Wireline operations

Quick refresher

Slickline is a single steel strand 1.8–3.2 mm thick. It is cheap and fast, and it passes jar blows (sharp knocks) well, but it gives no live link to the instrument. Braided line is twisted from several wires and is stronger. We use it for heavy jobs and fishing. Electric line has conductors inside steel armour. It gives power and a real-time data link, and we use it for perforating and for setting packers and plugs.
On a well under pressure, the tool is run in through a lubricator, a pipe above the Christmas tree that holds the whole tool string. A stuffing box seals slickline. A grease injection head seals braided line and electric line. The wireline BOP (blowout preventer) closes on the line if the seal fails.
The tool string, from the top, is: rope socket, sinker bars, jar, knuckle joint, working tool. The bars give weight and the force of the blow. A mechanical jar strikes up and down when the line is moved sharply. A hydraulic jar strikes hard upward after a delay.
Wellhead pressure pushes the line out of the well with a force $p_{wh}A$ , like a plunger. This is tens or hundreds of newtons for slickline and thousands for thick cable. The tool string must weigh more than this, with a margin for friction in the seal.
The weight indicator shows the weight of the tool string and of the line in the well, less the ejection force (the force that pushes the line out). We limit the tension at the wellhead to a fraction of the breaking load. As the depth grows, the weight of the line uses up the margin at the tool. Overpulled wire usually breaks at the wellhead.
Before we pull a plug, we equalise the pressures above and below it. We check the depth on the counter against the nipples, the collars and the tubing tally (the list of tubing joints with their lengths).

Key relationships

F_{\text{ej}} = p_{wh}\,A_{\text{line}},\quad A_{\text{line}} = \pi d^2/4

the force that pushes the line out of the well; MPa × mm² = N

T = (W_{\text{tool}} + wL)\left(1 - \rho_{\text{L}}/\rho_{\text{steel}}\right) - p_{wh}A_{\text{line}}

weight indicator reading with the string hanging freely

F_{\text{pull}} = k_{\text{all}}\,F_{\text{br}} - T

the static force that can be applied to a stuck tool

Common pitfalls

Running cable into a high-pressure well without calculating the weights. The well pressure will push the tool string out, and the lubricator may not be able to hold the bars that are needed.
Pulling the wire “to the limit” to free the tool. It will part (break) at the wellhead, and kilometres of wire will fall into the tubing.
Pulling a plug without equalising the pressure above and below it. The pressure difference will throw the tool string upward.
Using carbon-steel wire in a well with hydrogen sulfide.
Treating the depth counter as exact: heating of the line and changes in its tension in the well produce errors of metres.

After this lesson you can

compare slickline (a single smooth steel wire), braided line (twisted wire rope) and electric line (a cable that carries power and signals), and choose a line for the job
describe the wellhead equipment for work under pressure (the lubricator, the stuffing box, the grease injection head and the wireline blowout preventer) and the order in which a tool is run into the well and pulled out
name the parts of the tool string (the assembly of tools that hangs on the line): the rope socket, sinker bars, jars and knuckle joint, and explain how we use jar blows to set and retrieve devices
describe the typical operations: gauge runs (checking that the tubing is open), setting and pulling plugs and locks, shifting sliding sleeves, changing gas lift valves, surveys and sampling, and fishing
calculate the upward force that the well pressure puts on the line, the tool string weight needed to resist it, the weight of the line in the well, the weight indicator reading, and the margin of overpull (spare pull) at the tool

What is done on wireline, and why it pays

The equipment of the tubing string — landing nipples, sliding sleeves, side-pocket mandrels and subsurface safety valves that can be retrieved on wireline — is designed to be serviced from above, without pulling the tubing (Completion string equipment). We lower a tool on the line into the tubing and operate it there by blows or by tension. We seat a device in the profile of a nipple, shift a sleeve, or latch a device and retrieve it. In Russian-language practice this work is called “rope work” or “work with rope equipment”. Surveys with downhole instruments on slickline are part of well testing.

Learn about the well and the reservoir: record pressure and temperature along the wellbore, leave a gauge in a nipple for the duration of a test, take a bottomhole sample, record an inflow profile.
Keep the tubing passable: check the bore with a gauge cutter (a tool of the full tubing size that shows whether the bore is clear), cut away wax and deposits, bail out (scoop out) sand.
Change devices in the tubing string: set or retrieve a plug, a choke (a restriction that sets the flow rate), a check valve or a wireline-retrievable safety valve, replace a valve in a side-pocket mandrel, open or close a sliding sleeve.
Open the reservoir and set equipment on electric line: perforate, and set packers and plugs — including a permanent packer before the tubing is run.
Deal with accidents: retrieve tools left in the well and broken wire — fishing.

The main advantage is that the work is done on a well under pressure. We do not kill the well, so there is no risk of damaging the reservoir with kill fluid (Formation damage and damage skin). The limits are also clear. The tool goes down under its own weight. At an inclination of more than about 60–70° (the angle from the vertical), friction against the tubing stops it (limiting angle). Beyond that, we need stem on rollers (rigid bars fitted with wheels), a downhole tractor on electric line, or coiled tubing (a long continuous pipe that is unreeled into the well). The tool must pass through the narrowest point of the tubing. Finally, the operator at the wellhead cannot see the tool. The operator judges it by the tension in the line and the depth on the counter, so the work calls for experience and calculation.

Slickline, braided line and electric line

Slickline is a single smooth steel strand, usually 1.8–3.2 mm in diameter and sometimes up to 4.8 mm. Several kilometres of it are wound on the winch drum. In Russian-language practice it is also called “scraper wire”, because the same wire is used to run the scrapers that clean paraffin from the tubing. We choose the smallest diameter for which the high-strength steel still carries the required load. Thin wire is lighter, bends less on the sheaves (the wheels that guide the line), and is pushed out less by the pressure. A smooth strand is easy to seal, and it passes sharp movements to the jar well.

Braided line is twisted from several strands of thin wires and is 4.8–6.4 mm in diameter. It is two to three times stronger than slickline. We use it when slickline is not strong enough: heavy tool strings, fishing, and recovery of stuck equipment. Electric line has one or more insulated conductors under two layers of steel armour, and it is about 5.6–12 mm in diameter. Power goes down the cable to the instrument, data come up from it, and a command goes to a perforating gun or a setting tool. That is why we use electric line to perforate, to set packers and plugs at an exact depth (with depth correlation), and to run logs in cased hole (Perforating operations: gun conveyance and procedures). Braided line and electric line are twisted on the outside, so they are harder to seal at the wellhead than smooth slickline.

Lines for wireline operations (typical data)

Line: Slickline 0.092 in
Diameter: 2.34 mm
Breaking load: 6.9 kN
Used for: light work, instruments

Line: Slickline 0.108 in
Diameter: 2.74 mm
Breaking load: 9.4 kN
Used for: the main line for plugs, locks and valves

Line: Slickline 0.125 in
Diameter: 3.18 mm
Breaking load: 12.6 kN
Used for: heavy strings, deep wells

Line: Braided line 3/16 in
Diameter: 4.75 mm
Breaking load: 22 kN
Used for: fishing and heavy work

Line: Electric line 0.330 in
Diameter: 8.38 mm
Breaking load: 46–50 kN
Used for: perforating, setting, logging

Line	Diameter	Breaking load	Used for
Slickline 0.092 in	2.34 mm	6.9 kN	light work, instruments
Slickline 0.108 in	2.74 mm	9.4 kN	the main line for plugs, locks and valves
Slickline 0.125 in	3.18 mm	12.6 kN	heavy strings, deep wells
Braided line 3/16 in	4.75 mm	22 kN	fishing and heavy work
Electric line 0.330 in	8.38 mm	46–50 kN	perforating, setting, logging

The breaking load given for slickline is for carbon steel to API 9A. Slickline made of corrosion-resistant alloys has a lower breaking load, and for electric lines it depends on the construction. In calculations, take the value from the data sheet of the particular line.

Kinks made during rig-up, bending on sheaves of small diameter, and fatigue from jar blows all damage slickline. We test it for torsion (the number of turns that a sample withstands before breaking), and we regularly cut off the worn length near the rope socket. In wells with hydrogen sulfide, hydrogen makes carbon-steel wire brittle, and it can break at a load far below the rated one. If the partial pressure of hydrogen sulfide (its share of the total pressure) is above 0.3 kPa (0.05 psi), the threshold adopted in ISO 15156, we use wire made of corrosion-resistant alloys. This matters for the sour fields of Kazakhstan (fields whose gas contains hydrogen sulfide).

Instruments run on slickline record their data in memory and run on batteries. Examples are a downhole pressure–temperature gauge, a casing collar locator (a tool that finds the pipe joints) and a flowmeter. We read the data after the instrument has been pulled out. Electric line sends the data up at once. This costs more, but we can then run the job on the instrument's real-time readings.

Wellhead equipment: the lubricator, the line seal and the BOP

To run a tool into a well under pressure without letting fluid escape, the crew assembles a system above the Christmas tree that holds the pressure around the line and the tool.

SchematicOn the left, the wellhead equipment for wireline operations (schematic, not to scale). On the Christmas tree there is the wireline BOP. Above it is the lubricator, which holds the whole tool string. At the top is the stuffing box with a sheave. The wire runs over the bottom sheave, which carries a tension sensor, to the winch. The pressure in the lubricator pushes the wire out through the stuffing box with a force

p_{wh}A

. On the right, the tool string on slickline above a nipple with a lock and a plug.

Lubricator: One or several pipes joined by quick-connect unions, rated for the well pressure. The lubricator works as an airlock between the well and the atmosphere. We load the tool into the lubricator with the Christmas tree closed. So the length of the lubricator limits the length of the tool string, and with it the weight of the bars and the length of a perforating gun. At the bottom there is often a tool catcher, which stops the tool falling into the well if the line breaks at the stuffing box, and a valve for bleeding off the pressure.
Stuffing box: It seals smooth wire with packing, which is tightened by hand or hydraulically. A sheave sits above it. If the wire breaks and is blown out of the stuffing box, the well pressure forces the internal shut-off element into place and closes the opening.
Grease injection head: We cannot seal braided line or cable with packing, because gas would escape along its surface as along a thread. The line passes through several tubes with a small clearance. A pump feeds thick grease into them at a pressure higher than the wellhead pressure. The grease fills the gaps between the wires and constantly escapes in small amounts, and we collect it.
Wireline BOP: Rams (sliding steel blocks) with rubber seals close around the line and shut in the well. We use them when the packing of the stuffing box has to be replaced or the grease seal has failed. For braided line and cable we fit two pairs of rams and pump grease in between them. On hazardous wells we add a device that cuts the line and seals the bore. This is the last resort.
Sheaves, weight indicator and depth counter: The bottom sheave is attached to the Christmas tree. A tension sensor on it, the weight indicator, measures the load. The winch, with a measuring wheel, counts the length of line run in.

How a tool goes into the well and comes out

1
Rigging up
With the tree valves closed, the crew places the tool string in the lubricator and puts the lubricator, with the BOP, on the tree.
2
Pressure testing
The crew tests the lubricator and the connections at a pressure not lower than the expected wellhead pressure.
3
Equalising
The crew opens the tree valves slowly. The pressure in the lubricator becomes equal to the wellhead pressure, and from this moment the force $p_{wh}A$ pushes the line out.
4
Running in and working
The crew runs the tool string in, watching the tension and the depth. They slow down at nipples and restrictions.
5
Retrieval
The crew pulls the tool string into the lubricator and closes the tree valves. They bleed off the pressure in the lubricator, and only then do they break the connections.

The tool string and how a jar works

The crew assembles a tool string at the end of the line. Its top part is the same for almost all jobs. At the bottom goes the tool for the particular operation. The parts are joined by quick-connect unions, and each has a fishing neck on top. This is a profile by which the part can be caught if it is left in the well.

Rope socket: It attaches the line to the tool string. Slickline is plaited round a spring-loaded disc. Braided line and cable are clamped with wedges. The cable head contains a weak point. It breaks at a set force before the cable does, so that we can detach the cable from a stuck tool string and pull it out whole.
Sinker bars (stem, weight bars): Steel rods 0.6–1.5 m long and about 19 to 89 mm in diameter. They give the weight that makes the tool string go down against the pressure and friction. They also give the mass that speeds up and strikes when the jar is worked. For heavy tool strings in a narrow bore, we use bars made of heavier alloys. In deviated wells, we use bars on rollers.
Mechanical jar (spang jar): A telescopic link (two parts that slide in and out) with a free stroke. The operator pulls the line up sharply. The bars speed up along the stroke, and at the end of it the jar strikes upward. The operator lets the line go sharply. The bars fall and strike downward. The force of the blow grows with the mass of the bars and with the speed.
Hydraulic jar: It strikes only upward. Oil flows slowly through a narrow channel, and the jar holds the tension for some time. The operator has time to stretch the line. When the channel opens, the energy of the stretch speeds up the bars. In deep wells, the stretch of the wire damps sharp movements at the wellhead, so it is hard to get a strong upward blow without a hydraulic jar. An accelerator (a spring or gas unit) above the jar strengthens the blow.
Knuckle joint: A ball joint that lets the tool string flex. It helps the tool string pass bends in the wellbore and restrictions, and it centres the tool over a nipple or over the pocket of a side-pocket mandrel.
Working tool: A running tool holds the device on shear pins and releases it when a blow shears the pin. A pulling tool catches the device by its fishing neck. It also has a pin. If the device does not come, we can shear the pin to disengage.

Why blows? The force of a blow at the tool can be many times greater than the static tension that the wire can bear. The blow lasts milliseconds, and the mass of the speeding bars creates the load, not the line. A downward blow “drives” the lock into the profile of the nipple and shears the setting pin. Upward blows tear the lock loose and bring it out. The price is fatigue of the wire and the rope socket. So after hard jarring, we retie the wire at the socket.

Typical operations

Gauge run: The first run of almost any job is a gauge cutter: a ring with a cutting edge, slightly smaller in diameter than the tubing bore. It checks that the tubing is clean and the tool will pass. It cuts away wax and deposits. It also finds the nipples and the bottom, and we use them to correct the depth counter.
Setting and retrieving locks and plugs: We run a lock (a lock mandrel) with a plug, a gauge, a choke or a valve on a running tool. Blows downward drive the keys of the lock into the profile of the nipple (landing nipples). Then we shear the pin and pull the tool string out. A pulling tool catches the lock by its fishing neck, and blows upward bring the keys out of the profile.
Shifting sliding sleeves: A tool with spring-loaded keys engages the inner sleeve and shifts it up or down by blows. Before opening, we equalise the pressures in the tubing and the annulus (sliding sleeves).
Changing valves in side-pocket mandrels: A kickover tool, which has a deflecting arm, swings its latch out sideways at the pocket of the mandrel. It pulls the gas lift valve or plug and sets a new one (Gas lift: principles and choice of operating conditions). In this way, without pulling the tubing, we convert the well to gas lift or change the point of gas injection.
Surveys: We run downhole pressure and temperature gauges to stops at set depths (pressure and temperature gradients: Reservoir pressure and temperature). Or we leave them in a nipple for the duration of a pressure build-up test (a test that records how the pressure rises after the well is shut in) (Pressure build-up analysis).
Taking bottomhole samples: We run the sampler opposite the reservoir or a little above the perforations (Reservoir fluid sampling).
Cleaning: We cut away paraffin wax with scrapers and wax cutters. We lift sand and sediment from the bottom, or from above a plug, with bailers (long tubes with a valve that scoop up the material).

Fishing begins when a tool, an instrument or broken wire has been left in the well. The first rule is to understand what exactly is down there and how it lies. For this we run an impression block, a lead block that takes an imprint of the top of the object left in the hole. We catch an object that has a fishing neck with a pulling tool. Broken wire coils into a tangle in the tubing. We first knock it down into a lump and then catch it with a hooked catcher, a wire grab. If the tool string is stuck and we cannot free it by blows or by shearing a pin, we cut the wire at the rope socket with a cutter dropped down the wire. We pull the wire out and then fish the tool string itself. Heavy fishing is done on braided line or on coiled tubing, where the permissible tension is greater.

Calculations: ejection force, weight and margin of overpull

The well pressure acts on the tool string and the line from all sides, and one force remains unbalanced. At the stuffing box the line passes out of a volume under pressure into the atmosphere. The line behaves like a plunger that is pushed into a vessel under pressure. To push each next metre into the well, we need a force equal to the pressure multiplied by the cross-sectional area of the line. The weight of the tool string must overcome this force.

The force with which the pressure pushes the line out of the well

F_{\text{ej}} = p_{wh}\,A_{\text{line}}, \qquad A_{\text{line}} = \frac{\pi d^2}{4}

where:

$F_{\text{ej}}$ ejection force, N
$p_{wh}$ wellhead pressure, that is, the pressure in the lubricator (gauge), Pa
$A_{\text{line}},\ d$ cross-sectional area and diameter of the line; for braided line and cable, the outer diameter, m², m

Units: SI; it is convenient to take the pressure in MPa and the area in mm², and their product is in newtons

Holds when

The pressure above the seal is atmospheric.
Friction in the stuffing box or the grease head is not included. A weight margin covers it.
The force does not depend on the depth of the tool string. It acts all the time that the line passes through the seal, but it matters most at the start of the run, when there is as yet no weight of line in the well.

The force grows with the square of the line diameter: an 8.38 mm cable is pushed out 9 times harder than a 2.74 mm slickline. That is why wells with high pressure favour slickline or thin cable.

In other unit systems (2)

Metric units

F_{\text{ej}}\,[\text{N}] = p_{wh}\,[\text{MPa}]\cdot A_{\text{line}}\,[\text{mm}^2]

pressure in MPa, area in mm².

There is no constant: 1 MPa·mm² = 1 N.

Oilfield units

F_{\text{ej}}\,[\text{lbf}] = p_{wh}\,[\text{psi}]\cdot A_{\text{line}}\,[\text{in}^2]

pressure in psi, area in square inches.

There is no constant: a psi is a pound-force per square inch.

Weight indicator reading with the string hanging freely

T = \left(W_{\text{tool}} + w\,L\right)\left(1 - \frac{\rho_{\text{L}}}{\rho_{\text{steel}}}\right) - p_{wh}\,A_{\text{line}}

where:

$T$ tension in the line above the seal — the weight indicator reading, N
$W_{\text{tool}}$ weight of the tool string in air, N
$w,\ L$ weight of a metre of line in air and the length of line in the well, N/m, m
$\rho_{\text{L}},\ \rho_{\text{steel}}$ density of the liquid in the well and of steel (7850 kg/m³); their ratio gives the buoyancy (the lifting effect of the liquid), kg/m³
$p_{wh}A_{\text{line}}$ ejection force, N

Units: SI; N, N/m, m, MPa and mm² are convenient

Holds when

The wellbore is vertical, and the tool string is stationary or moving uniformly. Friction is not included. On the way up it increases the reading, on the way down it decreases it, and in a deviated wellbore it can exceed the weight of the tool string.
The liquid density is the same along the whole wellbore. In gas the buoyancy factor is close to one.
For cable the factor is approximate, because a cable is lighter than steel of the same volume.

This is the “hook load” of wireline work (the load that hangs on the line). The operator uses it to see whether the tool string is moving freely. When the tool string sits down on a restriction, the tension drops. When it drags, the tension rises. With a stuck tool, whatever the indicator shows above $T$ is applied to the tool.

Margin of overpull at the tool

F_{\text{pull}} = k_{\text{all}}\,F_{\text{br}} - T

where:

$F_{\text{pull}}$ the largest static force that can be applied to a stuck tool in addition to its weight, N
$F_{\text{br}}$ breaking load of the line from the data sheet, N
$k_{\text{all}}$ allowable fraction of the breaking load; it is set by company rules, taking into account bending on the sheaves, wear and blows, —

Units: SI (any consistent units of force)

Holds when

The load is static. Jar blows create short-lived forces greater than the static ones.
The tension is greatest at the wellhead, where the line carries both itself and the tool string. So overpulled wire usually breaks at the wellhead, and almost the whole line falls into the tubing.

With depth, the weight of the line uses up the margin. At great depth we can hardly move anything with thin wire by static pull, so the work is done by blows.

Worked exampleIs the weight enough to enter the well: slickline and cable

Given (assumed data)

Well: gas well, shut-in wellhead pressure 23.0 MPa
String on 2.743 mm (0.108 in) slickline: rope socket, 3.0 m of 38.1 mm sinker bars, mechanical jar, knuckle joint, pulling tool — 45 kg
String on 8.382 mm (0.330 in) cable: instruments 60 kg without weights; 42.9 mm sinker bars
Conditions: weight margin for friction in the seal 20%; we ignore buoyancy in gas

Solution

Cross-sectional areas: slickline $\pi\cdot 2.743^2/4 = 5.91$ mm², cable $\pi\cdot 8.382^2/4 = 55.18$ mm².
Ejection force: on the slickline $23.0\cdot 5.91 = 136$ N (about 14 kgf), and on the cable $23.0\cdot 55.18 = 1269$ N, which is 9.3 times greater.
The tool string on slickline weighs $45\cdot 9.81 = 441$ N, which is 3.2 times the ejection force. It will go in.
The cable tool string needs at least $1.2\cdot 1269 = 1523$ N, that is, 155 kg. The instruments weigh 60 kg, so 95 kg is missing. A metre of 42.9 mm bar weighs 11.3 kg, so we need about 8.4 m of sinker bars.

Answer: The slickline tool string has enough weight, with a margin of 3.2 times. The cable tool string needs about 95 kg added, which is 8.4 m of bars, and the lubricator must hold them together with the instruments.

What it means. The ejection force matters at the start of the run. After that, the weight of the line itself in the well helps. The cable weighs 2.86 N/m and balances the ejection force at 444 m already, and the slickline does so at 299 m. But first the tool string has to enter through the tree, and its weight decides everything. This is why high-pressure gas wells have long lubricators, bars of heavy alloys and a choice of thin cable. Ignoring buoyancy in gas (the gas density is about 0.18 g/cm³ at this pressure) overstates the weight by about 2%. For the ejection check this is slightly on the unsafe side, but it does not change the conclusion. The data in this example are assumed for illustration.

Worked exampleWhat margin of overpull remains at depth

Given (assumed data)

Line: 2.743 mm carbon-steel slickline, breaking load 9.386 kN (2110 lbf)
Rule: tension at the wellhead not more than 50% of the breaking load
Job: pulling a plug from a nipple at 2450 m; tool string 45 kg; wellhead pressure 23.0 MPa; gas
For comparison: 3.175 mm (0.125 in) slickline, breaking load 12.63 kN (2840 lbf); depth 5000 m

Solution

A metre of wire has a mass of $7850\cdot 5.91\cdot 10^{-6} = 0.0464$ kg and a weight of 0.455 N/m. At 2450 m, the wire in the well weighs $0.455\cdot 2450 = 1115$ N.
Weight indicator reading with the tool string hanging: $1.115 + 0.441 - 0.136 = 1.42$ kN (wire, tool string, minus the ejection force).
Allowable tension: $0.5\cdot 9.386 = 4.69$ kN. Margin at the tool: $4.69 - 1.42 = 3.27$ kN.
At 5000 m: wire 2.276 kN, indicator 2.58 kN, margin 2.11 kN. This is a third less.
3.175 mm wire at 5000 m is heavier (0.610 N/m) but stronger. Allowable tension 6.32 kN, indicator 3.31 kN, margin 3.01 kN.

Answer: At 2450 m, we can apply about 3.27 kN of static force to a stuck tool, in addition to its weight. At 5000 m, we can apply 2.11 kN. The thicker wire at 5000 m gives 3.01 kN.

What it means. We choose release pins and setting forces for the jar blows that this tool string produces. We use static pull only within the margin that we found. If the pull goes beyond the allowable tension, the wire breaks at the wellhead, where the tension is greatest, and kilometres of wire fall into the tubing. That means heavy fishing. The calculation is for a vertical wellbore without friction. In a deviated wellbore, friction against the tubing on the way up adds to the indicator reading and reduces the force that reaches the tool. We ignored buoyancy in gas. It would lower the reading by about 2%, so the margin is in fact slightly greater. The data in this example are assumed for illustration.

CalculatedCalculated weight indicator reading for a freely hanging 45 kg tool string in a gas well with a wellhead pressure of 23 MPa, for 2.74 mm and 3.18 mm slickline. The dashed lines show the allowable tension at the wellhead (50% of the breaking load). The distance between the solid and the dashed line of the same colour is the margin of overpull at the tool. It decreases with depth.

Terms and symbols

Slickline, braided line, electric line: Three kinds of line: a smooth strand, a twisted rope of wires, and a cable with communication conductors inside armour.
Lubricator: The pipe that serves as an airlock above the Christmas tree. We load the tool into it before running the tool into a well under pressure.
Stuffing box and grease head: The stuffing box seals slickline with packing. The grease head seals braided line or cable with thick grease in tubes with a small clearance.
Wireline BOP: Rams that close around the line and shut in the well if the seal fails.
Tool string: The rope socket, sinker bars, jar, knuckle joint and working tool at the end of the line.
Jar: A link that turns the speed of the moving bars, or the energy of the stretched line, into a blow upward or downward.
Fishing: The recovery of tools, instruments and broken line left in the well.
$F_{\text{ej}} = p_{wh}A_{\text{line}}$: The force with which the pressure pushes the line out. The weight of the tool string overcomes it.

Common mistakes and misconceptions

Wrong

“The pressure in the well presses the string down and helps the run.”

In fact

Quite the opposite. The pressure pushes the line out through the seal with a force $p_{wh}A$ . The tool string must weigh more. A thick cable on a gas well needs hundreds of kilograms of weights.

Wrong

“As long as the wire has not broken, you can keep pulling.”

In fact

We limit the tension to a fraction of the breaking load, because bending on the sheaves, wear and blows lower the strength. A break at the wellhead drops almost all the wire into the tubing, and that means expensive fishing.

Wrong

“A plug is pulled as soon as it has been caught.”

In fact

We equalise the pressures above and below the plug first. Otherwise the pressure difference throws it up the tubing together with the tool string.

Wrong

“The depth counter shows where the tool is.”

In fact

The counter measures the line at the wellhead. In the well the line is heated, and friction, overpull and blows change how much it stretches. At a depth of 2–3 km this adds up to metres. We correct the depth against the nipples, the bottom, the collars and the tubing tally.

Wrong

“One wire serves for all wells.”

In fact

In hydrogen sulfide, carbon-steel wire becomes brittle. At a hydrogen sulfide partial pressure above 0.3 kPa, we use wire made of corrosion-resistant alloys, and its breaking load is lower.

In depth: advanced detail

Stretch of the line: the depth counter, the jar and the point of sticking

The counter measures the length of line passing through the measuring wheel, under whatever tension the line has at that moment. In a quiet vertical well, every part of the line goes on carrying the same load, which is the weight of everything that hangs below it. So the stretch under a constant load is already in the reading. The tool moves away from the counter reading in two cases: when the tension in the well changes after the line has passed the wheel, and when the line is heated:

In a deviated wellbore, friction reduces the tension on the way down and increases it on the way up, so the same counter mark corresponds to different depths going in and coming out.
Pulling on a stuck tool and jar blows stretch the line more, while setting the tool string down on a restriction unloads it.
In the well the line is heated from the surface temperature to the reservoir temperature and gets longer.
The measuring wheel slips and wears.

Magnetic marks on a logging cable work differently. They are placed at equal distances under a set tension, but in the well the tension along the cable is different. So we correct the depth read from the marks for stretch (wireline logging). Comparing the counter with the marks also reveals slippage of the wheel.

Elongation of the line under a change of tension and heating

\Delta L = \frac{\Delta F\,L}{E\,A_{\text{line}}} + \alpha\,L\,\Delta\bar T

where:

$\Delta L$ elongation of the line in the well beyond what the counter showed, m
$\Delta F$ change of tension along the whole length $L$ after the line has passed the counter — for example, an overpull beyond the weight, N
$L$ length of line on which $\Delta F$ acts: to the tool or to the point where the line is held, m
$E$ Young's modulus of steel (how stiff it is), 207,000 MPa = 2.07·10¹¹ Pa, Pa
$A_{\text{line}}$ cross-sectional area of the wire, m²
$\alpha$ coefficient of linear expansion of steel, $1.24\cdot 10^{-5}$ , 1/°C
$\Delta\bar T$ heating of the line, averaged along its length, relative to the temperature at the counter, °C

Units: SI

Holds when

The wire is solid and the wellbore is vertical. For braided line and cable the elongation under load is greater, and we take it from the data sheet. The stretch coefficients of slickline in the data sheets are also somewhat greater than a calculation from the modulus of steel gives.
$\Delta F$ is the same along the whole length $L$ . This happens when the tool or the line is held and is pulled from the wellhead. Friction along the wellbore reduces it with depth.
The effect of pressure on the elongation is not included.

A long wire is a spring of stiffness $EA/L$ . At depth, to pass an overpull to the tool, the operator has to take up metres of wire. A sharp movement at the wellhead hardly reaches the tool. This is why we need a hydraulic jar for a strong blow upward. The inverse problem (finding the cause from the result) is to find, from the elongation under an extra pull, the length over which the line is free.

Worked exampleOverpull, heating and the point of sticking at 2450 m

Given (assumed data)

Line: 2.743 mm slickline; the tool is stuck in the nipple at 2450 m
Overpull: 1.0 kN beyond the weight with the tool string hanging freely
Heating: 40 °C on average

Solution

Section stiffness: $EA = 207{,}000\cdot 5.91 = 1223$ kN (MPa × mm² = N).
An overpull of 1.0 kN stretches 2450 m of wire by $1.0\cdot 2450/1223 = 2.00$ m. We have to take up that much wire before the tool feels 1 kN.
Heating: $1.24\cdot 10^{-5}\cdot 2450\cdot 40 = 1.22$ m. The tool is deeper than the counter reading.
If the same overpull produced an elongation of only 1.40 m, the free length would be only $1223\cdot 1.40/1.0 \approx 1710$ m. The wire would be held above the tool, for example because it is tangled or stuck in deposits.

Answer: An overpull of 1 kN stretches the wire at 2450 m by 2.00 m, and heating by 40 °C lengthens it by 1.22 m. An elongation of 1.40 m under the same overpull would point to a sticking point at about 1710 m.

What it means. For seating in a nipple, a counter error is harmless, because we find the nipple by the seating of the tool. For correlating an instrument to the reservoir, or a perforating gun to an interval, such an error is unacceptable, and we correct the depth against collars and the tubing tally. The estimate of the sticking point is approximate. Friction in the wellbore, and the imprecisely known stiffness of the wire, can shift it by hundreds of metres. The stretch of wire given in data sheets can be 15–20% greater than a calculation from the modulus of steel gives, and then the same elongation would point to a sticking point 200–300 m higher. The data in this example are assumed for illustration.

The weight indicator needs attention too. Its sensor sits on the bottom sheave, over which the line turns from the lubricator to the winch. It senses the resultant of two tensions, $R = 2T\cos(\beta/2)$ , where $\beta$ is the angle between the directions of the line to the lubricator and to the winch. The scale is calibrated for a particular angle, usually 90°. If the winch has been placed so that the angle is different, we must recalculate the reading: at a smaller angle the instrument overstates the tension, and at a larger angle it understates it.

Connections to other topics

Completion string equipment — nipples, locks, sliding sleeves, side-pocket mandrels and subsurface safety valves, which are serviced on wireline.
Well completion: opening the reservoir and the tubing string — the functions of the tubing string and well barriers.
Perforating operations: gun conveyance and procedures — perforating on electric line and slickline, depth correlation.
Gas lift: principles and choice of operating conditions — gas lift and side-pocket mandrels, whose valves are changed on wireline.
Reservoir fluid sampling — bottomhole samplers on electric line and slickline.
Pressure build-up analysis — surveys with a downhole gauge in a nipple.
Open-hole logging: why, how and when — cable, depth from the cable and delivery of instruments into deviated wellbores.
Well control: primary barrier and kick indicators — barriers and well control for work under pressure.

Check yourself

1
In a gas well with a wellhead pressure of 30 MPa, a downhole gauge has to be set in a nipple for two weeks for a pressure build-up test. Which line should be chosen?
1. A.Only coiled tubing
2. B.Braided line 6.4 mm
3. C.Electric line 11.8 mm
4. D.Slickline with an instrument that records its data in memory
Show the answer
Correct answer: D. Slickline with an instrument that records its data in memory
An instrument with memory and batteries needs no live link. Slickline is pushed out least by the pressure and is the easiest to seal. A thick cable at 30 MPa would need hundreds of kilograms of weights, and we would still have to leave the instrument in the nipple without the cable.
2
The wellhead pressure is 30 MPa and the slickline is 3.175 mm. What force does the pressure create, and how much must the tool string weigh with a 20% margin for friction in the stuffing box?
Show the answer
The area is $\pi\cdot 3.175^2/4 = 7.92$ mm², and the force is $30\cdot 7.92 = 237.6$ N (about 24 kgf). With a 20% margin it is about 285 N, so the tool string must weigh at least about 29 kg. An ordinary tool string with sinker bars weighs more, so for slickline this limit seldom decides the choice.
3
A plug in a nipple has been caught with a pulling tool. Below the plug is reservoir pressure; above it, in the tubing, the pressure is much lower. What must be done before pulling?
1. A.Nothing: the pressure difference will help lift the plug
2. B.Close the BOP and pull
3. C.Strike upward with the jar immediately
4. D.Equalise the pressure above and below the plug — by pumping into the tubing or through the equalising valve of the plug
Show the answer
Correct answer: D. Equalise the pressure above and below the plug — by pumping into the tubing or through the equalising valve of the plug
A plug pulled free under a pressure difference will fly upward together with the tool string. It can jam in the tubing, and the wire will tangle and break. We equalise the pressure first, and then pull the plug loose by upward blows.
4
While running in at 1500 m, the weight indicator reading suddenly dropped, although the winch keeps paying out wire. What has happened, and what should be done?
Show the answer
The tool string has stopped: it has sat down on a restriction, on deposits or on a landing profile, and the wire no longer carries its weight. If the run continues, the wire will pile up in loops on top of the tool string, kink and may tangle. We stop the run, pull up carefully until the weight comes back, and find the cause, for example with a gauge cutter or an impression block.
5
Why, in deep wells, is a strong upward blow obtained with a hydraulic jar and not only with a mechanical one?
1. A.A hydraulic jar works from the well pressure
2. B.Long wire stretches and damps sharp movements at the wellhead; a hydraulic jar holds the tension while the line stretches and then releases it — the energy of the stretch accelerates the bars
3. C.A hydraulic jar is lighter
4. D.A mechanical jar does not strike upward
Show the answer
Correct answer: B. Long wire stretches and damps sharp movements at the wellhead; a hydraulic jar holds the tension while the line stretches and then releases it — the energy of the stretch accelerates the bars
With a mechanical jar, the force of the blow depends on the speed of the bars. At depth, the elasticity of several kilometres of wire absorbs a sharp jerk at the wellhead. A hydraulic jar delays its action, so the operator stretches the line, and the energy stored in it speeds up the bars for the upward blow.
6
The wire was overpulled while trying to free a stuck tool string. Where is it most likely to break, and why is that bad?
Show the answer
At the wellhead. The tension is greatest there, because the line carries both itself and the tool string, and on the sheaves it is also bent. The broken wire falls into the tubing and coils into a tangle above the tool string. First we fish the wire with a wire grab, then the tool string. That is why we limit the tension to a fraction of the breaking load, and why tools have shear pins for disengaging.

Updated: 26 October 2026