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Types of completions:
Perforating is required
Open hole Uncemented
liner
Cemented liner
Perforating is not required Cemented
casing
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PERFORATING TECHNIQUES
Jet Charge or Lined Shape Charge
The lined shape charge has four parts. They are the
liner, the main explosive charge, the booster, and the
charge case.
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The formation of a jet stream occurs very quickly. As the explosive wave-front
travels along the liner, small metallic particles from the inner half of the liner
are spalled off at a very high velocity
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These particles converge along the cone axis, and with the proper standoff
(distance from first target interface to the base of the liner
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For a specific charge, the size of the hole and the depth of penetration
depends on the strength of the target, that is, a smaller hole will be
produced in high strength casing than in a lower strength casing.
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When jet charges are assembled in a gun or carrier to be
run into a well, the charge is placed in contact with a
detonating cord, to which is attached an electric detonator
or blasting cap. When an electric current is applied to the
detonator, the explosive force of the detonator causes the
detonator cord to detonate.
This sets up the explosive energy wave which travels along
the detonating cord, detonating each charge as it passes.
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Parameters Affecting Performance
The fluid clearance
yield strength of casing
and the compressive strength of the formation .
The data shown for those factors would apply to only
the specific charge; however, other charges would have
very similar relationships
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Fluid clearance is the distance between the fluid
barrier and target material (casing). Two inches of
clearance does not reduce penetration by two inches
since water takes less energy to penetrate than
formation material
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Higher yield strength casing will reduce entry hole diameters slightly
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Formation compressive strength has a significant
effect on the depth of penetration.
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Hollow Carriers
Hollow carrier guns are available in both casing
and through-tubing sizes.
The casing guns are retrievable and reusable or
expendable. Carriers are usually from 3 1/8 in. to 7 in.
in diameter and range from a few inches up to I6 ft. in
length. They can be run in tandem on wireline under
most conditions.
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FACTORS AFFECTING PERFORMANCE
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CLEARANCE
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The through-tubing hollow carriers range from 1 3/8
in. to 2 3/4 in. in diameter and are retrievable but not
reusable because the charges actually perforate the
carrier.
Shot densities of from one to six per foot are available
with 0°, 60°, or 180° phasing; however, 0° phasing is
recommended when the gun is decentralized for
minimum clearance.
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The advantages of hollow carrier guns are as follows:
1. Provide a fixed and precise perforation pattern .
2. Protect the charges from well bore fluids and
pressures.
3. Absorb the shock and gas energy from the charge
detonation, thus protecting casing and cement sheath
from possible damage.
4. Provide a rugged charge alignment system that
permits spudding through light bridges in the well
bore.
5. Contain the debris from the charge case and
alignment systems; preventing plugging of chokes,
valves or flow lines.
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Wireline Expendable Guns
Advantages or expendable guns are as follows:
1. Allow greater charge explosive weight for fixed
running diameter to be used.
2. Provide flexibility to allow running through crooked
casing or tubing.
3. Allow a longer gun to be run when necessary.
4 . Provide the ability to pass through small
restrictions and give good performances in varying
casing sizes.
5. Provide numerous perforation patterns and
practical ly any shot spacing desired.
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The disadvantages are as follow
1. Debris left in wellbore.
2. The casing must absorb the shock and pressure
generated by the charge detonation .
3. Lower pressure and temperature ratings than
hollow carriers.
4. Not recommended for use in hostill environments
such as caustic fluids or certain types of acid.
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Factors Affecting Perforating Efficiency
1- Wellbore Conditions
When perforating in mud-filled casing with a pressure
differential to the formation , the perforations are
plugged with mud, crushed formation material , and
charge debris. The plugging could be reduced by using
a clean fluid such as salt water, kerosene or oil as a
completion fluid , but the filtration of wellbore fluids
caused by the pressure differential to the formation
still carried some crushed formation and charge debris
into the perforation.
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2- Formation Conditions
The variation of penetration with the compressive
strength of the formation has been discussed
previously.
However, indications are that the perforation is made
by plastic flow of the formation.
Removal of the crushed zone gives a perforation
diameter larger than that of the entrance hole in the
casing, depending upon the porosity and rock
strength. In dense formations, the perforation will be
smaller and tapered similar to the shape of the jet
stream itself
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The effect of fluids on depths of perforation clean-out.
For a given pressure differential to the wellbore, the
lower the density of the fluid in the wellbore, the
greater the depth to which the perforation is cleaned
out.
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Drilling and Perforating Damage
Perforating damage, along with formation damage by
drilling mud and cement contaminants, can be a block
to higher production.
Jet perforating with large casing guns produces a deep
perforation tunnel beyond the mud damaged zone.
The jet charges plug the holes they have just made and
compact the rock around the perforotion tunnel
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Perforation immediately after creation.
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Wellhead Pressure Control Equipment
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Types of Perforating
Casing Guns
Hollow Steel Carriers, more commonly called casing
guns, have the advantage of larger charges, providing
deep penetration or large hole size .These guns, run on
wireline, are normally used to perforate in a balanced or
slightly over-balanced condition. The formation
pressure is held by the hydrostatic while the guns are
pulled from the well and the production Equipment
well head are installed. The well is then swabbed to
induce flow from the formation.
In most cases, stimulation of t he well and hydraulically
opening the perforations is required to achieve the
production desired from the completion
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Extreme overbalance perforations.
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Through-Tubing
Through-tubing perforating is widely used and is quite
effective in high porosity and high permeability zones.
However, small gun perforators have shallow
penetration into the formation because of inadequate
explosive force.
In tighter formations, normal through-tubing
pressure limitations may not allow sufficient
differential pressures to clean the flow channels.
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Tubing-Co nveyed
Tubing-Conveyed Perforating, using large guns and
high differential pressures, combines the best of
casing gun and through-tubing perforating
techniques. It is becoming widely recognized as a
simple, safe, and efficient system for completing wells
and removing the drilling and perforating damage.
Using the proper differential when perforating will
remove the compacted zone around the perforation
tunnel and expulse the debris from the perforation.
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Optimum underbalance from Behrmann’s criteria.
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Example perforation penetration prediction
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Perforation spacing and geometry
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Determining Optimum Completion and
Under balance Pressure
Designing the optimum completion program for a well
includes selecting (1) the most effective perforating
system, (2) perforation size and shot density, and {3}
calculation of the maximum underbalanced pressure
that the formation and/or well mechanical system will
permit.
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Productivity Ratio
The productivity ratio (PI) of a formation is equal to
the production flow of a perforated interval divided by
the open hole production of the same interval. Figure
is based on no skin damage and 6 in. diameter open
hole. Therefore, the chart represents the PR to be
obtained if total skin damage is overcome.
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Designing for these high production rates reduces the
need for workovers in the early life of a well.
It also maintains laminar flow through the perforations.
By maintaining laminar flow, sand production is often
avoided , even in cases of unconsolidated formations.
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Effects of Skin Factors on Productivity
Ratio
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Formation Definitions
To choose the proper underbalance pressure required
to overcome total skin damage at the time of
perforating, it is necessary to first define
consolidated and unconsolidated formations.
Consolidated
A consolidated formation is defined as one with the
sand grains cemented or compacted sufficiently that
they remain intact and do not flow even if there is
some turbulent flow of fluids in the pore spaces.
We identify the degree of sandstone consolidation by
use of sonic or density logs.
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A consolidated formation is identified as one that has
adjacent shales (above and/or below) that are so
compacted that the sonic log travel time in the shales
is 100 microseconds or less per foot.
Unconsolidated
An unconsolidated formation is defined as a sandstone
formation with adjacent shales having a sonic or
acoustic log travel time greater than 100 microseconds
per foot.
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DETERMINE SHOT DENSITY
First we will assume that the perforating service
company and the oil company engineer have agreed on
the following:
1. They will use tubing-conveyed perforating.
2. They will use 5 in. OD guns In 7 in. casing.
3. They will use 5 in. BH (big hole) charge for hole
diameter of 0. 71 in.
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Consolidated formations are those having a sonic log
travel time in the adjacent clean shale (above or below
of 100 micro-seconds per foot or less). When working
from a density log, consolidated formations are
identified as those having a density log bulk density in
the adjacent clean shale (above or below) of 2.40 g /cc
or more.
Unconsolidated formations are those having a sonic
log travel time in the adjacent clean shale (above or
below) greater than 100 micro-seconds per foot. When
working from a density log unconsolidated formations
are those having a density log bulk density in the
adjacent clean shale of less than 2.40 g /cc.
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DETERMINING PERFORATING UNDERBALANCE
TO OVERCOME TOTAL SKIN DAMAGE
For Consolidated Formations
a. Choose maximum △P underbalance from the
lesser value of the mechanical system, (i.e., casing, tubing,
packer, and downhole tools). Our experience indicates that
consolidated furmatiolls can be perforated with as much
underbalance pressure as the mechanical system will
withstand, since sand flow is not 1a problem.
1- The maximum under balance to safely use is the lesser
value of safe casing or tubing collapse pressure limit, packer
differential pressure limit, and other tool string differential
limits.
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For new casing or tuhing we use a limit of 80% of
collapse rating. This gives us a 20% safety factor.
b. Another method to choose maximum underbalance
is the use of formation compressive strength. From lab
tests of formation cores, it has been found that there is
no formation matrix movement until the effective
stress exceeds 1.7 times the formation compressive
strength. In this case, effective stress is defined as
overburden pressure minus pore pressure. Therefore,
minimum pore pressure is overburden minus 1.7
times compressive strength. Thus, maximum
underbalance is formation pressure minus minimum
pore pressure.
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2- Choose minimum △P underbalance for
consolidated formations from Figure 5.43 or 5.44 .
Figures 5.43 and 5.44 are graphs of permeability of the
formation vs. under balance pressure. Note that there
is a separate graph (or oil wells and gas wells.
This chart also indicates that a minimum of 500 psi
underbalance is required for oil zones with 200 md or
more. The highest permeability oil zone tested was
approximately 600 md .
Minimum Undcrbalance, psi = 3500/K0.37 where K is
permeability in millidarcies. (for oil)
Minimum Undcrbalance, psi = 2500/K0.17 where K is
permeability in millidarcies. (for gas)
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Having determined the maximum and minimum △P
for the consolidated formation in I and 2, choose the
midpoint pressure between minimum ,and maximum
△P . If the logs indicate shallow invasion, and low
water loss cement was used, the UN DERBALANCE
△P range to use is minimum △P to midpoint ± P.
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SUMMARY OF OPTIMUM COMPLETION DESlNG
I. Determine Shot Density
• Use API Section II penetration corrected for formation strength and
find shot density for PR-l.
• To assure laminar flow and minimum workover, use two to three
times shot density from above.
2. Classify Formation as Consolidated or Unconsolidated
• Use sonic log or density log values in adjacent shales.
• Consolidated formation
• Sonic log △t shale 100 microsecond / ft or less
• Density log bulk density of shale 2.40 g /cc or more
• Unconsolidated formation
• Sonic log △t shale greater than 100 microseconds/ft.
• Density log bulk density of shale less than 2.40 glee.
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3. Perforating Underbalance to Overcome Total Skin
Damage
• For consolidated formations Choose minimum △P
underbalance by ,use of formation permeability charts
for gas or oil sands, or use equations.
• Choose maximum △P from lesser value of safe
casing collapse limit , tubing collapse Iimit, packer and
other tools limit, or equations
Determine midpoint pressure between minimum and
maximum △ P.
If logs indicate shallow invasion and you used low
water loss cement, UNDERBALANCE △P range is
minimum △P to midpoint △P.
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If logs indicate medium to deep invasion, and/or used
medium to high water loss cement, the
UNDERBALLANCE RANGE is midpoint △p to
maximum △p.
• For Unconsolidated Formations
Choose minimum △P underballane by use of
formation permeability charts for gas or oil sands or
equations.
• Choose maximum △P from chart of Unconsolidated
Sands Using Sonic Log, travel Time or Density Log
Bulk Density, or equations.
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• Determine midpoint pressure between minimum
and maximum △p If logs indicate shallow invasion
and you used low water loss cement,
UNDERBALANCE △p at range is minimum △p to
midpoint △p
If logs indicate medium to deep invasion and/or used
medium to high water loss cement, the UNDEHBA
LANCE RANGE is mid point △p to maximum △p
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Tubing-Conveyed Perforating
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Selective Dual Zone Single String
Completions
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Dual Completions
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Standard Rod Pump
Completions
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Horizontal Completions of Highly
Deviated well Completions
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Perforation Cleaning
Perforation debris removal is considered an important
step in well completion.
In addition to removing perforating debris, such
methods should also recover mud cake and mud
pockets from the cement-formation interface.
These have been shown to cause problems with
injectivity. This is very important when attempting to
consolidate formations with sequentially injected resin
systems.
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When mechanical means of control are used , well
preparation is equally important. To obtain a uniform
pack and to provide for adequate flow of fluid when
the well is being produced, a maximum number of
perforations should be open.
Removal methods include underbalance perforating,
backsurge, and perforation washing.
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Perforation Washing
Perforation washing is a positive method of removing
perforation debris, mud and formation sand from the
perforation tunnel and from behind the casing, Figure.
An opposed cup packer or sucker type tool is employed
to isolate as little as 1 foot of perforated Interval at a
time. Acids or other clean fluids can be injected
through the isolated perforations to establish
communication through all perforations.
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Back surging
The most common method Involves a back surge tool,
consisting of a treating packer, an atmospheric air
chamber which can be one or more joints of drill string
or tubing, and two valves - one above and one below
the air chamber.
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To operate the tool, the lower quick-opening valve is
opened to suddenly expose the perforated interval
below the packer to lower pressure.
The sudden surge of fluid into the wellbore withdraws
plugging material from perforations.
Air chamber volumes of at least five gallons per
perforation are reported to have improved productivity
four to five times that of non back surged completions.
Even larger air chamber volumes are desirable
occasionally.
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Thank you
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