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 zone.

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.

0 μsec

4 μsec

9.4 μsec

16.6 μsec

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 energy.

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

Density

(g/cc)

Detonation

Velocity (ft/sec)

Detonation

Pressure (psi)

RDX Cyclotrimethylene trinitramine C3H6N6O6 1.80 28,700 5,000,00

0

HMX Cyclotrimethylene tetranitramine C4H8N8O8 1.90 30,000 5,700,00

0

HNS HexanitrostilbeneC14H6N6O12 1.74 24,300 3,500,00

0

PYX Bis(picrylamino)-3,5-dinitropyridine C17H7N11O16 1.77 24,900 3,700,00

0

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.

A Pivot gun system

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.

Carrier gun arrangement

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.

Effect of centralizati

on

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 permeability gas wells.

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

ep

w

kh P Pq

rB 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

Hw

rs

r

for 04

for 0

perf

w

o w perf

l

r

a r l

Constant ao depends on the perforation phasing.

a1a2a1b2b1c2c

The Vertical Converging Effect

110a b bV D Ds h r

1 2log Da a r a 1 2Db b r b

12perf V

Dperf H

r kr

h k

1

shot densityperfh perf H

Dperf V

h kh

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

rr

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.