Petroleum Production Engineering - Perforation

PET 325
James A.
Craig
Omega 2011
PERFORATION

TABLE OF CONTENTS
 Introduction
 Shaped Charged Perforation
 Explosives
 Perforating Guns
 Perforation Efficiency & Gun Performance
 Well/Reservoir Characteristics
 Calculations
 References

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.

 Well productivity & injectivity depend
primarily on near-wellbore pressure drop
called Skin.
 Skin is a function of:
 Completion type
 Formation damage
 Perforation
 Skin is high & productivity reduced when:
 Formation damage is severe (drilling &
completion fluids invasion ranges from several
inches to a few feet)
 Perforations do not extend beyond the invaded

 Deep penetration:
 Increases effective wellbore radius
 Intersects more natural fractures if present
 Prevents/reduces sand production by reducing
pressure drop across perforated intervals.
 High-strength formations & damaged
reservoirs benefit the most from deep-
penetrating perforations.

SHAPED CHARGED PERFORATION
 The shaped charge evolved from the WW2
military bazooka.
 Perforating charges consist of:
 A primer
 Outer case
 High explosive
 Conical liner connected to a detonating cord.

 The detonating cord initiates the primer &
detonates the main explosive
 The liner collapses to form the high-velocity
jet of fluidized metal particles that are
propelled along the charge axis through the
well casing & cement & into the formation.

 The detonator is triggered by:
 Electrical heating when deployed on wireline
systems or,
 A firing pin in mechanically or hydraulically
operated firing head systems employed on
tubing conveyed perforating (TCP) systems

 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
 3 x 105 psi on formation.
 These jet impact pressures cause steel,
cement, rock, & pore fluids to flow plastically
outward.

 Elastic rebound leaves shock-damaged rock,
pulverized formation grains & debris in the
newly created perforation tunnels.
 Hence, perforating damage can consist of
three elements:
 A crushed zone
 Migration of fine formation particles
 Debris inside perforation tunnels.

 The crushed zone can limit both productivity
& injectivity.
 Fines and debris restrict injectivity & increase
pump pressure, which:
 Decreases injection volumes
 Impairs placement or distribution of gravel &
proppants for sand control or hydraulic fracture
treatments.

 The extent of perforation damage is a
function of:
 Lithology
 Rock strength
 Porosity
 Pore fluid compressibility
 Clay content
 Formation grain size
 Shaped-charge designs

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

 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)

Explosive
Chemical
Formula
Densit
y
(g/cc)
Detonatio
n Velocity
(ft/sec)
Detonatio
n
Pressure
(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

 RDX is the most commonly used explosives
for shaped charges (up to 300 oF).
 In deep wells when extreme temperature is
required & where the guns are exposed to
well temperatures for longer periods of time
HMX, PS, HNS or PYX is used.

 It is important to respect the explosives used
in perforating operations.
 They are hazardous.
 Accidents can occur if they are not handled
carefully or if proper procedures are not
followed.

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

Wireline Conveyed Casing Guns
Generally run in the
well before
installing the tubing.

 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
CCL/Gamma Ray

Through-tubing Hollow Carrier Guns
Smaller versions of
casing guns which
can be run through
tubing.

 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.

Through-tubing Strip Guns
Semi-expendable
type guns
consisting of a
metal strip into
which the charges
are mounted.

 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.

Tubing Conveyed Perforating Guns (TCP)
TCP guns are a
variant of the
casing gun which
can be run on
tubing.

 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.

PERFORATION EFFICIENCY &
GUN PERFORMANCE
 Optimizing perforating efficiency relies
extensively on the planning & execution of
the well completion which includes:
 Selection of the perforated interval
 Fluid selection
 Gun selection
 Applied pressure differential
 Well clean-up
 Perforating orientation

 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

 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.

 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.

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.

Shot Density
 Shot density is the number of holes specified
in shots per foot (spf).
 An adequate shot density can reduce
perforation skin & produce wells at lower
pressure differentials.
 Shot density in homogeneous, isotropic
formations should be a minimum of 8 spf but
must exceed the frequency of shale
laminations.

 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.
 Note: Too many holes can weaken the
casing strength.

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.

 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.

 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.

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 which will impact
penetration.

WELL/RESERVOIR CHARACTERISTICS
 Pressure differential between a wellbore and
reservoir before perforating can be described
by:
 Underbalanced
 Overbalanced
 Extreme overbalanced (EOB)

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.

 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

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.

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.

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.
 For example, under steady-state conditions:
 
141.2 ln
e wf
e
p
w
kh P P
q
r
B s
r



  
  
  

 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
 The wellbore effect, swb
 The total perforation skin effect is then:
p H V wbs s s s  

The Plane-flow Effect
 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)
 
ln w
H
w
r
s
r 
 
    
 
 
for 0
4
for 0
perf
w
o w perf
l
r
a r l





  
  

 Constant ao depends on the perforation
phasing.
a1a2a1b2b1c2c

The Vertical Converging Effect
1
10a b b
V D Ds h r

 1 2log Da a r a  1 2Db b r b 
1
2
perf V
D
perf H
r k
r
h k
 
   
 
1
shot density
perfh 
perf H
D
perf V
h k
h
l k


 a1, a2, b1 & b2 are obtained from the table
above.
 kH = horizontal permeability
 kV = vertical permeability
 rperf = radius of perforation (ft)
 sV is potentially the largest contributor to sp.

The Wellbore Effect
 c1 & c2 are obtained from the table above.
 1 2expwb wDs c c r 
 
w
wD
perf w
r
r
l r



REFERENCES
 Gatlin, C.: “Drilling Well Completion,”
Prentice-Hall Inc., New Jersey, 1960.
 ENI S.p.A. Agip Division: “Completion Design
Manual,” 1999.
 Halliburton: “Petroleum Well Construction,”
1997.
 Ott, W. K. and Woods, J. D.: “Modern
Sandface Completion Practices Handbook,”
1st Ed., World Oil Magazine, 2003.

 Schlumberger: “Completions Primer,” 2001.
 Golan, M. and Whitson, C. H.: “Well
Performance,” 2nd Ed., Tapir, 1995.
 Karakas, M. and Tariq, S.: “Semi-Analytical
Productivity Models for Perforated
Completions,” paper SPE 18271, 1988.
 Clegg, J. D.: “Production Operations
Engineering,” Petroleum Engineering
Handbook, Vol. IV, SPE, 2007.
 Bellarby, J.: “Well Completion Design,” 1st
Ed., Elsevier B.V., 2009.

Petroleum Production Engineering - Perforation

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