back off procedures

7/22/2019 Back Off Procedures

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BACK-OFFProcedures

FISHING TECHNOLOGY

Bernie Springer, Abu Dhabi, May 5t, 2002


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A. Safety Precautions

While applying left hand torque with the rotary slips, the slip handles are to be tiedtogether with a length of wire rope. This will ensure that the slips are not thrown outof the rotary in case the pipe breaks high, causing the pipe to jump due to the instan-taneous loss of strain.

In cases where the wellbore fluids are not balanced between the inside and outside ofthe drill string, or where it is suspected that formation fluids may have migrated intothe drill string annulus, the string shot is to be run using a wireline lubricator and

stuffing box.

The wireline safety procedures for perforating operations are to be followed whilemaking up and running the string shot.

B. Determining the Free Point

Two techniques are commonly used to determine the free point of a stuck drilling as-sembly. The first technique simply involves the measurement of pipe stretch for a

given over-pull and then calculation of the length of free pipe. The second techniquerequires the use of an electronic free point indicating system run on electric line.When possible, the free point tool and back off shot should be run in combination.

In general, all our back-offs will be performed based on the free point established us-ing the electronic free point indicator. However, the calculated free point from pipestretch data is useful in planning fishing and subsequent operations since the actualfree point will typically be within +/- 500.

1. Determining Free Point based on Pipe Stretch Data

Free point calculations using pipe stretch data are most accurate in vertical holeswhere the impact of hole drag is minimal. This method may also be used in directorialwells, but will probably underestimate true length of free pipe due to the effects ofhole drag. The following procedure is to be followed when using this technique:

1.1. Pick up the drill string to the normal pickup weight and mark the pipe at aconvenient reference point. Apply an overpull of 10,000 lbs and return to thenormal pickup weight. Make a second mark on the Kelly. The second markshould be distinct from the first. The difference is caused by the friction of thestring in the hole. Draw a datum line midway between the two marks.


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1.2. Take an overpull of e.g. 50,000 lbs on the pipe and mark the pipe. Increaseoverpull by 10,000 lbs (but do not exceed 80% of the minimum yield strength ofthe pipe) and return to 50,000 lbs. Make a second mark on the Kelly. Draw adatum line midway between the two marks and set the brake.

1.3. Measure and record the distance (inch) between the two datum lines (amount ofstretch that resulted from the over-pull).

1.4. Estimate the length of free pipe using the following equation:

Lf = (L x Ap x E) / (12 x P)

where:

L f = length of free pipe, feetL = length change due to over-pull, inchesAp = cross-sectional areas of pipe tube, sq. in.E = modulus of elasticity, psi

= 30,000,000 psi for steelP = applied over-pull force, pounds

1.5. The amount of over-pull can be increased if desired, to check the free pipe

length calculated. However, it must be remembered to measure the amount ofstretch from the initial reference point.

2. Determining Free Point using Electronic Free Point Indicator.

The most accurate and commonly used technique for determining the free point isthrough the use of an electronic free point indicator. Electronic free point services areoffered by Schlumberger and Atlas, as well as a number of other electric logging com-panies. The technology involves measurement of the axial and torsional strain in thepipe body through application of tension and torque to the drill string. Provided the

pipe is completely free, a fixed and measurable degree of strain will be recorded by thesystem's electronics. Once the degree of strain diminishes, or reduces to zero, the pipeis stuck. To achieve a successful back off, the pipe must be 100% free.

The following procedures and guidelines are to be followed determining the free pointusing electronic instruments. I n general, a combination free-point tool / back-off shotshould be run unless tool limitations necessitate performing two separate runs.

2.1. When using a bow spring anchored free point tool, be certain a wireline Swivel /expansion joint has been fitted to isolate possible cable movement from themeasuring device. This is critical, particularly in directional wells, where the

cable will tend to move upward as the pipe is stretched. Lowering the free pointtool on depth and then picking up on the logging cable to open the tool to mid-


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stroke most effectively uses the expansion joint. If the strain indicating needlejerks when the pipe is tensioned, then the expansion joint will have to be col-lapsed slightly until the correct space-out is achieved for the amount of pipetension that is applied.

2.2. I f an expanding arm free point tool is used, the expansion joint can stillbe used, however it is not so critical since these tools firmly anchor thefree point tool inside the pipe and are less prone to slippage. Generallythe cable is slackened to provide the necessary allowance for pipe move-ment.

2.3. When free point readings are being taken, the Drilling Supervisor shouldbe present in the logging unit to witness and supervise the operation. Becertain to maintain a written record of all tool readings as well as tensionand torque applied to the drill string.

2.4. The free point instrument is to be run into the drill string to 1000' belowthe surface and reference free point readings taken in both tension andtorque. These figures will then be used to indicate the degree of free pipefor readings taken close to the stuck point. With the spring bow free

point tool used by Schlumberger and Atlas, 100% free pipe is generallytaken to be 80 units of meter deflection.

2.5. Once calibrated, the free point tool is run in to 500 ft above the estimatedfree point depth calculated using the pipe stretch technique discussedearlier. Once on bottom, tension readings are to be taken first at eachmeasurement point. With the pipe tensioned on surface to the estimatedpick-up weight at the stuck point, apply over-pull to the pipe in even loadincrements (+/- 50,000 lbs) until pipe movement is noted at the free pointtool. Smooth deflection of the strain indicating needle should be noted ifthe tool is functioning properly. If the needle jerks erratically, the cable

has probably moved or the springs have slipped causing interferencewith the instrument reading.

2.6. Repeat readings as required until a consistent deflection of the free pointindicating needle is noted. The repeatability of the reading may not beprecisely consistent and, in these cases, the average of the measuredreadings should be used.

2.7. The degree of free pipe at any survey point is then taken as a percentageof the free pipe meter deflection measured previously. Half scale deflec-

tion would be 50% free, 3/4-scale deflection 75% free, etc.


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2.8. I f the pipe is found to be 100% free at a given survey point, drop downthe pipe in +/- 100 ft increments repeating the procedure until the meterdeflection reduces to zero.

2.9. Once the free point in tension has been located, repeat the procedureworking up the hole taking torque measurements. Be certain to apply thesame amount of torque at each depth, taking note of the number of turnsrequired to achieve the desired torque. Once a reading has been taken ata given depth, completely release the torque prior to taking the nextreading.

2.10. The depth where both the torque and tension readings indicate 100% freepipe is taken as the free point. This is the depth, above which the pipeshould be backed off. When analyzing stuck point readings, the followingguidelines will prove useful:

In a highly deviated well with a significant degree of hole drag, a widediscrepancy between the degree of axial and torsional deflection of thefree point meter may be noted. This will likely be due to an inability towork sufficient torque down the hole to achieve full-scale deflection. In

this case attempt to work the torque down and repeat the readings. I f thetorque cannot be worked down to the right, it will also be difficult to worktorque down to the left; therefore, a shallower back-off point should beselected.

In cases where the bottom-hole assembly is differentially stuck, a sharpdistinction between free pipe and stuck pipe will generally be noted. Thelength of partially stuck pipe will be short, however the pipe should stillbe backed off at the point where the pipe is 100% free.

In those cases where the formation has progressively fallen in behind thedrilling assembly, a very gradual transition between 100% free and 100%stuck will be noted. The tendency here is to try for a deeper back-off. Thisshould not be attempted, as the time wasted trying to back-off below the100% free depth will generally result in more of the pipe becoming stuck.

2.11. Once the free point is located, pickup to the tool joint that will be backedoff and mark the wireline cable for later reference while running in withthe string shot.


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C. Determining String Tension Required to Back-Off

Under ideal conditions the desired pipe tension at the back-off point is zero. Recog-nizing that this cannot be achieved in practice, it is preferred to have the pipe slightlyin tension at the back-off point as opposed to compression.

Calculation of the required surface tension, and how this tension is applied prior tofiring the string shot is critical to successfully backing off the pipe at the intendeddepth. I f too much tension is applied, or if the pipe is in compression, it will probably

not back off. I f it does, it will probably be further up the string than planned. Thismay require that the pipe has to be made up again and the back off procedure re-peated.

To determine the necessary surface tension to optimize the tension at the back-off point, the following information is required.

Pickup, slack-off and off bottom rotating weights of the drill sting prior to be-coming stuck. These measurements are to be taken with the pumps off. I f thepipe became stuck while tripping, the required figures may have to be esti-mated based on previously recorded pickup, slack-off and rotating weights re-corded while drilling at the depth of the stuck point.

The weight/foot and length of individual components of the drilling assembly

The density of the drilling fluid in the well

The average hole inclination through the section of the well where the drillingassembly is stuck.

Once the hook load required putting the back-off point in zero tension has been de-

termined, an additional e.g. 5000 lbs of overpull is then applied to ensure that theconnection is in tension rather than compression.


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The required additional overpull can be calculated as follows:

AO = MW * VD * TJ A

Where:

AO = Additional Overpull (lb)MW = Mud weight (ppg)VD = Vertical depth at backoff point (ft)J A = Tool J oint Mating Surface Area (sq. inch)

Tool J oint Mating Surface Area (API Spec 7)

Type ofConnection

Mating SurfaceArea JA inch2

Type ofConnection

Mating SurfaceArea JA inch2

NC 23 1.653 2 3/8 REG 1.464

NC 26 1.591 2 7/8 REG 2.690

NC 31 2.908 3 1/2 REG 3.095NC 35 4.609 4 1/2 REG 4.754

NC 38 3.400 5 1/2 REG 8.419

NC 40 4.948 6 5/8 REG 13.290

NC 44 8.151 7 5/8 REG 21.443

NC 46 6.801

NC 50 6.717 5 1/2 FH 8.081

NC 56 14.116

NC 61 21.892

NC 70 23.156

NC 77 31.678


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A generalized equation for determining the correct weight indicator reading when thestring shot is fired is as follows:

Weight Indicator Reading =--+

Pick-up WeightBuoyed Weight of Stuck ToolsHole Drag of Stuck Toolse.g. 5,000 lbs Over-pull

Where:

Pickup Weight = Pickup weight of entire drilling assem-

bly before becoming stuck, lbsBuoyed Weight of Stuck Tools =

=

Buoyed weight of drilling assembly be-low back-off point taking account of holeinclination, lbs.Air weight x buoyancy factor x cosine ofhole angle

Hole Drag of Stuck Tools = Proportion of total pickup drag based onlength of stuck tools

The following examples illustrate the correct calculation procedure for determiningthe required hook load for backing off.

1. VERTICAL WELL

In this example it will be assumed that the drilling assembly has become differ-entially stuck while drilling on bottom.

Well depth: 10,500' MD (vertical well)Hole size: 12-1/4"

Drill String: 500' x 5" x 150 1b/ft drill collars450' x 5 x 50 lb/ft HWDP9,450' x 5 x 19.5 lb/ft S-135 drill pipe

Mud weight: 10.5 ppgBuoyancy Factor: 0.84Pickup Weight: 370,000 lbs *)Slack-off Weight: 335,000 lbs *)Rotating Weight: 350,000 lbs *)Weight of Blocks: 100,000 lbs *)Stuck Point: 9,900' MD (top of drill collars)

Avg. Hole Angle: 0 deg.*) I ncludes weight of blocks, hook, etc.


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In this example it will be assumed that the hole drag is distributed uniformly alongthe length of die drilling assembly, that 5,000 lbs of tension will be maintained in thepipe at the back-off point when the string shot is fired, and that the pipe will bebacked off at the stuck point (i.e. 9,900' MD).

1.1. Calculate buoyed weight of drill collars below stuck point:

Air Weight of Dril l Collars: 90,000 lbsBuoyancy factor: 0.84

Buoyed weight: 75,600 lbsCosine (0 deg): 1.00Net buoyed weight: 75,600 lbs

1.2. Calculate hole drag of stuck BHA:

The hole drag figure to use here is the drag experienced while picking upthe drilling assembly since the pipe will be picked up to the correctweight indicator reading to perform the back-off.

Total hole drag moving up: 20,000 lbsLength of drilling assembly: 10,500 ftHole drag per 1,000 feet: 1,905 lbsLength of stuck pipe: 600 ftHole drag along stuck BHA: 1,143 lbs

1.3. Calculate required weight indicator reading at time string shot is fired:

Total Pickup weight: 370,000 lbsMinus BHA weight: -75,600 lbsMinus hole drag: - 1,143 lbs

Plus over-pull requirement: + 5,000 lbsRequired pickup weight: 298,257 lbs

Therefore, prior to firing the string shot, the pipe will be slacked off to below the ini-tial slack-off weight and then picked up and tensioned to a weight indicator reading of298,257 lbs. For practical purposes this figure can be rounded off to 298,000 lbs.

It should be noted that in order to apply the correct pick-up weight, the weight of alltools and equipment that were suspended below the hook are accounted for in thispickup weight calculation. For instance, if the kelly has been removed, this weightmust be subtracted from the required pickup weight.


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2. Directional Well

In this example it will be assumed that the drilling assembly has become differ-entially stuck while making a connection on bottom.

Well depth: 12,500' MD (directional well)Hole size: 8-1/2"Drill string: 400' x 5-1/2" x 90 ppf drill collars

1,000' x 5" X 5O ppf HWDP11,100 x 5" x 19.5 ppf S-135 DP

Mud Weight: 12.5 ppgBuoyancy Factor: 0.809Pickup Weight: 355,000 lbs *)Slack-off Weight: 160,000 lbs *)Rotating Weight: 295,000 lbs *)Weight of Blocks: 100,000 lbsStuck point: 12,200' MD (300 above bit)Kick-off Point: 2,000' MD

End of Build: 4,000' MDHole Angle: 45 Deg. (below KOP to TD)

*) Includes weight of blocks, hook, etc.

In this example it will be assumed that the hole drag is distributed uniformlyalong the length of the drilling assembly and that 5000 lbs of tension will bemaintained in the pipe at the back-off point when the string shot is fired.

As with the previous example, the required weight indicator reading when thestring shot is fired will be determined by deducting the weight of the section of

BHA to be left in the well taking account of hole inclination.

2.1. Calculate buoyed weight of BHA to be left in hole following back-off:

Air weight of DC Below stuck point: 27,000 lbsCosine of 45 deg.: 0.707New air weight of DC: 19,089 lbsBuoyancy factor: 0.809Net buoyed weight of DC fish: 5,443 lbs


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2.2. Calculate hole drag of pipe to be left in hole:

The hole drag figure to use here is the drag experienced while picking upthe drilling assembly since the pipe will be picked up to the correctweight indicator reading to perform the back-off.

Hole drag moving up: 60,000 lbsTotal length of drilling assembly: 12,500 ftHole drag per 1000 ft (moving up): 4,800 lbsLength of stuck pipe: 300 ft

Hole drag along stuck pipe: 1,440 lbs

2.3. Calculate required weight indicator reading at the time string shot isfired:

Pickup weight of drilling assembly: 355,000 lbsMinus along-hole weight of DC fish: - 15,443 lbsMinus hole drag: - 1,440 lbsPlus over-pull requirement: + 5,000 lbsRequired Pick Up Weight: 343,117 lbs

Therefore, prior to firing the string shot, the pipe will be slacked off tobelow the initially recorded slack-off weight and then picked up to aweight indicator reading of 343,117 lbs. For practical purposes this figurecan be rounded off to 343,030 lbs.

D. String Shot Charge Size

The size of the explosive used to make-up the string shot is critical to creating suffi-cient hammer effect to cause the tool joint to break at the desired connection. As a

rule, the number of strands of primacord to the used for a specific connection shouldbe 2 - 3 greater than the number specified by either Schlumberger or Atlas.

The primacord selected should be an 80 gm/ft RDX or HDX specification. The RDXexplosive is good to a nominal maximum bottom-hole temperature of 325 deg, whilethe HDX charge can be used up to 400 deg. F.

The following table gives nominal primacord requirements for back-off operations.Again, the actual number of primacords used should be 2 - 3 more than listed.


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MEASURED DEPTH TO BACK-OFF POINT(FEET)

CONNECTION 0to

3,000

3,000to

6,000

6,000to

9,000

9,000to

12,000

over12,000

TUBING

2-3/8 1 1 1 2 22-7/8 1 1 2 2 3

3-1/2 1 2 2 3 3

4 & 4-1/2 2 2 2 3 3

DRILL PIPE

2-3/8 & 2-7/8 1 2 2 3 3 4 4 5

3-1/2 & 4 2 3 3 4 4 6 5 8

4-1/2 to 6-9/16 2 3 4 4 6 5 9 6 12

6-5/8 3 4 - 5 5 - 7 6 - 10 7 14

DRILL COLLARS

3-1/2 & 4 2 4 2 5 3 7 3 8 4 5

4-1/8 5-1/2 2 4 3 6 4 8 4 10 5 12

5-3/4 7 3 6 4 8 5 10 6 12 7 15

7-1/4 to 8-1/2 4 - 6 5 - 9 6 - 12 7 - 15 8 18

CASING

4-1/2 to 5-1/2 3 3 3 3 3

6 to 7 3 3 3 4 4

7-5/8 4 4 4 4 5

8-5/8 5 5 5 5 5


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E. Working Right Hand Torque Down the Hole

Prior to performing a downhole back-off it will be necessary to be certain sufficientmake-up torque has been applied to the pipe to avoid a shallow back-off or a deep openhole back-off at the incorrect depth.

The correct amount of right hand torque to work down the drill string will depend onseveral factors including well depth, wellbore profile, and degree of hole drag in bothtension and torsion. The correct string tension to be applied at the surface in order toeffectively work torque down to the back-off point should have been determined while

locating the free point. I n this regard, it does not necessarily follow that the pipeshould be worked between the calculated pickup and slack-off weights at the back-offpoint. For this reason the torque measurements taken while determining the freepoint should be used as an indicator of the correct surface tension to use for applyingboth left hand and right hand torque.

When working right hand torque down the hole, the following practices are to be fol-lowed:

1. The amount of right hand torque that must be worked into the pipe should be

sufficient to generate a full scale torque reading on the free point tool at theplanned back-off depth plus 30%. This will ensure that maximum left handtorque can be applied to the pipe when the string shot is fired.

2. The torque is to be worked down the pipe from surface to the free point. Sincetool joints can only be torqued correctly with minimum axial tension the pipe isto be raised and lowered between the calculated free pipe pickup weight at theplanned back-off point and zero surface tension while progressively increasingthe applied right hand torque.

3. The torque is to be applied in 3 -4 steps until the maximum make up torque has

been worked into the string.

Be certain to record the number of total turns necessary to achieve maximummake up torque, as well as the number of turns that can be anticipated whenthe required left hand torque is applied.

4. Complete make up of all connections will be indicated when no loss of trappedtorque occurs as the pipe is cycled between the range of pick-up and slackoffweights being used.


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F. Working Left Hand Torque down the Hole

The amount of left hand torque to work down the hole should be the maximum possi-ble within the constraint of avoiding a premature or shallow back-off. This is a haz-ardous operation and safety precautions as outlined must be closely followed. Whenworking left hand torque down the hole the following guidelines and procedures haveto be followed.

1. As a rule of thumb, the amount of left hand torque applied at the surface shouldnot exceed 70% of the right hand makeup torque.

2. Since a tool joint is more inclined to break when the axial tension on the con-nection is zero, left hand torque should be worked down the hole stating withmaximum surface tension and working progressively down to planned back-offtension. This should be performed in several stages starting with approxi-mately half of the required left hand torque. Only when a given amount oftorque has been worked into the entire string, the torque should be increased tothe next stage.

3. Be certain to maintain a record of the number of cumulative left hand turns

that have been put in the pipe as the torque is worked down to the plannedback-off point.

4. The string shot is to be loaded into the drill string prior to applying any torqueat surface. Do not load the string shot into the string shot while torque istrapped in the pipe. This could result in serious injury to personnel if the pipewere to break prematurely and backlash at surface.

G. Running the String Shot and Backing Off

Once the number of turns required to work the desired left hand torque downhole hasbeen determined, the string shot can be loaded into the pipe in preparation for run-ning downhole.

1. With the surface torque on the pipe completely released, load the string shotand collar locator assembly into the drill string and run in to 1000 ft below sur-face. Do not run in deeper since if the pipe were to back-off shallow, the wire-line cable may be severely damaged or part downhole resulting in anotherfishing job.


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NOTE:

In cases where a combination free point/back-off shothave been run, the tool assembly should be pulled upto 1000 ft below the rotary table prior to working lefthand torque into the string.

2. Apply left hand torque to the pipe and work it down-hole to the planned back-off point. The toque will be

completely worked down to the back off point whenthe number of previously measured left hand turnshave been achieved. If holding the torque with therotary or top drive, make sure to lock the rotary ta-ble or power swivel once the final torque is achieved.

3. Apply left hand torque to the pipe and work it down-hole to the planned back-off point. The torque will becompletely worked down to the back-off point whenthe number of previously measured left hand turns

have been achieved. If holding the torque with therotary or a top drive, make sure you lock the rotarytable or power swivel once the final torque isachieved.

4. Run in with the string shot and position the chargeopposite the planned back-off point.

5. Clear all personnel from the rotary table area.

6. Fire the string shot and monitor the torque gauge for

a loss of torque. I f successful, DO NOT unscrew thepipe.

7. Pull the wireline cable back to +/- 1000 feet.

8. Release the trapped left hand torqueand make note of the number of righthand turns necessary to reduce thetorque to zero.

9. POOH with the wireline and pull thelogging cable clear.

BACKOFF STRING-SHOT

Running Hardware


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10. Reduce string tension to anticipate rotating weight at back-off point and rotatepipe to the left to complete back-off. Pick up on string and check the pick-upweight.

10.1. I f pipe backed off at planned depth, POOH.

10.2. I f the pipe backed off high, screw into connection looking up and repeatentire back-off procedure. Since the pipe backed off high, it may be nec-essary to increase the amount of right hand torque, work the torque

down for a longer period, and/or adjust the string tension to be held whenthe string shot is fired.

If the pipe backed off close to the planned back-off point, check thepickup, slackoff and rotating string weights prior to screwing back intothe fish These measurements should then be used to determine the cor-rect pickup and slackoff weights for working torque down the hole, aswell as the correct tension to hold when the string shot is fired.

NOTE:

In certain cases where the pipe has backed off high, it may be desirableto pull out of the hole and run an alternative fishing assembly. This willbe decided on a case by case basis following consultation with the drillingsuperintendent.


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Technical Data


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Standard Fishing Tool Inventory

Fishing Tools for 26"-17-1/2" & 12-1/4" holes

8" hydraulic jar, 6-5/8" reg pin x box8" accelerator, 6-5/8" reg pin x box8" fishing bumper sub 6-5/8" reg pin x box11.1/4" overshot c/w extension subs and 15"-22" guides to catch 9-1/2" and 8-1/4DC

and 5" DP with 6-3/8" tool joints9-1/2" junk sub 6-5/8" reg pin x box8-1/8 overshot c/w extension sub and 11"-15" guides to catch5" DP and 6-3/8" tool jointsJ unk mills (as required) 6-5/8" reg. pinADT Hydraulic junk retrieverCrossover subs - as required

Fishing Tools for 8-1/2" Hole

6-1/4" hydraulic jar, 4-1/2" IF pin x box6-1/4" accelerator, 4-1/2" IF pin x box6-1/4" fishing bumper sub, 4-1/2" IF pin x box8-1/8" overshot c/w extension subs to catch 5" DP, 6-1/4 DC and 6-3/8" OD tool joints.7" junk sub 4-1/2" reg box x pinBe sure required crossovers are available8-3/8" junk mill 4.1/2" reg pinADT hydraulic junk retriever

Fishing Tools for 6" Hole

4-3/4" hydraulic jar, 3-1/2" IF pin x box4-3/4" accelerator, 3-1/2" IF pin x box4-3/4" fishing bumper sub, 3-1/2" IF pin x boxCrossover subs5-3/4" overshot c/w extension subs to catch 3-1/2" DP and 4-3/4" DC + tool joints5-1/2" junk sub 3-1/2" reg box x pin5-15/16" junk mill, 3.1/2" reg pinADT Hydraulic J unk Retriever

back off procedures

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