Breakdown and drilling
problems
Matanović Davorin, professor
Faculty of mining, geology and petroleum
engineering, Zagreb
Introduction
Wellbore 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.
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.
Introduction
Under 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.
Introduction
LOST
CIRCULATION
Wellbore
Pressure
(a)
FRIABLE
SANDSTONE
OVERGAUGE
HOLE
BRITTLE
SHALE
HOLE
CLOSURE
SALT
(b)
Introduction
Fishing – 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
Introduction
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
The 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 =
V f + Cs
R f + Cd
Introduction
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.
Twist 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.
Twistoff
Twist off
The 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 pipe
There 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)
WASHOUT
CUTTINGS
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 other
as 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.
IMPERMEABLE
FORMATION
MUD
FILTRATE
PERMEABLE
FORMATION
IMPERMEABLE
FORMATION
MUD CAKE
DIFFERENTIAL
PRESSURE
MUD
FILTRATE
MUD CAKE
Differential
Sticking
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 PIPE
Referring 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 nonproductive 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 pipe
increases when drilling depleted reservoirs
avoids when drilling underbalanced
Differential sticking - definition
A situation in which the drilling assembly (pipe, drill collars and bottom
hole 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 = ∆P·A·µ
µ
where
F - pullout force, N
∆P - differential pressure, Pa
A - contact area, m2
µ
– 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 increase
with time
METHODS USED TO GET THE PIPE FREE
...., in addition to pulling and torquing the
pipe, include:
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.
(1)
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 coefficient
combined with environmentally-safe lubricants (twophase 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 F1 and F2, Hooke’s law,
the length of the drill string from the surface to the free point (Lf) is
E ⋅ A ⋅ ∆L
Lf =
F2 − F1
where
E is the Elastic Modulus (Young’s 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),
F1 is the force to place the entire drill string in tension (N),
F2 is a force greater than F1 but less that the force limited by the yield
stress of the pipe grade (N).
Laboratory research
Laboratory 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
Lubricant
The 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 fluidloss control (resulting in thinner cakes), and
Can be incorporated into the filter cake to reduce the yield stress
of the cake.
Lignosulphonate mud formulation
C0, C3, C5 - CMC concentration in g/L of the mud
L0, L20, L40 - lubricant concentration in ml/L of the mud
Composition
Units
C0L0
C3L0
C5L0
C0L20
C3L20
C5L20
C0L40
C3L40
C5L40
Water
mL
1000
1000
1000
1000
1000
1000
1000
1000
1000
Bentonite (MIL-GEL)
g/L
80
80
80
80
80
80
80
80
80
FCL (Spersen CFI)
g/L
20
20
20
20
20
20
20
20
20
NaOH
g/L
3
3
3
3
3
3
3
3
3
CMC
g/L
0
3
5
0
3
5
0
3
5
Viscosifer (DUO-VIS)
g/L
5
5
5
5
5
5
5
5
5
Biocide
g/L
0,6
0,6
0,6
0,6
0,6
0,6
0,6
0,6
0,6
Lubricant (Lube 167)
ml/L
0
0
0
20
20
20
40
40
40
Defoamer
ml/L
1
1
1
1
1
1
1
1
1
Barite
g/L
500
500
500
500
500
500
500
500
500
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 tester
marketed by OFI Testing Equipment
International.
The test device consists of
filtration 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
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
Coefficient (Ksc) is
calculated by dividing the
Sliding Force (Fs) by the
Normal Force (Fn):
Ksc = Fs / Fn
For r = 1":
Ksc = 0,001 × Tu
Tu – torque (lbf-inch, 0,1129 N—m)
(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
(Ksc) multiplied by the variable stuck
area.
Kst = Ksc × (Variable Stuck Area)
LAB RESULTS - TORQUE
Effect of Time on Torque
14
12
60 min
Torque (Nm)
10
120 min
180 min
8
240 min
6
300 min
16 sati
4
2
0
C0L0
C3L0
C5L0
C0L20
C3L20
C5L20
C0L40
C3L40
C5L40
Mud
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
Bulk Sticking Coefficient
Bulk Sticking Coefficient
0,120
300 min
Sticking Time
0,100
16 hours
Spotting Time
0,080
0,060
0,040
0,020
0,000
C0L0
C3L0
C5L0
C0L20
C3L20
C5L20
C0L40
C3L40
C5L40
Mud
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
0,140
Stuck Tendency Coefficient
0,120
0,100
0,080
60 min
120 min
180 min
0,060
240 min
300 min
16 hours
0,040
0,020
0,000
C0L0
C3L0
C5L0
C0L20
C3L20
C5L20
C0L40
C3L40
C5L40
Mud
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)
Effect of CMC on Bulk Sticking Coefficient
Bulk Sticking Coefficient
0,120
0,100
0,080
0,060
0 g CMC
3 g CMC
5 g CMC
0,040
0,020
0,000
60 min
120 min
180 min
240 min
300 min
Time
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
Effect of Lubricant on Bulk Sticking Coefficient
of Mud with 5 g of CMC
Bulk Sticking Coefficient
0,100
0,093
0,090
0,080
0,084
0,075
0,070
0,060
0,050
0,040
0,030
0,020
0,059
0,038
0,030
0,028
0,054
0,050
0,064
0,058
0,071
0,066
without lubricant
2 % lubricant
4% lubricant
0,043
0,034
0,010
0,000
60 min
120 min
180 min
240 min
300 min
Time
After 300 min test bulk sticking coefficient of mud with 5 g CMC and with 4%
lubricant is 29 % less than without lubricant.
7
6
Fluid loss (ml)
5
4
30 min
3
300 min
2
1
FLUID
LOSS TORQUE
0
C0L0
C3L0
C5L0
C0L20
C3L20
C5L20
C0L40
C3L40
C5L40
Mud
There is correlation
between torque and fluid
loss
The lower fluid loss value
– the lower value of
torque
API Fluid loss decreases
with addition of CMC
(and lubricant)
14
12
Torque (N m)
10
8
6
4
2
0
C0L0
C3L0
C5L0
C0L20
C3L20
C5L20
C0L40
C3L40
C5L40
Mud
60 min
120 min
180 min
240 min
300 min
16 hour
API Fluid loss-30 min (ml)
CAKE
THICKNESS -
0,8
0,6
0,5
0,4
30 min
0,3
16 sati
TORQUE
0,2
0,1
Cake thickness
0
C0L0
C3L0
C5L0
C0L20 C3L20 C5L20 C0L40 C3L40 C3L40
8
0,8
7
0,7
6
0,6
5
0,5
4
0,4
3
0,3
2
0,2
1
0,1
0
0
C0L0
C3L0
C5L0
C0L20
C3L20
C5L20
C0L40
C3L40
Mud
60 min
Filter cake thickness
C5L40
Thicikness (mm)
Mud
Torque (Nm)
Cake thickness (mm)
0,7
and torque
decreases with
increasing
concentration of
CMC (and
lubricant)
The thinner cake
– the lower
torque
PLASTIC
VISCOSITY YIELD POINT
0,035
Plastic viscosity (Pa s)
0,03
0,025
0,02
0,015
0,01
0,005
0
C0L0
C3L0
C5L0
C0L20 C3L20 C5L20 C0L40 C3L40 C5L40
Mud
30
Yield Point (Pa)
25
20
15
10
5
0
C0L0
C3L0
C5L0
C0L20 C3L20 C5L20 C0L40 C3L40 C5L40
Mud
Plastic viscosity
and YP
increase with
increasing
concentration
of CMC (and
lubricant)
16,00
14,00
Gel strenght (Pa)
12,00
10,00
8,00
10 sec
6,00
10 min
GEL
STRENGHTS pH VALUE
4,00
2,00
0,00
C0L0 C3L0 C5L0 C0L20 C3L20 C5L20 C0L40 C3L40 C5L40
Gel strength
increases and pH
value decreases
with increasing
concentration of
CMC (and
lubricant)
Mud
10,5
10
pH
9,5
9
8,5
8
7,5
C0L0
C3L0
C5L0
C0L20 C3L20 C5L20 C0L40 C3L40 C3L40
Mud
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.
a.
b.
c.
d.
e.
f.
g.
h.
Junk in hole, smaller fish, lost in the hole may include:
bit cones, bearings, or other parts lost when a bit breaks;
broken reamer or stabilizer parts;
metal fragments lost in a twist off;
metal fragments produced by milling the top of a fish to aid
in its retrieval;
naturally occurring pieces of hard, crystalline, or abrasive
minerals such as iron pyrite;
tong pins, wrenches, or other items that fall into the hole
because of rig equipment failure or by accident;
equipment such as packer, core barrels, and drill stem test
(DST) tools that become lodged downhole; and
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.
2.
3.
4.
5.
6.
7.
8.
What is to be fished out of the hole?
Is the fish stuck, or is it resting freely?
If stuck, what is causing it to stick?
What is the condition in the hole?
What are the size and condition of the
fish?
Could fishing tools be run inside or
outside the fish?
Could other tools be run through the
fishing assembly that is to be used?
Are there at least two ways to get
loose from the fish if it cannot be
freed?
Fishing tools
Fishing, either in open
hole or inside casing,
involves operation of the
following tools and
accessories:
1.
2.
3.
4.
5.
Spears and overshoots
Internal and external
cutters
Milling tools
Taps and die collars
Wash over pipe
a.
b.
c.
Wash over pipe overshot
(releasable)
Wash over pipe back-off
connector
Wash over pipe drill collar
spear
6.
Accessories
a.
b.
c.
d.
e.
f.
7.
8.
9.
Bumper jar
Mechanical jar
Hydraulic jar
Jar accelerator
Hydraulic pull tool
Reversing tool
Safety joints
Junk retrievers
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 fish’s location, the operator can
run an electric log.
Freeing
Flowchart
Locating the fish
1. 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 mark
the 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.
L – length of free pipe, m
E ⋅ e ⋅ As
L=
∆F
E – modulus of elasticity, 21—1010 Pa
e – differential elongation, m
As – pipe cross section area, m2
∆F – differential pull, N
Locating the fish
ELECTRIC
CABLE
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.)
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 freepoint is determined
Locating the fish
3. 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 cut
Mechanical 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 cut
The 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.
Impression block
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.
Mills
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.
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.
Fishing assembly
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.
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.
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.
External catch
tools
A typical circulating and
releasing
overshot
consists 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.
Overshot
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 wallhook 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.
KNUCKLE
JOINT
OVERSHOT
WALL
HOOK
Inside
fishing
When a drill collar separates, the
break usually occurs at a
tools
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
movement
movement
tool within
0,5 m.
device that permits vertical
of the upper section without
of the lower section of the
a limiting travel; usually about
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 jar
The 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.
One 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.
Fishing for junk
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.
WIRELINE
CONNECTOR
HOOK GUIDE
MILL GUIDE
Some 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.
Sidetracking
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 wireline
The 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 wireline
The 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 line
The 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.