Download pdf - Breakdown and drilling problems

Breakdown and drilling problems

Matanovi Davorin, professor

Faculty of mining, geology and petroleum engineering, Zagreb

IntroductionWellbore stability is the prevention of brittle failure or plastic deformation of the rock surrounding the wellbore due to mechanical stress or chemical imbalance.

Prior to drilling the mechanical stresses in the formation are less than the strength of the rock.

The chemical action is also balanced, or occurring at a rate relative to geologic time.

Rocks under this balanced or near balanced state are stable.

Introduction

After drilling, the rock surrounding the wellbore undergoes changes in tension, compression, and shear loads as the rock forming the core of the hole is removed.

Chemical reaction also occur with exposure to the drilling fluid.

IntroductionUnder these conditions the rock surrounding the wellbore can become unstable, begin to deform, fracture, and cave into the wellbore or dissolve into the drilling fluid.

Excessive rock stress can collapse the hole resulting in stuck pipe.

Hole-squeezing mobile formations produce tight hole problems and stuck pipe.

Cavings from failing formation makes hole cleaning more difficult and increases mud and cementing costs.

Wellbore

Pressure

(a)

(b)

BRITTLE

SHALE

SALT

FRIABLE

SANDSTONE

OVERGAUGE

HOLE

LOST

CIRCULATION

HOLE

CLOSURE

IntroductionFishing is the process of removing a fish or junk from the borehole.

A fish is a part of the drill string (tubing, sucker rods, wire, rope or cable) that separates from the upper remaining portion of the drill string while the drill string is in the well.

This can result from the drill string failing mechanically, or from the lower portion of the drill string becoming stuck or otherwise becoming disconnected from drill string upper portion.

Such an event will instigate an operation to free and retrieve the lower portion (or fish) from the well with a strengthen specialized string

Junk is usually described as small items of non-drillable metals that fall or are left behind in the borehole during the drilling, completion, or workover operations.

These non-drillable items must be retrieved before operations can be continued.

It is important to remove the fish or junk from the well as quickly as possible.

The longer these items remain in a borehole, the more difficult these parts will be to retrieve.

Further, if the fish or junk is in an open hole section of a well the more problems there will be with borehole stability.

Introduction

There is an important tradeoff that must be considered during any fishing operation.

Although the actual cost of a fishing operation is normally small compared to the cost of the drilling rig and other investments in support of the overall drilling operation, if a fish or junk cannot be removed from the borehole in a timely fashion, it may be necessary to sidetrack (directionally drill around the obstruction) or drill another borehole.

Introduction

Thus, the economics of the fishing operation and the other incurred costs at the well site must be carefully and continuously assessed while the fishing operation is underway.

It is very important to know when to terminate the fishing operation and get on with the primary objective of drilling a well.

Introduction

IntroductionThe number of days that should be allowed for a fishing (Df) operation is:

Where:

Vf the replacement value of the fish (dollars or other money)

Cs estimated cost of the sidetrack or the cost of restarting the well (money)

Rf the cost per day of the fishing tool and services (money/day)

Cd the cost per day of the drilling rig (and appropriate support) (money/day)

df

sff CR

CVD

++

=

The most causes of fishing jobs, not necessarily in order of frequency, are::

1. twist off, a parting of the drill string caused by metal fatigue or washout;

2. sticking of the drill string;

3. bit and tool failures; and

4. foreign objects such as hand tools, logging instruments, and broken wireline or cable lost in the hole.

Introduction

TwistoffTwist off is the result of:rough pipe handling,

faulty drill string,

stress reversals in a sharply deviated hole drilling with drill pipe in compression,

poorly stabilized drill collars

scarring by tong dies,

improper makeup torque,

erosion caused by washout, and

other damage that create weak spots where cracks can form and enlarge under the constant bending and torque stresses of routine drilling.

Twist offThe pipe often separates in a helical break or in a long tear or split.

Surface signs of a twist off include:

loss of drill string weight,

lack of penetration,

reduced pump pressure,

increased pump speed,

reduced drilling torque, and

increased rotary speed.

Stuck pipeThere are two general categories of drill string sticking:

mechanical and

differential.

In mechanical sticking, the drill string is lodged in place by solid material.

In differential sticking, the cause is pressure difference of fluid column in the hole and formation pressure.

Among the ways the drill string can become mechanically stuck are the following:

Sloughing hole;that results from shale absorbing water from the drill fluid, expanding, sloughing off, and falling downhole.

Large masses may lodge around drill collars and stabilizers, sticking the drill string and blocking circulation.

Abnormally pressured shale, steeply dipping shale beds, and erosion by drilling fluid can also cause hole wall to cave in.

Pipe stuck in under gauge hole often happens in shale formations.

If the formation swells but does not slough off, the deformed layer may close around the drill pipe, cutting off circulation and preventing passage of the tool joints, drill collars, or bit.

A buildup of mud solids can have the same effect, especially in a permeable zone where water is lost to the formation.

Blowout sticking occurs when a large volume of sand or shale is driven up hole by formation fluids entering the wellbore.

Inadequate hole cleaning that is,

failure of the circulating system to clean cuttings or other material from the hole

can result from sloughing shale,

drill string washout above the bit,

a low circulation rate in a large hole having unweighted mud,

or lost returns.

Inadequate hole cleaning permits a buildup of solids around the bit and collars. (cuttings, washout)

CUTTINGS

WASHOUT

Key seating occurs when drill pipe in tension wears an under gauge groove in the wall of a curved section, or dogleg, of the hole.

When the drill string is raised or lowered, tool joints or drill collars may become lodged in the lower or upper end of the key seat.

Junk in the hole such as metal fragments or broken-off or dropped equipment, may lodge between the hole wall and drill pipe, tool joints, or drill collars.

Except when the drill string pulls around the object or the object can be pushed into the hole wall, serious fishing problems can develop.

This is especially true if the drill pipe gets jammed to one side in a cased hole.

(Never leave the hole unprotected or leave loose objects lying around the rotary area.)

Bit stuck in tapered hole is the result of drilling with worn bit (under gauge) in hard, abrasive formations.

Tripping in a new bit without reaming it to bottom can jam it partway down the tapered section of hole.

Crooked pipe stacked in hole is often the result of dropping the drill string or applying excessive weight to stuck pipe, may jam against the hole wall and become impossible to raise, lower, or rotate.

Multiple string wrap around occurs when tubing strings twist and wrap around each otheras they are run into the hole.

Differential Sticking

This is a condition in which the drill stem becomes stuck against the wall of the wellbore because part of the drill stem (usually the drill collars) has become embedded in the filter cake (wall cake).

Necessary conditions for differential sticking are;

a permeable formation and

a pressure differential across a nearly impermeable filter cake and drill stem.

MUD CAKEDIFFERENTIAL

PRESSURE

MUD CAKE

MUD

FILTRATE

IMPERMEABLE

FORMATION

IMPERMEABLE

FORMATION

MUD

FILTRATE

PERMEABLE

FORMATION

DifferentialSticking

Normally, the drill string is differentially stuck when:

the drill string cannot be rotated, raised or lowered, but circulating pressure is normal,

the drill collars are opposite a permeable formation, and

sticking was instantaneous when the pipe was stationary after drilling fast hole.

STUCK PIPEReferring to the varying degrees of inability to move or remove the drill string from the wellbore. Some level of sticking occurs routinely in drilling operations and these events only become problematic

if the force required to initiate pipe movement exceeds what can be delivered to the stuck point.

Usually, even if the stuck condition starts with the possibility of limited pipe rotation or vertical movement, it will degrade to the inability to move the pipe at all.

STUCK PIPE CAUSES

1. MECHANICAL

Key seats

Wellbore geometry

Inadequate hole cleaning

Under gauge borehole

Unconsolidated formations

Settled cuttings...

2. DIFFERENTIAL STICKING

DIFFERENTIAL PRESSURE STICKING (DPS)

Is common worldwide

Results in a significant amount of non-productive time

May result in abandonment of the current hole and force a sidetrack, and

Ends up as one of the major causes of increased well costs all over the world

DPS - Costs

It is estimated that the cost of stuck pipe in deep oil and gas wells can be approximately 25% of the overall budget.

In some areas, events related to differentially stuck pipe can be responsible for as much as 40% of the total well cost.

The risk of differentially stuck pipeincreases when drilling depleted reservoirs

avoids when drilling underbalanced

Differential sticking - definitionA situation in which the drilling assembly (pipe, drill collars and bottomhole assembly) is stuck in filter cake that was previously deposited on a permeable zone. The pipe is held in the cake by a difference in pressures (P) between

the hydrostatic pressure of the mud (Pm) and the formation (pore)

pressure (Pf) in the permeable zone.

P = Pm

Pf

Differential pressure sticking is usually indicated when the drill string cannot be rotated, raised or lowered, but full circulation at normal pressure can be established.

The force required to pull the pipe free can exceed the strength of the pipe.

DRILL STRING BEFORE AND AFTER STUCK

DIFFERENTIAL

STICKING - CAUSES

Relatively high differential pressure

Mud cake characteristics (thickness, permeability, lubricity..)

In the situations when it is not possible to reduce the differential pressure by reducing the mud weight the option is to act on the mud cake.

DIFFERENTIAL STICKING

TENDENCIES OF MUDS

.... depend on mud filter cake properties: Thickness,

Shear strength, and

Lubricity.

These filter cake properties are influenced by a combination of variables such as: Mud overbalance

Solids content of the mud (both high-gravity and low-gravity solids)

Mud type (e.g., oil-based, polymer water-based, gel water-based),

Specific mud composition, and

Fluid loss

MINIMIZING RISK OF STICKING

Early detection of DPS risks could be made through observation of torque and drag levels while drilling to detect any sign of deviation from a normal trend for the well.

To mitigate DPS events, operators often Minimize: the overbalance (by decreasing mud weight) the stationary time, drilled length through low pressure formations,

Increase drill collar and drill string stabilization, and Optimize mud properties

However, despite the best efforts of operators a DPS event may still occur.

PULLOUT FORCE

Pullout force needed to free a stuck pipe depends on:

F = PA

whereF - pullout force, NP - differential pressure, PaA - contact area, m2

coefficient of friction (adhesion) between the collars and the cake

Value of F is also increased with compressibility and thickness of the filter cake hole deviation, and diameter of the drill collars

Value of F is decreased with increase in diameter of the hole

Contact area and coefficient of friction increasewith time

METHODS USED TO GET THE PIPE FREE

...., in addition to pulling and torquing the pipe, include:

(1) lowering hydrostatic pressure in the wellbore (by reducing the mud weight; this will reduce the differential pressure; should not be used if well control is a problem)

(2) placing a spotting fluid next to the stuck zone and

(3) applying shock force just above the stuck point by mechanical jarring, or

(4) all the above.

METHODS USED TO GET THE PIPE FREE

The most common approach, however, to getting pipe free is to place:

a spot of oil,

oil-base mud, or

special spotting fluid.

A spotting fluid (spot)

a small volume or pill of fluid placed in a wellbore annulus

SPOTTING FLUID

MECHANISM OF ACTION

Breaks the capillary forces that hold the drill string against the wellbore wall, Penetrates, dehydrates, and cracks

(breaks up) the filter cake, Migrates into cracks in the cake and

between the pipe and the cake, Reduces the contact (stuck) area

between pipe and wall Reduces the forces needed to work the

pipe free Increases drill string lubricity

throughout stuck zone Allows pipe to be pulled free

Spotting fluids need to be in place as quickly as possible (within six hours after pipe becoming stuck)

.

SPOTTING FLUIDS

Water-based spotting fluids

Drill-in fluids (Low and ultra-low solids fluids; the sealing mechanism is generated inside the rock, leaving just a thin film on the outside.

Salt solutions with a low activity coefficientcombined with environmentally-safe lubricants (two-phase spot) produce low torque levels

Diesel-based spotting fluids

Synthetic-based spotting fluids

Acid based spotting fluid

SPOTTING FLUIDS

Spotting pills:

Unweighted or

Weighted (spotting fluid + viscosifier+ weighting material)

Spotting fluids (pipe-freeing agents) consist of

detergents,

soaps,

oils,

surfactants and

other chemicals (wall cake cracking material).

PIPE RECOVERY AND FREE POINT

Before placement of spotting fluid, the depth at which the pipe is stuck must be determined

FREE POINT

depth to where the drill string is free and

depth where sticking starts.

This free point can be calculated using measurements taken on the rig floor.

LENGTH TO THE FREE POINT

Knowing the stretch L and the forces applied F1and F

2, Hookes law,

the length of the drill string from the surface to the free point (Lf) is

whereE is the Elastic Modulus (Youngs Modulus) of steel (i.e., 200 GPa),A is the cross-sectional area of the pipe body (m2),L is the stretch distance (elastically stretch of the free portion of the drill string (m),F1is the force to place the entire drill string in tension (N),

F2is a force greater than F

1but less that the force limited by the yield

stress of the pipe grade (N).

12 FF

LAEL f

=

Laboratory researchLaboratory tests were run to evaluate the effectiveness of mud system additives: Carboxymethylcellulose- CMC (filtration control additive)

Lubricant

on differential sticking tendency of the tested fluids

Selected mud: Lignosulphonate mud

LubricantThe addition of certain lubricants to water- and oil- based mud will Reduce the risk of differential sticking and, should sticking still occur,

Reduce the force needed to free the stuck pipe or tool.

Depending on their chemical composition and state of dispersion or solubility in the base mud, lubricants: Can coat metal surfaces, reducing the adhesion of steel to the mud cake,

Can be incorporated into the filter cake and provide better fluid-loss control (resulting in thinner cakes), and

Can be incorporated into the filter cake to reduce the yield stress of the cake.

Lignosulphonate mud formulationC0, C3, C5 - CMC concentration in g/L of the mud

L0, L20, L40 - lubricant concentration in ml/L of the mud

500500500500500500500500500g/LBarite

111111111ml/LDefoamer

404040202020000ml/LLubricant (Lube 167)

0,60,60,60,60,60,60,60,60,6g/LBiocide

555555555g/LViscosifer (DUO-VIS)

530530530g/LCMC

333333333g/LNaOH

202020202020202020g/LFCL (Spersen CFI)

808080808080808080g/LBentonite (MIL-GEL)

100010001000100010001000100010001000mLWater

C5L40C3L40C0L40C5L20C3L20C0L20C5L0C3L0C0L0UnitsComposition

Spotting fluid composition

Diesel: 620 ml

Pipe Lax W: 80 ml

Water: 280 ml

Barite: 73 g

Spotting time: 16 hours

Lab tests

...were carried out according to API Recommended Practice Standard Procedure for Testing Drilling Fluids, API RP13B:

API fluid loss

Cake thickness

Rheological properties

pH value

DIFFERENTIAL STICKING TENDENCY

...of the tested lignosulfonate mud was evaluated using differential sticking testermarketed by OFI Testing Equipment International.

The test device consists offiltration cell capable of holding 200 mL of fluid, perforated bottom capable of holding filter paper and screen, plate (on a plunger) and torque wrench.

Torque necessary to break the plate free is measured

Test conditions:- pressure: 3 291 kPa (477.5 PSI) - temperature: room temperature

BULK STICKING COEFFICIENT (KSC)

The Bulk Sticking Coefficient (Ksc) is calculated by dividing the Sliding Force (F

s) by the

Normal Force (Fn):

Ksc

= Fs / F

n

For r = 1":

Ksc

= 0,001 Tu

Tu

torque (lbf-inch, 0,1129 Nm)

The Sliding Force (Fs)

of the plate is a function of the measured torque (Tu):

Fs

= 1,5 Tu

The Normal Force (Fn)

on the plate is determined by multiplying the area by the differential pressure (This assumes that a pressure of 477,5 PSI was used during the test)

Fn = 1 500 r2

STUCK TENDENCY COEFFICIENT (Kst)

The Stuck Tendency Coefficient (Kst) is

equal to the Bulk Sticking Coefficient (K

sc) multiplied by the variable stuck

area.

Kst= K

sc (Variable Stuck Area)

LAB RESULTS - TORQUE

Effect of Time on Torque

0

2

4

6

8

10

12

14

C0L0 C3L0 C5L0 C0L20 C3L20 C5L20 C0L40 C3L40 C5L40

Mud

To

rqu

e (N

m)

60 min

120 min

180 min

240 min

300 min

16 sati

Torque - increases with time regardless of concentration of CMC and lubricant- decreased with increasing concentration of CMC and lubricant and - decreased after placement of spotting fluid for16-hours

BULK STICKING COEFFICIENT

0,000

0,020

0,040

0,060

0,080

0,100

0,120

Bu

lk S

tick

ing

Co

effi

cien

t


Mud

Bulk Sticking Coefficient

300 minSticking Time

16 hoursSpotting Time

Bulk Sticking Coefficient is decreased with increasing concentration of CMC and lubricant Bulk Sticking Coefficient of C5L40 mud is 35,3 % lesser than of C0L0 mud For C5L40 mud sticking coefficient is 2,3 times less after 16 hours spotting time

STUCK TENDENCY COEFFICIENT

0,000

0,020

0,040

0,060

0,080

0,100

0,120

0,140


Mud

Stuc

k T

ende

ncy

Coe

ffic

ient

60 min

120 min

180 min

240 min

300 min

16 hours

Stuck tendency coefficient is increased with time regardless of concentration of CMC and lubricant, but decreased with increasing concentration of CMC and lubricant

EFFECT OF CMC (MUD WITHOUT LUBRICANT)

0,000

0,020

0,040

0,060

0,080

0,100

0,120B

ulk

Stic

kin

g C

oef

ficie

nt

60 min 120 min 180 min 240 min 300 min

Time

Effect of CMC on Bulk Sticking Coefficient

0 g CMC3 g CMC5 g CMC

Bulk sticking coefficient of mud without lubricant and with 5 g CMC after 300 min is

9,2 % less than without CMC but 2,4 times higher than after 60 min.

EFFECT OF LUBRICANT ON

STICKING COEFFICIENT

0,0380,0300,028

0,059

0,0430,034

0,075

0,0540,050

0,084

0,0640,058

0,093

0,0710,066

0,000

0,010

0,020

0,030

0,040

0,050

0,060

0,070

0,080

0,090

0,100

Bu

lk S

ticki

ng

Co

effic

ien

t

60 min 120 min 180 min 240 min 300 min

Time

Effect of Lubricant on Bulk Sticking Coefficient of Mud with 5 g of CMC

without lubricant

2 % lubricant

4% lubricant

After 300 min test bulk sticking coefficient of mud with 5 g CMC and with 4% lubricant is 29 % less than without lubricant.

FLUID

LOSS -

TORQUE

0

2

4

6

8

10

12

14


Mud

To

rqu

e (N

m)

60 min 120 min 180 min 240 min 300 min 16 hour API Fluid loss-30 min (ml)

0

1

2

3

4

5

6

7


Mud

Flui

d lo

ss (m

l)

30 min

300 min

There is correlation between torque and fluid lossThe lower fluid loss value the lower value of torqueAPI Fluid loss decreases with addition of CMC (and lubricant)

CAKE

THICKNESS -

TORQUE

0

1

2

3

4

5

6

7

8


Mud

To

rqu

e (N

m)

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

Th

icik

nes

s (m

m)

60 min Filter cake thickness

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8


Mud

Cake

thi

ckne

ss (m

m)

30 min

16 sati

Cake thickness and torque decreases with increasing concentration of CMC (and lubricant) The thinner cake

the lower torque

PLASTIC

VISCOSITY -

YIELD POINT

0

0,005

0,01

0,015

0,02

0,025

0,03

0,035


Mud

Plas

tic

visc

osit

y (P

a s)

Plastic viscosity and YP increase with increasing concentration of CMC (and lubricant)

0

5

10

15

20

25

30


Mud

Yiel

d Po

int

(Pa)

GEL

STRENGHTS -

pH VALUE

0,00

2,00

4,00

6,00

8,00

10,00

12,00

14,00

16,00


Mud

Gel

str

engh

t (P

a)

10 sec

10 min

Gel strength increases and pH value decreases with increasing concentration of CMC (and lubricant)

7,5

8

8,5

9

9,5

10

10,5


Mud

pH

Cement Sticking

Although cement sticking can result from a mechanical malfunction such as pump failure or leak in a string of pipe, there are three primary causes:

displacement has been miscalculated,

the hole has washed out as a result of efforts to contain a downhole blowout, and

efforts have been made to prevent excessive lost circulation.

Junk in hole, smaller fish, lost in the hole may include:

a. bit cones, bearings, or other parts lost when a bit breaks;

b. broken reamer or stabilizer parts;

c. metal fragments lost in a twist off;

d. metal fragments produced by milling the top of a fish to aid in its retrieval;

e. naturally occurring pieces of hard, crystalline, or abrasive minerals such as iron pyrite;

f. tong pins, wrenches, or other items that fall into the hole because of rig equipment failure or by accident;

g. equipment such as packer, core barrels, and drill stem test (DST) tools that become lodged downhole; and

h. wireline tools and parted wireline.

Preparing for a fishing job

When it becomes necessary to fish drilling equipment out of an uncased hole, one has to find out as much as possible about the situation before taking action.

The questions to answer are:

1. What is to be fished out of the hole?

2. Is the fish stuck, or is it resting freely?

3. If stuck, what is causing it to stick?

4. What is the condition in the hole?

5. What are the size and condition of the fish?

6. Could fishing tools be run inside or outside the fish?

7. Could other tools be run through the fishing assembly that is to be used?

8. Are there at least two ways to get loose from the fish if it cannot be freed?

Fishing toolsFishing, either in open hole or inside casing, involves operation of the following tools and accessories:1. Spears and overshoots

2. Internal and external cutters

3. Milling tools

4. Taps and die collars

5. Wash over pipea. Wash over pipe overshot

(releasable)

b. Wash over pipe back-off connector

c. Wash over pipe drill collar spear

6. Accessoriesa. Bumper jar

b. Mechanical jar

c. Hydraulic jar

d. Jar accelerator

e. Hydraulic pull tool

f. Reversing tool

7. Safety joints

8. Junk retrievers

9. Impression blocks

In a fishing job involving the drill string, the operator can often ascertain whether or not the lost drill pipe is stuck in the hole by determining what happened just before it was lost.

If the bit was on bottom and drilling, and if there was no sudden, unexplained increase of torque or decrease in rotary speed before the drill string broke, the most likely explanation is the occurrence of a twist off and the pipe is probably not stuck.

If the pipe was motionless in the hole or if it was being raised or lowered but not rotated, it is probably stuck -either mechanically or differentially.

The operator must determine, as accurately as possible, the depth at which the top of a broken drill string can be found.

The upper section of the string is measured as it is removed from the hole.

If the bit was on bottom when the drill string broke, or if the drill string become stuck off bottom, the length of the upper part is the same as the measured depth of the top of the fish.

If the drill string broke with the bit off bottom and the fish then fell downhole, the remainder of the drill string in the hole must be measured as it is set back.

The depth of the top of the fish can then be closely estimated, assuming that the fish fell all the way to bottom.

If there is any doubt about the fishs location, the operator can run an electric log.

FreeingFlowchart

Locating the fish1. Pipe stretch method One of the oldest and

quickest free point methods is the drill pipe stretch test (the method should be used only as a guide, for preliminary decisions or when the drill string is plugged);

- to run the test, pick up the weight of the drill string and markthe drill pipe opposite the rotary top,

- pull up additional 100000 to 200000 N (depending on hole depth) and measure the distance from original mark and the rotary top.

F

AeEL s

=

L length of free pipe, m

E modulus of elasticity, 211010 Pa

e differential elongation, m

As pipe cross section area, m2

F differential pull, N

Locating the fish

2. Electrical free-point Surveys have eliminated most of the guess work about where to back off and start fishing;- The free-point detector is lowered

through the drill pipe on an electric line to locate the lowest point where the pipe is free.

- That is essentially a strain gauge device to measure molecular rearrangement located between drag strings or electromagnets. Stops are made at increasing depths, torsion or stretch is applied to the drill string, and the degree of pipe movement is measured at the surface. (Torsional movement is more important in deviated wells.)

ELECTRIC

CABLE

TOOL JOINT

LOCATOR

FREE-POINT

INDICATOR

COIL

STRING

SHOT

Locating the fish

- After the free-point detector passes the stuck point, no movement will register when strain is applied.

- Free-point devices can be run in conjunction with a string shot to permit a back-off as the free-point is determined

Locating the fish3. Drill pipe recovery log The drill pipe

recovery log is an acoustical survey used to determine stuck points in the drill pipe, collars, casing and tubing.

- The survey is normally run when the fish is extremely long.

- Results are used to evaluate the severity of the fishing job prior to undertaking or sidetracking.

Back-off procedures

String shot The string shot is a length of prim cord explosive which is detonated by an electrical cap.

The shot is run into the well to the desired depth opposite a tubing coupling or tool joint as indicated by the collar locator at the top of the assembly.

Back-off is completed by applying left-hand torque to the string and holding while the shot is exploded.

If the correct torque is applied, the jar of the explosion will cause the threaded connection to unscrew.

Making a cut

Electric line cutters are used where conditions prevent backing off at the required depth.

Two basic types of cutters are available: jet cutters and chemical cutters.

Jet cutters This cutters use a high velocity beam of gases created by the shape charge to cut the pipe.

Making a cut

Hydraulic pipe cutters -can be:

erosional (A) or

mechanical cutters (B)Erosional perforators enable limited penetration in to the formation with great differences in opening shape and penetration depth.Their use is limited because they are long-lasting and expensive. Mechanical cutters cut section of the pipe completely.They are also limited according penetration and last long.

(A) (B)

Making a cut

Chemical cutters The chemical cutter uses a strong acid to make the cut.

Oxidation destroys the pipe.

When this cutter is used, acid concentrations and quantity are designed so that the acid is completely consumed in the cutting process.

Both cutters perform the same function, but chemical cutter gives a smoother cut.

Making a cutMechanical cutters can be internal and external.

Internal hydraulic (mechanical) pipe cutter is used to cut single or multiple strings of casing in one or more runs.

The tool consists of a:top sub,

a body, a piston, a piston spring, a flow indicating device and

cutter arms.

Upon pumping fluid through the tool, a pressure drop is created across the piston forcing it downwards.

The lower end of the piston stem pushes on lugs at the upper end of the arms, making them pivot and forcing their lower tips into contact with the casing.

The hard metal with which the arms have been dressed cuts the casing.

Making a cutThe section mill is a hydraulically actuated tool used to mill a section or window in casing or tubing.

It can also be used as a pipe cutter. The tool consists of a top sub, a body, a piston with cam, a piston spring, a flow indicating device, cutter knives and arm stop stabilizers.

Upon circulating through the tool a pressure drop is created across the piston which forces the cam down the ramp of the knives, welding them into contact with the casing.

Half the number of knives part the casing and all the knives participate in the subsequent milling effort.

Lower standpipe pressure indicates that the pipe has been cut.

When circulation is stopped the piston spring will lift the piston, making the cam withdraw from between the knives, and they will collapse into the body.

Internal mechanical pipe cutter is used to cut strings of tubing, casing or drill pipe; particularly useful in cutting small diameter strings.

By rotating the mandrel and cutter assembly clockwise relative to other assemblies the slips move axially due to a screw drive between the lower end of the mandrel and the friction assembly.

This forces the slip segments to expand, traveling up on a cone.

Once the slips are set, weight can be applied, causing wedges to force cutters into contact with the pipe.

The screw drive thread of the friction block assembly is cut on spring loaded segments, and to disengage the knives and slips and to reset the tool for another cut it is only necessary to elevate the string.

External mechanical pipe cutter is employed in cutting tubular strings from the outside.

It is used to externally cut and retrieve drill pipe or tubing, regardless of the tool joint.

The tool consists of a top sub, a body, a piston assembly, knives and a guide.

The piston assembly consists of a rubber ring, conical piston segments and a feed ring.

The piston segments are kept together by the rubber ring and each has a hole drilled through it.

These holes are sized to permit circulation through the tool as well as providing adequate pressure drop to make it function.

When running in the hole the piston assembly is kept in the up position by two shear pins..

Upon kicking in the pumps and building pressure, the pins shear, permitting the feed ring to wedge in behind the knives, thereby forcing them into contact with the pipe to be cut.

A secondary way of actuating the knives is to raise the string until the piston segments engage below a connection and then pull 4500 N tension on the string in order to shear the pins.

Hole conditions, in particular whether circulation can be maintained, are important considerations in recovering a fish.

If drill pipe is stuck by a cave-in or by swelling shale circulation my be restricted or cut off altogether.

However, circulation is usually not affected if the drill string is stuck in a key seat or if pipe is wall stuck.

The condition of the fish also is essential information.

Most pipe recovery tools are designed with close tolerances; that if specific component sizes are needed for specific jobs.

Irregularities likely to hamper recovery of a fish must be dealt with.

For example, if the top of a section of broken-off drill pipe is burred or split, it may be necessary to clean up, or dress, the pipe before trying to retrieve it.

And junk may have to be broken apart so that it can be picked up.

Inspecting the break on the part of the pipe that is pulled from the hole may provide a good reverse image of the top of a twist off.

One method that is sometimes used to assess the condition of the top of a fish is to run an impression block.

A typical impression block consists of a block of lead, having a circulation port, molded onto a steel body.

The block is made up on drill pipe and collars and run into the hole until it is just above the fish.

Circulation is started to wash all settlings off the top of the fish so that a good impression can be obtained.

The block is lowered gently to touch the fish, and weight is then applied.

The top of the fish indents the bottom of the soft lead block, leaving an impression that can be examined and measured at the surface.

Impression block

If a part of drill string has broken off in open hole and it is not stuck, the fishing job consists mainly of locating and engaging the top of the fish with an appropriate tool.

If the top of the broken-off pipe is badly split or twisted, it requires that the damaged metal is removed to give a fish a more acceptable shape, because most fishing techniques require a section of straight undamaged pipe to make a firm catch.

Mills

During milling, the drill string must be rotated quickly and carefully.

It may be necessary to use a pilot mill, which will not jump off the top of a fish and go down beside it. High-volume circulation should be maintained to flush the cuttings and cool the mill.

A ditch magnet can be placed in the return line between the shale shaker screen and the mud pit to capture most cuttings from the milling operation.

Removal of cuttings reduces wear on mud pumps and other equipment.

A typical fishing string consists of, from bottom to top:

an overshot,

a bumper sub,

a hydraulic jar,

a series of drill collars,

and a jar accelerator made up on drill pipe.

Fishing assembly

It is recommended to install the safety joint as the part of the fishing string.

Because when engaged, and the jarring does not free the fish, the safety joint can be broken out by rotating to the left and lowering the drill string.

External catch tools

Die collars are used to externally engage and retrieve a tubular fish.

The fish must offer some rotational constraint as the gripping force depends upon the amount of torque with which the die collar can be made up to the fish.

Its bor permits the passage of wire line tools

The tool is a solid one-piece construction conical cylinder furnished with a fine case hardened thread on its inner circumference.

Upon being screwed over the fish the hardened threads embed in the softer material of the fish securing a good grip for retrieval.

OvershotA typical circulating and releasing overshotconsists of three outside parts:

a top sub,

a bowl, and

a guide.

The top sub connects the overshot to the fishing string.

The bowl may be fitted with different types of equipment to grasp the fish and different guides to help center the fish beneath the tool.

TOP SUB

BOWL

BASKET

GRAPPLE

FISH

GUIDE

Two kinds of grapples are possible:

spiral grapple;if the diameter of the fish is close to the maximum catch size for the overshot, and

backet grapple;if the fish diameter is below the maximum catch size.

Both types of packers seal around the fish, allowing drilling fluid to be pumped down to clean out the bottom of the hole.

The fishing string is run to within a few feet of the top of the fish.

Circulation starts to clean cuttings and settlings off the top of the fish and to clean out mud cake from inside the overshot.

The fishing string is slowly lowered to touch the top of the fish and establishes its exact depth.

When the fish has been tagged, hook load decreases;

the position is marked on the kelly.the string is raised, and, with slow rotation to the right, lowered slowly without circulation,if the overshot is centered over the fish, the lowering and right-hand rotation of the string forces the grapple upward within the tapered helix of the bowl, allowing the grapple to expand and the fish to enter the overshot.

After the string has been lowered, the weight indicator should register a decrease.

When the fish is engaged, rotation is stopped and all torque of the string relieved.

Than upward strain is taken.This causes the fish to pull the grapple downward and the wickers on the grapple to bite into the fish.

If the fish is gripped tightly, the weight indicator will show an increase.

Circulation is started, without rotation, to clean out the hole before the fish is brought to the surface.

If it is not possible to pull out the string it must be stacked.

To break out a string from the fish, the overshot is bumped down and rotated to the right and gradually raised until it is clear of the fish.

When coming out of the hole with the fish engaged in the overshot, the procedure is the same, but the rotary slips in rotary table are used and the overshot is bumped on them.

If the upper end of the fish cannot be engaged, an extension sub is installed between the top sub and the bowl of the overshot to allow the damaged top of the fish to go past the grapple.

Different guides can be used.

If the top of the fish is in a washed-out section of the hole, a wall-hook guide may be used in place of the regular guide.

When a wall-hook guide is used, the distance from the bottom of the guide to the top of the wall-hook opening and from there to the stop in the overshot are measured.

The string is run in to a point just above the fish, then lowered with slow rotation until the guide tags the fish.

Downward movement is then stopped, but rotation is continued.

Torquing up of the fishing string is a sign that the fish is caught in the wall-hook opening.

The rotary table is locked and the fishing string is raised.

A release of torque signals that the top of the fish has slipped beneath the top of the wall-hook opening and is centered beneath the overshot.

To engage the fish previously defined measures are taken.

FISH

OVERSHOT

GUIDE

In a very large washout, a knuckle joint may be made up above the overshot to extend the wall hook and overshot out into the cavity.

WALL

HOOK

OVERSHOT

KNUCKLE

JOINT

Inside fishing tools

When a drill collar separates, the break usually occurs at a connection ; thee pin breaks off in the box, or the box breaks off and comes out with the top part of the string. If the diameter of the drill collar is very close to that of the wellbore (as in packed hole assembly), an overshot may not have enough clearance to go over the collars. Either a fishing neck must be milled on the top drill collar or an inside fishing tool must be used. The simplest inside fishing tool is the taper tap (without or with guide).

The tap is lowered into the collar bore and slowly rotated to make its own threads as it engages the fish.

The taper tap is no releasing, and it is used only when a releasing tool cannot be run.Some taps have open tips, allowing limited circulation for cleaning off the top of the fish; others have small side jets that move the point of the taper tap to help locate the top of the fish.

Once the tap is made up in the fish, the pump pressure and torque increase, and the fishing string and fish are tripped out.

The taper tap should always be run with a safety joint and jar, because once a tap is engaged it cannot be backed out of a stuck collar.If jarring does not free the fish, the safety joint can be broken out by rotating to the left and lowering the drill string.

The most common inside fishing tool is the releasing spear .

The spear is made up on the fish string and lowered, with circulation, to the top of the fish.

Circulation is then stopped, and the spear is lowered slowly inside the fish until the weight indicator shows a decrease, indicating that the bumper ring or stop sub has seated on top of the fish.

Rotate sufficiently to move the mandrel one full turn to the left.

This turns the mandrel down through the grapple, placing the grapple into engaging position.

A straight pull will then wedge the grapple into positive engagement with the fish

The spear is made up on the fish string and lowered, with circulation, to the top of the fish.

Circulation is then stopped, and the spear is lowered slowly inside the fish until the weight indicator shows a decrease, indicating that the bumper ring or stop sub has seated on top of the fish.

To release:

Bump down to break the freeze, then rotate two or three turns to the right.

This moves the mandrel up through the grapple, forcing the grapple against the release ring and putting the spear in released position.

A straight upward pull will then generally free the spear, however, it is recommended that the spear be rotated slowly to the right when coming out.

Stop sub is used to locate the top of the fish when stop is required. It is installed in the box connection at the top of the mandrel.

Oversize stop ring is used with stop sub when use of larger stop is required. It is installed on the stop sub with set of screws.

Mill type nut is used to mill away burrs from the top of fish.

Side hill type nut is used to center the spear in greatly oversize holes to assure entry of the spear into the fish.

Pack off assemblies are used to pack-off all sizes of tubing, drill pipe and casing.

The assembly is attached to the sub type nut below the spear where it packs-off the fish in order to circulate through the fish.

Fishing of stuck pipe

After a fish has been caught in the overshot, the usual procedure is to circulate out the settled cuttings without rotation.

If circulation cannot be fully established and the fish cannot be pulled, the fish is almost certainly stuck mechanically in the hole; in such cases the jar must be used.

Jars can be mechanical or hydraulic.

The simplest is a bumper jar. That is a device that permits vertical movement of the upper section without movement of the lower section of the tool within a limiting travel; usually about 0,5 m.

Raising the upper section with the working string to the limit of travel will produce a slight upward jar on the lower section.

Dropping the string quickly will produce a sharp downward jar or bump on the lower part.

The jarring blow will be more pronounced if a few drill collars are placed in the string just above the bumper jar.

Mechanical jarThe force of the mechanical jar depends on the amount of torque turned against the trip mechanism.

The greater the torque, the harder the jarring blow when the tool trips.

Hydraulic jar depends for its intensity upon the pull taken on the tool before it trips.

It is placed directly below the drill collars in the fishing string.

The intensity of each blow is controlled by the amount of stretch placed in the drill string.

The more pull, the harder the blow.

The jarring effect is enhanced by placement of drill collars above the jar.

The hydraulic jar enables only upward jarring, so if there is a need for downward jarring combine it with mechanical jar.

To activate the oil jar, pull to a predetermined distance above the weight of the string at the jar.

Hold this position while the oil jar bleeds off and the blow is delivered.

Jar, surface bumper is designed to be installed in the drill string at the surface. It permits the operator to deliver sharp descending impact or downward jarring blows against fish at its stuck point.

At last 1300 to 1800 N weight must be installed on top of the jar in order to be able to reset the tool by closing it.Upon extending the tool from its closed position the friction mandrel pulls the friction slip into the upper (narrow) part of the bowl until the slip stops against the control ring.The control ring is threaded and translates axially upon being rotated.Its position determines how far into the tapered bowl the slip is allowed to travel and consequently the tripping load. When the preset load is exceeded the jar opens, permitting the drill string to drop the length of 1,22 m stroke.

The Intensifier is essentially a fluid spring which stores energy when a strain is pulled on the running string.

When the strain is removed by the free stroke of the jar, the stored energy is released, accelerating the drill collars and jar end upward until a blow of high impact is struck.

Its function is to supply acceleration to the upper end of the jar and lower portion of the work string during the jarring stroke.

Circulating fluid through the wash over assembly flushes out sand or shale in the space outside the fish.

The wash over drill collar spear is used in conjunction with a wash over string to engage and retrieve a fish which has become stuck off bottom.

The spear is connected to the fish before the wash over operation starts and will engage in the wash over pipe if the fish drops free, thereby preventing it from falling to the bottom of the well.

Fishing for junkOne of the simplest rotary fishing tools is the junk basket.

It is run into the hole on the bottom of the drill string to within a meter of the bottom, then lowered over the junk while being slowly rotated.

If the basket is nearly hole size, its finger link catchers will gather junk toward the center of the hole, and when weight is applied, bend inward to trap the junk inside.

Most effective for a small, solid mass lying loose on bottom, such as a bit cone.

The core-type junk basket is used to retrieve junk such as cones that may or may not be embedded in the formation.

A mill shoe is made up on the bottom of the tool.After it is run nearly to bottom, mud is circulated at reduced pressure, and the tool is slowly rotated and lowered to touch the junk.Weight is gradually increased.The mill shoe cuts away the protruding edges of the junk, as well as the formation, forcing the junk and a short core into the barrel.Rotation and circulation are stopped, torque is released from the drill string, and an upward strain is taken to break off the core.Upper and lower catchers in the basket hold the core and junk on the trip out.A magnet insert can be used in the tool to pick up small pieces of ferrous metal

A boot basket, also called a junk sub or junk boot, may be run just above the bit during routine drilling to collect small pieces of junk that may damage the bit or interfere with its operation.

Usually a but basket is run above a mill while it is milling away a metallic object such as the top of a fish.

During drilling or milling with circulation, the mud flowing upward in the narrow space between the boot basket cup and the hole wall flows rapidly enough to carry pieces of junk with it.

When it reaches the annulus above the cup, however, it slows down, and the larger bits of junk drop out into the cup, to be retrieved when the bit or mill is pulled.

CUP

Ferrous metallic junk can often be retrieved using a fishing magnet, a powerful permanent magnet having passageways for circulation.

A fishing magnet is lowered into the hole wit circulation to wash cuttings off the top of the junk.The magnet is encased on the top and sides by a nonmagnetic brass sleeve to prevent junk from clinging to the side of the magnet.A skirt on the bottom of the magnet keeps the junk from being knocked off during the trip out.If there is no fill on top of the fish, magnets can also be run on wireline, a much faster operation than tripping, the drill string in and out.

HOOK GUIDE

MILL GUIDE

WIRELINE CONNECTOR

SidetrackingSome fishing jobs can go on for months before the fish is retrieved.Than the sidetracking could be the option.Knowing the rate of penetration and the length of the original hole to bypass, one can estimate the cost of drilling new hole to reach the original tool depth.

Packer milling and retrieving tool

The tool is used to retrieve all types of full bore production packers.

The tool consists of a:

canfield type drive bushing

a long wash over show with

a wash over pipe extension, and

a packer retriever spear with threaded extension rods between it and the canfield bushing.

Packer milling and retrieving tool

The tool is assembled and connected to the running string, lower into the hole, and spear carefully run through packer.

By having run the spear through the packer the slip has been automatically set in the catch position by friction contact between it and packer bore:

Verify by picking weight;Lower, establish circulation and start milling the packer;

When the slips and the sealing element have been milled away, stop rotation and elevate the fishing string to engage and retrieve the packer.

Wireline fishing

One of the most challenging of all fishing jobs is the recovery of wireline and the tool or instrument run with it.

The first consideration is whether the line is parted or is still intact.

If a conductor line has not parted, it is good practice to avoid pulling it out of the rope socket.

If this occurs, contact with the tool or instrument will be lost, possibly permanently

Wireline fishing

To fish intact wireline, either the:cable-guide method or

the side-door overshot method can be used.

The cable-guide method should be chosen for all deep, open-hole situations or when a radioactive instrument is stuck in the hole.

This is the safest method and offers a high probability of success.

Cable-guide fishing

The tools consist of: a cable clamp with a T-bar,

rope sockets for each end of the line,

one or more sinker bars,

a special quick-connector-type overshot for the line on the reeled,

and a spear point for the well end.

Also included is a slotted plate to set on top of the pipe, a sub with a recess or retainer to hold the rope socket and an overshot to run on the pipe to catch the instrument or tool stuck in the wellbore.

Side-door overshot method

The side-door overshot is a special overshot with a gate or door in the side that can be removed to allow the line to be feed into the tool, after which the door is put back into position as part of the bowl.

The overshot is run on drill pipe or tubing until the fish neck or body of the stuck tool is engaged.

The advantage of this recovery method is that the line does not need to be cut.

Fishing parted wirelineThe Rope Spear is a reliable and efficient wireline and wire rope retrieval tool. The rope spear retrieves all sizes of electric wireline, slick line, braided line, or other types of wire rope that have been left downhole. It can also be used to retrieve control line or ESP cable that has been left downhole. This tool has been very successful in recovering these items in cased or open hole.The Rope Spear is one of the easiest tools in the industry to operate. The tool is dressed with the proper lower shank, run downhole on two or three drill collars to the required depth, and rotated to the right until ample contact with the wireline is achieved.As the work string is pulled up, the wireline that is wrapped around the spear slips down and latches onto the barbs. Assuming satisfactory contact is made with the wireline, the retrieval operation can be successfully completed in one run.

Fishing parted wirelineThe Latch Jack is a wireline fishing tool designed for fishing wire through restrictions or wire that has become balled up.

The tool is manufactured from high tensile steel so that the prongs may be forced down into or around a reasonably compacted ball of wire.

The design incorporates barbs on the ID only so that wire on the outside of the tool may be easily pulled down rather then having to yield the wire creating smaller unfishable pieces.

Similar spears are manufactured without the latch in two and three prong designs.

Cutting the lineThe Mechanical Wireline Cutter is used to cut a wireline just above the rope socket of a stuck tool.

This allows the cut line to be retrieved in one piece while providing a desirable fishing top on the stuck tool.The cutter is installed over the line and run in on wireline

or pipe, if clearances permit.The cutter is installed over the wireline by removing the keeper block. The keeper block is replaced and the cutter attached to a run-in line by means of a rope socket or crossover sub. Cutting knives are then activated by applying an upward strain on the run-in line or pipe that continues until the cut is indicated. The cut wireline and run-in line (or pipe) are then retrieved and fishing operations begun on the stuck tool.

It may be desirable, when line condition is questionable, to make a dummy run with cutter knives removed to be certain that no obstacles are encountered from surface down to the stuck tool.