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.
