Perforation
Perforation
Perforation
2
REFERENCES
Golan, M. and Whitson, C. H.: “Well Performance,” 2nd Ed.,
Tapir, 1995.
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OUTLINES
Introduction
Shaped Charged Perforation
Explosives
Perforating Guns
Perforation Efficiency & Gun Performance
Well/Reservoir Characteristics
Calculations
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PERFORATION
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INTRODUCTION
Objective of perforation is to establish communication
between the wellbore & the formation.
This is achieved by making holes through the casing,
cement & into formation.
The inflow capacity of the reservoir must not be
inhibited.
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INTRODUCTION
Well productivity & injectivity depend primarily on near-
wellbore pressure drop called Skin.
Skin is a function of:
Completion type
Formation damage
Perforation
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8
INTRODUCTION
Deep penetration:
Increases effective wellbore radius
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PERFORATING METHODS
4 Perforating Methods
1 Bullet Gun Perforating
2 Abrasive Perforating Methods
3 Water Jets
4 Shaped Charges
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SHAPED CHARGED PERFORATION
The shaped charge evolved from the WW2 military
bazooka.
Outer case
High explosive
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SHAPED CHARGED PERFORATION
The detonating cord initiates the primer & detonates the
main explosive
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SHAPED CHARGED PERFORATION
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SHAPED CHARGED PERFORATION
The jet penetrating mechanism is one of “punching” rather
than blasting, burning, drilling or abrasive wearing.
This punching effect is achieved by extremely high impact
pressures –
3 x 106 psi on casing
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0 μsec
4 μsec
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9.4 μsec
16.6 μsec
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SHAPED CHARGED PERFORATION
Elastic rebound leaves shock-damaged rock, pulverized
formation grains & debris in the newly created perforation
tunnels.
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SHAPED CHARGED PERFORATION
The crushed zone can limit both productivity & injectivity.
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SHAPED CHARGED PERFORATION
The extent of perforation damage is a function of:
Lithology
Rock strength
Porosity
Pore fluid compressibility
Clay content
Formation grain size
Shaped-charge designs
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EXPLOSIVES
Explosives used in perforation are called Secondary high
explosives.
Reaction rate = 22,966 – 30,000 ft/s.
Volume of gas produced = 750 – 1,000 times original
volume of explosive.
These explosives are generally organic compounds of
nitrogen & oxygen.
When a detonator initiates the breaking of the molecules'
atomic bonds, the atoms of nitrogen lock together with
much stronger bonds, releasing tremendous amounts of
energy.
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EXPLOSIVES
Typical explosives are:
RDX (Cyclotrimethylene trinitramine)
HMX (Cyclotrimethylene tetranitramine)
HNS (Hexanitrostilbene)
PYX Bis(Picrylamino)-3,5-dinitropyridine
PS (Picryl sulfone)
Composition B (60% RDX, 40% trinitrotoluene)
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EXPLOSIVES
Detonation Detonation
Chemical Density
Explosive Velocity Pressure
Formula (g/cc)
(ft/sec) (psi)
RDX Cyclotrimethylene
trinitramine C3H6N6O6 1.80 28,700 5,000,000
HMX Cyclotrimethylene
tetranitramine C4H8N8O8 1.90 30,000 5,700,000
HNS Hexanitrostilbene
C14H6N6O12 1.74 24,300 3,500,000
PYX Bis(picrylamino)-3,5-
dinitropyridine C17H7N11O16 1.77 24,900 3,700,000
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EXPLOSIVES
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EXPLOSIVES
It is important to respect the explosives used in
perforating operations.
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PERFORATING GUNS
Perforating guns are configured in several ways.
There are four main types of perforating guns:
Wireline conveyed casing guns
Through-tubing hollow carrier guns
Through-tubing strip guns
Tubing conveyed perforating guns
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WIRELINE CONVEYED CASING GUNS
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WIRELINE CONVEYED CASING GUNS
The advantages of casing guns over the other wireline
guns are:
High charge performance
Low cost
Highest temperature & pressure rating
High mechanical & electrical reliability
Minimal debris & minimal casing damage
Instant shot detection
Multi-phasing
Variable shot densities of 1 – 12 spf
Speed & accurate positioning using Casing Collar
Locator (CCL)/Gamma Ray
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THROUGH-TUBING HOLLOW CARRIER GUNS
Smaller versions of
casing guns which
can be run through
tubing.
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THROUGH-TUBING HOLLOW CARRIER GUNS
They have lower charge sizes &, therefore lower
performance, than all other guns.
They only offer 0o or 180o phasing
Maximum shot density of 4 spf on the 2-1/8” OD gun & 6
spf on the 2-7/8” OD gun.
Due to the stand-off from the casing which these guns
may have, they are usually fitted with
decentralizing/orientation devices.
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THROUGH-TUBING STRIP GUNS
Semi-expendable type
guns consisting of a
metal strip into which the
charges are mounted.
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THROUGH-TUBING STRIP GUNS
Charges have higher performance.
They also cause more debris, casing damage & have less
mechanical & electrical reliability.
They also provide 0o or 180o phasing.
By being able to be run through the tubing, underbalance
perforating can possibly be adopted but only for the first
shot.
A new version called the Pivot Gun has even larger
charges for deep penetration.
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A Pivot gun
system
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TUBING CONVEYED PERFORATING GUNS
(TCP)
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TUBING CONVEYED PERFORATING GUNS (TCP)
Longer lengths can be installed.
Lengths of over 1,000 ft are possible (especially useful for
horizontal wells).
The main problems associated with TCP are:
Gun positioning is more difficult.
The sump needs to be drilled deeper to accommodate
the gun length if it is dropped after firing.
A misfire is extremely expensive.
Shot detection is more unreliable.
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PERFORATION EFFICIENCY &
GUN PERFORMANCE
Optimizing perforating efficiency relies extensively on the
planning & execution of the well completion which
includes:
Selection of the perforation interval
Fluid selection
Gun selection
Applied pressure differential
Well clean-up
Perforating orientation
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PERFORATION EFFICIENCY &
GUN PERFORMANCE
API RP 19B, 1st Edition (Recommended Practices for
Evaluation of Well Perforators) provide means for
evaluating perforating systems (multiple shot) in four ways:
Performance under ambient temperature &
atmospheric pressure test conditions.
Performance in stressed Berea sandstone targets
(simulated wellbore pressure test conditions).
How performance may be changed after exposure to
elevated temperature conditions.
Flow performance of a perforation under specific
stressed test conditions
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PERFORATION EFFICIENCY &
GUN PERFORMANCE
Factors affecting gun performance include:
Compressive strengths & porosities of formations.
Type of charges used (size, shape).
Charge alignment.
Moisture contamination.
Gun stand-off.
Thickness of casing & cement.
Multiple casings.
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PERFORATION EFFICIENCY &
GUN PERFORMANCE
It is necessary for engineers to obtain as much accurate
data from the suppliers & use the company’s historic data
in order to be able to make the best choice of gun.
Due to the problem of flow restriction, the important
factors to be considered include:
Hole diameter to achieve adequate flow area.
Shot density to achieve adequate flow area.
Shot phasing, Penetration, Debris removal.
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HOLE SIZE
The hole size obtained is a function of the casing grade &
should be as follows:
Between 6 mm & 12 mm for natural completions.
Between 15 mm & 25 mm in gravel packed
completions.
Between 8 mm & 12 mm if fracturing is to be carried
out & where ball sealers are to be used.
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SHOT DENSITY
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SHOT DENSITY
A shot density greater than this is required where:
Vertical permeability is low.
There is a risk of sand production.
There is a risk of high velocities & hence turbulence.
A gravel pack is to be conducted.
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SHOT PHASING
Phasing is the radial distribution of successive
perforating charges around the gun axis.
Simply put, phasing is perforation orientation or the
angle between holes.
Perforating gun assemblies are commonly available in
0o, 180o, 120o, 90o & 60o phasing.
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Carrier gun
arrangement
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SHOT PHASING
The 0o phasing (all shots are along the same side of
the casing) is generally used only in small outside-
diameter guns.
60o, 90o & 120o degree phase guns are generally
larger & provide more efficient flow characteristics
near the wellbore.
Optimized phasing reduces pressure drop near the
wellbore by providing flow conduits on all sides of the
casing.
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SHOT PHASING
Providing the stand-off is less than 50mm, 180o or
less, 120o, 90o, 60o is preferable.
If the smallest charges are being used then the
stand-off should not be more than 25mm.
If fracturing is to be carried out then 90o and lower
will help initiate fractures.
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Effect of
centralization
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PENETRATION
In general, the deeper the shot the better, but at
the least it should exceed the drilling damage
area by 75mm.
However, to obtain high shot density, the guns
may be limited to the charge size which can
physically be installed that will impact
penetration.
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WELL/RESERVOIR CHARACTERISTICS
Overbalanced
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UNDERBALANCED PERFORATING
Reservoir pressure is substantially higher than the
wellbore pressure.
Adequate reservoir pressure must exist to displace
the fluids from within the production tubing if the well
is to flow unaided.
If the reservoir pressure is insufficient to achieve this,
measures must be taken to lighten the fluid column
typically by gas lifting or circulating a less dense fluid.
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UNDERBALANCED PERFORATING
The flow rates & pressures used to exercise control during
the clean up period are intended to maximize the return of
drilling or completion fluids & debris.
This controlled backflush of perforating debris or filtrate
also enables surface production facilities to reach stable
conditions gradually.
Standard differential pressure ≈ 200 – 400 psi.
Differential pressures up to 5,000 psi in low permeability
gas wells.
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OVERBALANCED PERFORATING
Perforating when the wellbore pressure is higher than the
reservoir pressure.
This is normally used as a method of well control during
perforating.
The problem with this method is it introduces wellbore
fluid into the formation causing formation damage.
Use clean fluid to prevent perforation plugging.
Use of acid in carbonates.
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OVERBALANCED PERFORATING
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OVERBALANCED PERFORATING
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EXTREME OVERBALANCED PERFORATING
The wellbore is pressured up to very high
pressures with gas (usually nitrogen).
When the perforating guns are detonated the
inflow of high pressure gas into the formation
results in a mini-frac, opening up the formation to
increase inflow.
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CALCULATIONS
A mechanism to account for the effects of perforations on
well performance is through the introduction of the
perforation skin effect, sp in the well production equation.
kh Pe Pwf
q
re
141.2 B ln s p
rw
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CALCULATIONS
Karakas and Tariq (1988) have presented a semi-
analytical solution for the calculation of the perforation
skin effect, which they divide into components:
The plane-flow effect, sH
The vertical converging effect, sV
l perf
for 0
rw 4
a r l for 0
o w perf
rw = wellbore radius (ft).
r’w(θ) = effective wellbore radius (ft). It is a function of the
phasing angle θ.
lperf = length of perforation (ft)
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THE PLANE-FLOW EFFECT
a a1 log rD a2 b b1rD b2
rperf kV 1 hperf kH
rD 1 hperf hD
2hperf kH shot density l perf kV
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THE VERTICAL CONVERGING EFFECT
a1, a2, b1 & b2 are obtained from the previous table.
kH = horizontal permeability
kV = vertical permeability
rperf = radius of perforation (ft)
sV is potentially the largest contributor to sp.
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THE WELLBORE EFFECT
rw
rwD
l perf rw
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THE WELLBORE EFFECT
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PERFORATION SKIN EFFECT CALCULATION
Example 1: Assume that a well in the reservoir has a radius of
rw = 0.328ft is perforated with 2 SPF, rperf = 0.25 in. (0.0208ft),
lperf = 8 in. (0.667 ft), and φ = 1800. Calculate the perforation skin
effect if kH/kV = 10. Repeat the calculation for φ = 00 and φ = 600.
If φ = 1800, show the effect of the horizontal-to-vertical
permeability anisotropy with kH/kV = 1.
Solution: using the equations:
l perf
for 0
rw 4
a r l for 0
o w perf
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PERFORATION SKIN EFFECT CALCULATION
Therefore:
Then,
rw 0.328
s H ln ln 0 .4
rw ( ) 0 .5
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PERFORATION SKIN EFFECT CALCULATION
For hD:
Remembering that hperf = 1/SPF = ½ = 0.5,
hperf kH
hD
l perf kV
Then,
0 .5
hD 10 2 .37
0 .667
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PERFORATION SKIN EFFECT CALCULATION
For rD:
rperf kV
rD 1
2hperf kH
Then,
0 .0208
rD (1 0 .1 ) 0 .027
( 2 )( 0 .5)
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PERFORATION SKIN EFFECT CALCULATION
For a,
a a1 log rD a2
For b,
b b1rD b2
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PERFORATION SKIN EFFECT CALCULATION
For sV,
sV 10a hDb 1rDb
s wb ( 2 .6 10 2 ) e ( 4.532 )( 0.33 ) 0 .1
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PERFORATION SKIN EFFECT CALCULATION
The total perforation skin effect is then,
s p 0 .4 4 .3 0 .1 4
If θ = 0o, then sH =0.3, sV =3.6, swb =0.4
Therefore, sp =4.3
If θ = 60o, then sH =0.9, sV =4.9, swb =0.004
Therefore, sp =4
For, θ = 180o and kH/kv =1, then sH and swb do not change;
sV, though, is only 1.2, leading to sp =0.9, reflecting the
beneficial effects of good vertical permeability even with
relatively unfavorable perforation density (2 PF).
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PERFORATION SKIN EFFECT CALCULATION
Exercises: Using typical perforation characteristics such as
rperf = 0.25 in., (0.0208ft), lperf = 8 in. (0.667 ft), φ = 1200, in a
well with rw = 0.328ft, develop a table of sV versus
perforation density for permeability anisotropies kH/kV =10,
5, 1. Populate the table with the result from the exercise.
Table: Vertical Contribution to Perforation Skin Effect
sV
SPF kH/kV =10 kH/kV =5 kH/kV =1
0.5
1
2
3
4
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