4. 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.
5. 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
6.
7. 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.
8. 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.
9. 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.
10. 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
11.
12. 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.
16. 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.
17.
18.
19. 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.
20. The extent of perforation damage is a
function of:
Lithology
Rock strength
Porosity
Pore fluid compressibility
Clay content
Formation grain size
Shaped-charge designs
21. 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
24. 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.
25. 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.
26. 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
30. 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.
32. 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.
35. 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.
36. 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
37. 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
38. 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.
39. 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.
40.
41. 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.
42. 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.
43. 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.
44. 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.
47. 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.
48. 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.
50. 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.
51. WELL/RESERVOIR CHARACTERISTICS
Pressure differential between a wellbore and
reservoir before perforating can be described
by:
Underbalanced
Overbalanced
Extreme overbalanced (EOB)
52. 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.
53. 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
54.
55. 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.
56.
57.
58. 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.
59. 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
60. 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
61. 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
62. Constant ao depends on the perforation
phasing.
a1a2a1b2b1c2c
63. 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
64. 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.
65. 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
66.
67. 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.
68. 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.