FISHING TECHNOLOGY
BACK-OFF Procedures
th
Bernie Springer, Abu Dhabi, May 5 , 2002
A.
Safety Precautions
While applying left hand torque with the rotary slips, the slip handles are to be tied together with a length of wire rope. This will ensure that the slips are not thrown out of the rotary in case the pipe breaks high, causing the pipe to jump due to the instantaneous loss of strain. In cases where the wellbore fluids are not balanced between the inside and outside of the drill string, or where it is suspected that formation fluids may have migrated into the 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 while making 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 assembly. 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 technique requires 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 using the electronic free point indicator. However, the calculated free point from pipe stretch data is useful in planning fishing and subsequent operations since the actual free 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 holes where the impact of hole drag is minimal. This method may also be used in directorial wells, but will probably underestimate true length of free pipe due to the effects of hole 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 a convenient reference point. Apply an overpull of 10,000 lbs and return to the normal pickup weight. Make a second mark on the Kelly. The second mark should be distinct from the first. The difference is caused by the friction of the string 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. Increase overpull by 10,000 lbs (but do not exceed 80% of the minimum yield strength of the pipe) and return to 50,000 lbs. Make a second mark on the Kelly. Draw a datum line midway between the two marks and set the brake.
1.3.
Measure and record the distance (inch) between the two datum lines (amount of stretch 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: Lf L Ap E P
= = = = = =
length of free pipe, feet length change due to over-pull, inches cross-sectional areas of pipe tube, sq. in. modulus of elasticity, psi 30,000,000 psi for steel 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 of stretch 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 is through the use of an electronic free point indicator. Electronic free point services are offered by Schlumberger and Atlas, as well as a number of other electric logging companies. The technology involves measurement of the axial and torsional strain in the pipe 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 the system's electronics. Once the degree of strain diminishes, or reduces to zero, the pipe is 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 point using electronic instruments. In general, a combination free-point tool / back-off shot should 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 the measuring device. This is critical, particularly in directional wells, where the cable will tend to move upward as the pipe is stretched. Lowering the free point tool 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 needle jerks when the pipe is tensioned, then the expansion joint will have to be collapsed slightly until the correct space-out is achieved for the amount of pipe tension that is applied. 2.2.
If an expanding arm free point tool is used, the expansion joint can still be used, however it is not so critical since these tools firmly anchor the free point tool inside the pipe and are less prone to slippage. Generally the cable is slackened to provide the necessary allowance for pipe movement.
2.3.
When free point readings are being taken, the Drilling Supervisor should be present in the logging unit to witness and supervise the operation. Be certain to maintain a written record of all tool readings as well as tension and torque applied to the drill string.
2.4.
The free point instrument is to be run into the drill string to 1000' below the surface and reference free point readings taken in both tension and torque. These figures will then be used to indicate the degree of free pipe for readings taken close to the stuck point. With the spring bow free point tool used by Schlumberger and Atlas, 100% free pipe is generally taken to be 80 units of meter deflection.
2.5.
Once calibrated, the free point tool is run in to 500 ft above the estimated free point depth calculated using the pipe stretch technique discussed earlier. Once on bottom, tension readings are to be taken first at each measurement point. With the pipe tensioned on surface to the estimated pick-up weight at the stuck point, apply over-pull to the pipe in even load increments (+/- 50,000 lbs) until pipe movement is noted at the free point tool. Smooth deflection of the strain indicating needle should be noted if the tool is functioning properly. If the needle jerks erratically, the cable has probably moved or the springs have slipped causing interference with the instrument reading.
2.6.
Repeat readings as required until a consistent deflection of the free point indicating needle is noted. The repeatability of the reading may not be precisely consistent and, in these cases, the average of the measured readings should be used.
2.7.
The degree of free pipe at any survey point is then taken as a percentage of the free pipe meter deflection measured previously. Half scale deflection would be 50% free, 3/4-scale deflection 75% free, etc.
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2.8.
If the pipe is found to be 100% free at a given survey point, drop down the pipe in +/- 100 ft increments repeating the procedure until the meter deflection reduces to zero.
2.9.
Once the free point in tension has been located, repeat the procedure working up the hole taking torque measurements. Be certain to apply the same amount of torque at each depth, taking note of the number of turns required to achieve the desired torque. Once a reading has been taken at a given depth, completely release the torque prior to taking the next reading.
2.10. The depth where both the torque and tension readings indicate 100% free pipe is taken as the free point. This is the depth, above which the pipe should be backed off. When analyzing stuck point readings, the following guidelines will prove useful: ♦
In a highly deviated well with a significant degree of hole drag, a wide discrepancy between the degree of axial and torsional deflection of the free point meter may be noted. This will likely be due to an inability to work sufficient torque down the hole to achieve full-scale deflection. In this case attempt to work the torque down and repeat the readings. If the torque cannot be worked down to the right, it will also be difficult to work torque down to the left; therefore, a shallower back-off point should be selected.
♦
In cases where the bottom-hole assembly is differentially stuck, a sharp distinction between free pipe and stuck pipe will generally be noted. The length of partially stuck pipe will be short, however the pipe should still be backed off at the point where the pipe is 100% free.
♦
In those cases where the formation has progressively fallen in behind the drilling 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. This should not be attempted, as the time wasted trying to back-off below the 100% 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 backed off and mark the wireline cable for later reference while running in with the 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. Recognizing that this cannot be achieved in practice, it is preferred to have the pipe slightly in tension at the back-off point as opposed to compression. Calculation of the required surface tension, and how this tension is applied prior to firing the string shot is critical to successfully backing off the pipe at the intended depth. If too much tension is applied, or if the pipe is in compression, it will probably not back off. If it does, it will probably be further up the string than planned. This may require that the pipe has to be made up again and the back off procedure repeated. ♦ To determine the necessary surface tension to optimize the tension at the backoff point, the following information is required. ♦ Pickup, slack-off and off bottom rotating weights of the drill sting prior to becoming stuck. These measurements are to be taken with the pumps off. If the pipe became stuck while tripping, the required figures may have to be estimated based on previously recorded pickup, slack-off and rotating weights recorded 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 drilling assembly is stuck. Once the hook load required putting the back-off point in zero tension has been determined, an additional e.g. 5000 lbs of overpull is then applied to ensure that the connection is in tension rather than compression.
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The required additional overpull can be calculated as follows: AO
=
MW * VD * TJA
= = = =
Additional Overpull (lb) Mud weight (ppg) Vertical depth at backoff point (ft) Tool Joint Mating Surface Area (sq. inch)
Where: AO MW VD JA
Tool Joint Mating Surface Area (API Spec 7)
Type of Connection NC 23 NC 26 NC 31 NC 35 NC 38 NC 40 NC 44 NC 46 NC 50 NC 56 NC 61 NC 70 NC 77
Mating Surface Area “JA” inch2 1.653 1.591 2.908 4.609 3.400 4.948 8.151 6.801 6.717 14.116 21.892 23.156 31.678
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Type of Connection 2 3/8 REG 2 7/8 REG 3 1/2 REG 4 1/2 REG 5 1/2 REG 6 5/8 REG 7 5/8 REG
Mating Surface Area “JA” inch2 1.464 2.690 3.095 4.754 8.419 13.290 21.443
5 1/2 FH
8.081
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A generalized equation for determining the correct weight indicator reading when the string shot is fired is as follows: = +
Pick-up Weight Buoyed Weight of Stuck Tools Hole Drag of Stuck Tools e.g. 5,000 lbs Over-pull
Pickup Weight
=
Pickup weight of entire drilling assembly before becoming stuck, lbs
Buoyed Weight of Stuck Tools
=
Buoyed weight of drilling assembly below back-off point taking account of hole inclination, lbs. Air weight x buoyancy factor x cosine of hole angle
Weight Indicator Reading
Where:
= Hole Drag of Stuck Tools
=
Proportion of total pickup drag based on length of stuck tools
The following examples illustrate the correct calculation procedure for determining the required hook load for backing off. 1.
VERTICAL WELL In this example it will be assumed that the drilling assembly has become differentially stuck while drilling on bottom. Well depth: Hole size: Drill String: Mud weight: Buoyancy Factor: Pickup Weight: Slack-off Weight: Rotating Weight: Weight of Blocks: Stuck Point: Avg. Hole Angle:
10,500' MD (vertical well) 12-1/4" 500' x 5" x 150 1b/ft drill collars 450' x 5” x 50 lb/ft HWDP 9,450' x 5” x 19.5 lb/ft S-135 drill pipe 10.5 ppg 0.84 370,000 lbs *) 335,000 lbs *) 350,000 lbs *) 100,000 lbs *) 9,900' MD (top of drill collars) 0 deg.
*) Includes weight of blocks, hook, etc.
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In this example it will be assumed that the hole drag is distributed uniformly along the length of die drilling assembly, that 5,000 lbs of tension will be maintained in the pipe at the back-off point when the string shot is fired, and that the pipe will be backed off at the stuck point (i.e. 9,900' MD). 1.1.
Calculate buoyed weight of drill collars below stuck point: Air Weight of Drill Collars: Buoyancy factor: Buoyed weight: Cosine (0 deg): Net buoyed weight:
1.2.
90,000 lbs 0.84 75,600 lbs 1.00 75,600 lbs
Calculate hole drag of stuck BHA: The hole drag figure to use here is the drag experienced while picking up the drilling assembly since the pipe will be picked up to the correct weight indicator reading to perform the back-off. Total hole drag moving up: Length of drilling assembly: Hole drag per 1,000 feet: Length of stuck pipe: Hole drag along stuck BHA:
1.3.
20,000 lbs 10,500 ft 1,905 lbs 600 ft 1,143 lbs
Calculate required weight indicator reading at time string shot is fired: Total Pickup weight: Minus BHA weight: Minus hole drag: Plus over-pull requirement: Required pickup weight:
370,000 lbs -75,600 lbs - 1,143 lbs + 5,000 lbs 298,257 lbs
Therefore, prior to firing the string shot, the pipe will be slacked off to below the initial slack-off weight and then picked up and tensioned to a weight indicator reading of 298,257 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 all tools and equipment that were suspended below the hook are accounted for in this pickup weight calculation. For instance, if the kelly has been removed, this weight must 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 differentially stuck while making a connection on bottom. Well depth: Hole size: Drill string: Mud Weight: Buoyancy Factor: Pickup Weight: Slack-off Weight: Rotating Weight: Weight of Blocks: Stuck point: Kick-off Point: End of Build: Hole Angle: *)
12,500' MD (directional well) 8-1/2" 400' x 5-1/2" x 90 ppf drill collars 1,000' x 5" X 5O ppf HWDP 11,100’ x 5" x 19.5 ppf S-135 DP 12.5 ppg 0.809 355,000 lbs *) 160,000 lbs *) 295,000 lbs *) 100,000 lbs 12,200' MD (300’ above bit) 2,000' MD 4,000' MD 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 uniformly along the length of the drilling assembly and that 5000 lbs of tension will be maintained 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 the string 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: Cosine of 45 deg.: New air weight of DC: Buoyancy factor: Net buoyed weight of DC fish:
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27,000 lbs 0.707 19,089 lbs 0.809 5,443 lbs
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 up the drilling assembly since the pipe will be picked up to the correct weight indicator reading to perform the back-off. Hole drag moving up: Total length of drilling assembly: Hole drag per 1000 ft (moving up): Length of stuck pipe: Hole drag along stuck pipe:
2.3.
60,000 lbs 12,500 ft 4,800 lbs 300 ft 1,440 lbs
Calculate required weight indicator reading at the time string shot is fired: Pickup weight of drilling assembly: Minus along-hole weight of DC fish: Minus hole drag: Plus over-pull requirement: Required Pick Up Weight:
355,000 lbs - 15,443 lbs - 1,440 lbs + 5,000 lbs 343,117 lbs
Therefore, prior to firing the string shot, the pipe will be slacked off to below the initially recorded slack-off weight and then picked up to a weight indicator reading of 343,117 lbs. For practical purposes this figure can 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 sufficient “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 should be 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 RDX explosive is good to a nominal maximum bottom-hole temperature of 325 deg, while the 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) 0 to 3,000
3,000 to 6,000
6,000 to 9,000
9,000 to 12,000
over 12,000
2-3/8”
1
1
1
2
2
2-7/8”
1
1
2
2
3
3-1/2”
1
2
2
3
3
4” & 4-1/2”
2
2
2
3
3
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
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
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
CONNECTION
TUBING
DRILL PIPE
DRILL COLLARS
CASING
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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 sufficient make-up torque has been applied to the pipe to avoid a shallow back-off or a deep open hole back-off at the incorrect depth. The correct amount of right hand torque to work down the drill string will depend on several factors including well depth, wellbore profile, and degree of hole drag in both tension and torsion. The correct string tension to be applied at the surface in order to effectively work torque down to the back-off point should have been determined while locating the free point. In this regard, it does not necessarily follow that the pipe should be worked between the calculated pickup and slack-off weights at the back-off point. For this reason the torque measurements taken while determining the free point should be used as an indicator of the correct surface tension to use for applying both left hand and right hand torque. When working right hand torque down the hole, the following practices are to be followed: 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 the planned back-off depth plus 30%. This will ensure that maximum left hand torque 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. Since tool joints can only be torqued correctly with minimum axial tension the pipe is to be raised and lowered between the calculated free pipe pickup weight at the planned back-off point and zero surface tension while progressively increasing the 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 maximum make up torque, as well as the number of turns that can be anticipated when the required left hand torque is applied.
4.
Complete make up of all connections will be indicated when no loss of trapped torque occurs as the pipe is cycled between the range of pick-up and slackoff weights 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 possible within the constraint of avoiding a premature or shallow back-off. This is a hazardous operation and safety precautions as outlined must be closely followed. When working left hand torque down the hole the following guidelines and procedures have to be followed. 1.
As a rule of thumb, the amount of left hand torque applied at the surface should not exceed 70% of the right hand makeup torque.
2.
Since a tool joint is more inclined to break when the axial tension on the connection is zero, left hand torque should be worked down the hole stating with maximum surface tension and working progressively down to planned back-off tension. This should be performed in several stages starting with approximately half of the required left hand torque. Only when a given amount of torque has been worked into the entire string, the torque should be increased to the 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 planned back-off point.
4.
The string shot is to be loaded into the drill string prior to applying any torque at surface. Do not load the string shot into the string shot while torque is trapped in the pipe. This could result in serious injury to personnel if the pipe were 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 has been determined, the string shot can be loaded into the pipe in preparation for running downhole. 1.
With the surface torque on the pipe completely released, load the string shot and collar locator assembly into the drill string and run in to 1000 ft below surface. Do not run in deeper since if the pipe were to back-off shallow, the wireline cable may be severely damaged or part downhole resulting in another fishing job.
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NOTE: In cases where a combination free point/back-off shot have been run, the tool assembly should be pulled up to 1000 ft below the rotary table prior to working left hand torque into the string. 2.
Apply left hand torque to the pipe and work it downhole to the planned back-off point. The toque will be completely worked down to the back off point when the number of previously measured left hand turns have been achieved. If holding the torque with the rotary or top drive, make sure to lock the rotary table or power swivel once the final torque is achieved.
3.
Apply left hand torque to the pipe and work it downhole to the planned back-off point. The torque will be completely worked down to the back-off point when the number of previously measured left hand turns have been achieved. If holding the torque with the rotary or a top drive, make sure you lock the rotary table or power swivel once the final torque is achieved.
4.
Run in with the string shot and position the charge opposite 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. If successful, DO NOT unscrew the pipe.
7.
Pull the wireline cable back to +/- 1000 feet.
8.
Release the trapped left hand torque and make note of the number of right hand turns necessary to reduce the torque to zero.
9.
POOH with the wireline and pull the logging cable clear.
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BACKOFF STRING-SHOT Running Hardware
10.
Reduce string tension to anticipate rotating weight at back-off point and rotate pipe to the left to complete back-off. Pick up on string and check the pick-up weight. 10.1. If pipe backed off at planned depth, POOH. 10.2. If the pipe backed off high, screw into connection looking up and repeat entire back-off procedure. Since the pipe backed off high, it may be necessary 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 when the string shot is fired. If the pipe backed off close to the planned back-off point, check the pickup, slackoff and rotating string weights prior to screwing back into the fish These measurements should then be used to determine the correct pickup and slackoff weights for working torque down the hole, as well 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 desirable to pull out of the hole and run an alternative fishing assembly. This will be decided on a case by case basis following consultation with the drilling superintendent.
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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 box 8" accelerator, 6-5/8" reg pin x box 8" fishing bumper sub 6-5/8" reg pin x box 11.1/4" overshot c/w extension subs and 15"-22" guides to catch 9-1/2" and 8-1/4” DC and 5" DP with 6-3/8" tool joints 9-1/2" junk sub 6-5/8" reg pin x box 8-1/8” overshot c/w extension sub and 11"-15" guides to catch 5" DP and 6-3/8" tool joints Junk mills (as required) 6-5/8" reg. pin ADT Hydraulic junk retriever Crossover subs - as required Fishing Tools for 8-1/2" Hole 6-1/4" hydraulic jar, 4-1/2" IF pin x box 6-1/4" accelerator, 4-1/2" IF pin x box 6-1/4" fishing bumper sub, 4-1/2" IF pin x box 8-1/8" 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 pin Be sure required crossovers are available 8-3/8" junk mill 4.1/2" reg pin ADT hydraulic junk retriever Fishing Tools for 6" Hole 4-3/4" hydraulic jar, 3-1/2" IF pin x box 4-3/4" accelerator, 3-1/2" IF pin x box 4-3/4" fishing bumper sub, 3-1/2" IF pin x box Crossover subs 5-3/4" overshot c/w extension subs to catch 3-1/2" DP and 4-3/4" DC + tool joints 5-1/2" junk sub 3-1/2" reg box x pin 5-15/16" junk mill, 3.1/2" reg pin ADT Hydraulic Junk Retriever
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