pressure differential may prevent 
optimum cleanup of the perforations. The situation is aggravated when perforating in
 drilling mud. The mud plugs are difficult to remove even when subjected to high
 reverse pressure. Perforating in clean liquids such as salt water is recommended.

- The strength of the wireline and the weight of the casing guns limit the length 
of the assembly which can be run on each trip into the well.

There are three basic types of casing guns:

1. Port Plug Guns

2. Scalloped Guns

3. Slick Walled Guns

Each gun design has the same primary purpose to seal the guns from the wellbore
 pressure and fluids. The differences are in how this is achieved and how the 
individual charges are secured in place. Port Plug guns are re-usable carriers
 that use the port plug to secure the charge (Figures B17 and B18). The perforating 
charge has to penetrate the carrier before it can perforate the casing. Port Plug guns 
are designed so that the charge perforates a port plug which can be replaced and the
 gun re-used. Gun life is not indefinite but being able to distribute the carrier cost
 over 10 to 15 jobs reduces the overall cost of perforating. Fig. B18: Port Plug Gun.

Scalloped guns are typically used when high shot density perforating 
(greater than 4 spf, 13 spm) is required. The carrier is a metal tube
 with Flat bottomed holes milled on the outside, about 3 mm deep. These 
scallops are aligned with the perforating charges inside the gun so that the 
charges fire through the centre of the scallop. This does not significantly 
change the penetration of the charge but rather is to ensure

that any burring that may have occurred on the gun wall does not exceed 
the overall gun outside diameter.

The charges are held in place by a tube or triangular strip (Figure B19)

 which is slid into the gun itself. The scalloped gun is used for high shot 
density because the cost of machining so many port plug holes (up to 39 per meter)
is prohibitive and the chance of a port plug leaking and flooding the guns is obviously
 increased. Another common use for the scalloped carrier is for TCP work. 
Whenever a TCP system is used for a permanent completion, the guns will
 not be retrieved, for this case the cost of machining port plugs will not be
recovered. Fig. B20: Slick Walled Gun.

The Slick Walled guns are a unique carrier designed for a moderate environment (Figure B20).

 The gun carriers have neither port plugs nor scallops. It is simply smooth surfaced
 pipe through which the charges perforate. This causes some burrs on the outside 
of the carrier but as long as enough clearance exists no problems will be encountered.
 The carrier and explosives are rated for lower pressures and temperatures than
 other gun systems (27.5 MPa, 99 degree C for 1 hour) and can only be loaded to 
a maximum of 13 spm (4 spf). The charges are held in place by a moulded Styrofoam
 case which allows quick efficient loading. 
The system allows for cost effective perforating of shallow to medium (2500 m) depth wells

 2.5.2.2. THROUGH-TUBING PERFORATING TECHNIQUE

In 1953, Humble Oil and Refining Co. pioneered the permanent-type well completion.

This technique involves setting the production tubing and wellhead in place and 
then perforating the well with small diameter guns capable of running through tubing.
 The main advantages of this technique are as follows:

- The well may be perforated with the wellbore pressure below the formation pressure 
allowing the reservoir fluids to instantly clean up the perforating debris.

- Completion of a new zone or a workover of an existing zone does not require 
the use of a rig.

- A casing collar locator allows for accurate depth positioning.

The main limitations of this method are as follows:

- To allow the gun to run through tubing, smaller shaped charges, with reduced 
penetrations, must be used. To achieve maximum penetration with through tubing
 perforators the gun is usually positioned against the casing to eliminate the loss 
of performance when perforating through the liquid in the wellbore. 
This arrangement restricts the gun to 0o phasing.

- In an effort to improve the penetration performance, gun system designers 
eliminate the hollow steel carrier and place pressure tight capsule charges 
on a strip or wire. These guns are called expendable or semi-expendable 
depending on whether the wire or strip is retrieved. Removing the steel 
carrier allows a larger charge to be used; however, charge case
 debris is left in the well after perforating and the casing may be 
damaged by the detonation.

- The charges are exposed in the expendable and semi-expendable systems restricting 
these guns to less severe well environments and lower running speeds.

As stated earlier, these guns, designed to pass through tubing are used for a variety of reasons:

1. Critical sour gas wells where a permanent packer is to be in place before perforating occurs.

2. Older wells where a retrievable packer cannot be un-set due to failure.

3. Perforate slim casings or liners (89 mm).

.

The small outside diameter of through-tubing guns implies that if the charges are to 
be contained inside of a tube (HSC) the explosive load will have to be small. Such
 is the case with our hyper dome guns (Figure B21). With explosive weights of 
1.8 gm to 6.5 gm, penetrations can be limited but exposure to wellbore fluids
 and potential failure thereby is eliminated. Fig. B22: Enerjet Gun.

If deeper penetration is required, an expendable or semi-expendable carrier is required.
Because the gun outside diameter is governed by the charge size, a maximum load can 
be placed down hole after passing through the tubing. Care must be taken not to attempt 
too large of an explosive charge. If unenclosed charges in excess of 20 grams are
 allowed to detonate downhole, casing damage could result.

Typical of these carriers is our Enerjet gun where the charges are threaded into 
a steel bar (Figure B22). Explosive loads can go as high as 15.5 gms and after
 detonation the steel bar and threaded caps are retrieved from the well.
In this manner only a minimum amount of debris, in the form of powder is left behind.

2.5.2TUBING-CONVEYED PERFORATING
                         TECHNIQUE

Although various attempts were made to convey perforating guns into 
the well on tubing it was not until the early 1980's that widespread use 
of the service began. The basic technique involves assembling hollow 
carrier steel casing guns vertically with a firing head on top. There 
are several types of firing heads including drop bar, differential pressure, 
direct pressure, and electrical wet connect. On top of the firing head 
is a sub used to allow reservoir fluids to flow into the tubing. 
A production packer is attached above the fluid communication sub. 
This entire assembly is then lowered into the well on the end of the tubing string.
 The string is depth positioned usually with a gamma ray survey.

 After the guns are positioned,
 the packer is set, and the well is readied for production. 
This includes establishing the correct underbalanced condition in the tubing.
 The guns are then fired and the surge of reservoir fluids is used to clean
 up the perforations. Depending on the situation the guns may be retrieved or dropped to the bottom of the well.
 Many variations of the procedure described above are in use today.
 The main advantages of this technique are as follows:

- The well can be perforated with large diameter, high performance;
 high shot density casing guns with the wellbore pressure lower 
than the formation pressure (underbalanced) allowing instantaneous cleanup of the perforations.

- The wellhead is in place and the packer is set before the guns are fired.

- Large intervals can be perforated simultaneously on one trip into the well.

- Highly deviated and horizontal wells can be perforated by pushing the guns into the well.

The main limitations of the technique are as follows:

- Unless the guns are withdrawn from the well it is difficult to confirm whether the entire 
gun fired. Effective shot detection systems may overcome this limitation.

- Explosives degrade when exposed to elevated temperatures, 
reducing shaped charge performance. It takes many times longer to run a TCP string into the hole than a wire line gun. To compensate, a more expensive and, in some cases, less powerful explosive must be used on TCP operations.

- Selective perforating options with TCP are limited. Small intervals over large intervals may not be economical with TCP.

- Accurate depth positioning of the gun string is more difficult and time consuming than wire line depth positioning.

2.5.2.3.1. TCP FIRING SYSTEMS

Several firing techniques are available for various types of completion or testing operations. They include percussion, pressure, and electrically activated systems.

2.5.2.3.2. Percussion-Activated Firing Head (Drop-bar)

The drop bar is the simplest TCP firing system. A cylindrical weight or sinker bar is dropped into the tubing and strikes a percussion- type detonator in the gun firing head.

Hydraulic pressure on the tubing fluid column is adjusted to achieve the desired underbalance on the formation before dropping the bar. The bar can be dropped by hand through an open wellhead control valve or contained in a wireline lubricator and released when wellhead valves are opened and can also be run on a slickline.

2.5.2.3.3. Bar Actuated Pressure Firing System

The gun is not fired by the impact of the drop bar on the firing head. Instead, the drop bar shears a pin, which releases the catch on the firing piston. The firing piston is then driven hydrostatically towards the percussion cap to set off the detonating train. A minimum hydrostatic head of 500 psi is needed in the tubing to set the gun off. With this feature, it is not possible to accidentally fire the gun at surface by dropping anything on the firing head. If the well is perforated dry, the 500 psi required can be obtained by pressuring the tubing with nitrogen prior to dropping the bar.

2.5.2.3.4.  Differential-Pressure Firing Head

The differential-pressure firing head utilizes a flowtube through the packer to transfer annulus pressure above the packer to an isolated piston in the firing head located beneath the packer. The advantage of this firing method is that, after setting the packer, the tubing and packer can be tested in the direction of well pressure by internally pressuring the tubing and transmitting this pressure to casing beneath. After Pressure testing, the desired underbalance pressure is fixed before firing the guns. The annulus pressure forces the release piston downward, breaking the shear pins and releasing the locking lugs which secure the firing pin. The hydrostatic pressure in the rathole below the packer then drives the firing pin into the percussion cap, igniting the Primacord which fires the perforating guns.

2.5.2.3.5. Tubing - Pressure Firing System

This system uses a percussion-activated firing head similar in principle to the differential pressure and drop-bar systems, except that it is activated by internally pressuring the tubing. This same system is used, without modification, for DST’s or permanent completions. After setting the packer, it is tested by pressuring the tubing annulus. Next, the tubing pressure is raised through three specific pressure cycles to arm the gun. Two of these are redundant safety cycles built into the system to account for unanticipated excursions such as high pressure surges, swab pressures, and high circulating pressures. After the three cycle sequence, there is a variable time delay before firing in order to correct underbalanced pressure and adjust wellhead choke manifolds.

An advantage of the tubing-pressure system is that it can be fired in wells where the casing above the packer is leaking; e.g., split or corroded pipe and old squeeze perforations.

2.5.2.3.6. Electric - Wire line Firing System

The electric-wire line firing system uses electric current and logging cable for firing. Conventional wire line pressure-control equipment (lubricator, blowout preventer, etc.) is used during flow testing with cable in the hole. The wet connector contains a mechanical latch that secures it to the TCP firing head, preventing the cable from being blown uphole. With these firing systems, an electric current transmitted to a wet connector at the gun head fires the detonator. One of the major advantages of these systems is that they cannot be inadvertently armed and fired before the electric power source is connected with the firing head. A gamma ray and collar log can be run with the electric-wire line firing system.

2.5.2.3.7. JOB PLANNING AND OPERATIONAL CONSIDERATIONS FOR TCP

Personnel safety is one of our highest concerns; Schlumberger requires the use of a minimum three meter safety spacer above the gun. This ensures that the guns are below the rig floor when the firing head is connected. Cleanliness is the most important factor governing success or failure of a TCP operation. A dirty workstring with pipe scale, dope, or gelled mud with high solids content can prevent access to any of the firing head systems. Any workstring (new or used) should always be rattled or washed clean before picking up the TCP assembly. Pipe dope should be used sparingly. Once on bottom, circulation should be established to flush trash through the circulating sub. A joint of tubing filled with clean fluid should be run immediately below the circulating sub. If the TCP assembly contains a closed annular production valve and the workstring is filled on the floor with clean brine or diesel while going in hole, circulation is not necessary. In high-angle holes the drop bar should not be used, and pressure-type firing systems are recommended. Gun release subs should be used with permanent completions to allow production logging and access to perforations for remedial stimulation work. If sufficient casing rathole is not available to accommodate the fired guns, they can be pulled out of the well if a stabthrough TCP arrangement is used with a larger bore packer. However, this is not desirable since the well will have to be killed and equipment pulled and rerun. Such a system would likely require guns with smaller OD’s. The optimum underbalance pressure is dependent upon several factors such as perforation size and length, rock strength, reservoir permeability, and fluid viscosity. All of these, in theory, affect the ability of the perforation to be cleaned. As a practical matter, the underbalance pressure should be between a minimum of a few hundred pounds per square inch and a maximum of the design collapse rating for the casing. Low-permeability formations and zones producing gas require larger pressure differentials to clean up the perforations.

 Some of the most common TCP accessories are listed on the following pages.

2.5.2.3.7.1. Radioactive Marker Sub

The sub is run in line with the workstring above the packer, or can simply be a tubing collar or a drill pipe tool joint where one or two small cavities have been drilled and threaded to receive a sealing plug. A radioactive pip tag is installed in each cavity. A pip tag is a very weak gamma ray source (1 microcurie of Cobalt 60). The radiocative marker sub is run above the packer, and all the radioactive material is fully recovered when the string is pulled (Figure B23).

2.5.2.3.7.2.Cone-Type Debris Circulating Sub

The debris circulating sub (Figure B24) consists of a ceramic cone seated into a ported sub. The sub is positioned between the packer and the guns, typically 10 m above the firing head. The isolated space below the sub is filled with a clean fluid. Once the assembly above the sub has been circulated clean, the packer is set. A debris circulating sub is often used with a drop bar or a wet connected firing system to prevent debris from settling on the downhole portion of the firing head. The drop

bar or female wet connector will easily break the fragile cone to reach the firing head.

Fig. B23: Radioactive Marker Sub. Fig. B24: Debris Sub.

2.5.2.3.7.3.Mechanical Gun Release Sub

The operating principle of the mechanical gun release sub (Figure B25) is similar to other gun release subs. After a release sleeve is shifted, the lower sub locking fingers retract. The lower sub and the gun string are then released and fall to the bottom of the well. Fig. B25: Gun Release Sub. Fig. B26: Surge-Disc Sub

2.5.2.3.7.4. Surge-Disc Sub

The surge-disc sub (Figure B26) features a fragile, high strength, sealed, glass disc inside a sub. The disc is designed to withstand a high differential pressure.

The sub is positioned above the circulating sub, completely sealing off the portion of tubing above it. This portion of tubing can be dry or partially filled with a clean fluid cushion to create an underbalance condition after the packer is set and the disc broken. In the presence of old perforations, the sub can be used with a drop bar firing system. In this application, the underbalance will be established a few seconds before firing the guns. The underbalance will remain effective during firing and at the time of the surge immediately after firing.

2.5.2.4. COILED TUBING CONVEYED PERFORATING FOR HORIZONTAL WELLS

2.5.2.4.1. Principle

The principle of this system (Figure B27) is particularly simple: the guns are mounted directly on the end of tubing coiled on a reel in which the electric cable has first been inserted. The connection between guns and tubing ensures the mechanical and electrical bottom link, while on the surface; the cable outlet passes through the shaft of the drum by a rotating device. The lowering and raising movements are provided by the standard coiled-tubing injector head, and the depth measurements are made on the tubing near the injector. This technique is equally capable of conveying small-diameter guns and standard guns, but the performance capabilities will be affected by gun weight. In addition, circulation through the coiled tubing remains available, although the cross section is reduced, owing to the cable.

2.5.2.4.2. Procedure

The logging procedure with this system is exactly the same as that for normal use of coiled tubing. Should it be necessary to work under pressure, a lubricator adapted to the guns should be added. The weak link in this system is its relative fragility, rendering it incapable for pushing heavy guns over great distances. Fig. B28: Perforating Depth Control.