Avoiding stuck pipe is one of the objectives of the drilling operations because such a problem leads to many additional actions which are very expensive and increase the non-productive time like fishing and sidetracking.
Back off is the first action to perform after trying all the solutions to unstuck the pipe. There are many approach to perform the back off due to its difficulty. There are also other issues which can make back off more complicated like the uncertainty of the location of the free point, the transmission of the surface loads to the back off connection and the level of the torque and the tension which have to be applied while backing off.
Free Point Locating
There are two methods to estimate the depth where the string is stuck
- Stretch test method
- Running free point indicator (FPI)
Stretch Test Method
This is an inaccurate method and it can be used as guide to have an idea about the stuck depth. The uncertainty of stuck point determination increases in deviated wells where the effect of the frictional forces can mislead the calculations.
The stretch test can be performed as follows:
- Pull the string till getting the value of weight of the string in air. Make a mark at the string (Point A)
- Apply an over pull of 40 K Lbs. and make a mark (Point B)
- Release the weight to the original hook load and mark a third point (Point C)
- Apply again an over pull of 40 K Lbs. and mark the fourth point (Point D)
The stretch (e) in this approach will be the distance between the midpoint of (A and C) and the midpoint of (B and D)
The length of the free pipe can be deduced using the correlation below
L= (E e Wdp) / (40.8P)
Where
L = Length of the free pipe
E = Elasticity Modulus ( 30 x 10E6)
e = Stretch in pipe (Inches)
Wdp = pipe weight in air (lbs/ft)
P = Differential pull (Lbs.)
Free Point Indicator (FPI)
The free point indicator is a tool that gives readings of tension and torque and can transmit them to surface. It is run inside the drill string using the wire line. The determination of the free point using this tool is performed by applying surface torque and tension, then the tool measures these two parameters in the string. The part of the string which is free reacts to loading (torque and tension), the point where the string does not react, it is considered as the stuck point. The back off can be performed at the first connection above the stuck point. It has to be mentioned that back off is performed at a point which reacts to both torque and stretch. Normally 80% free reading in both torque and stretch is recommended for best chance of successful back-off, otherwise the use of the pipe cutter tool should be considered.
Backing Off
Backing off means separating the free part of the drill string from the stuck part at the threaded connection above the free point. Drill pipe can be unscrewed down hole by exploding charges inside the drill string at a selected connection.
There are some conditions for a successful back off:
- Sufficient large explosive charge located accurately at the connection. The charge has to be large enough without damaging the threaded connection.
- Zero or slightly positive tension at the back off point.
- Sufficient left hand torque applied at the selected connection which should not exceed 80% of the make-up torque.
Making the back off connection at the ideal tensile load is a difficult task due to the uncertainty of choosing the accurate surface over pull. The debate is about the effect of the buoyancy which is neglected in most approaches, so with this condition the reading of the weight required to maintain the back off connection in tension is the weight of the free part of the drill string in air taking in consideration the drilling jar firing-up load if it is a part of the free side of the drill string.
The required left hand torque for the backing off operation is related to the type, size, depth and the condition of the pipe, but there are some rules of thumb to choose this parameter:
- 0-4000 ft , apply 1/4 to 3/4 turn per 1000 ft
- 4000-9000 ft , apply 1/2 to 1 turn per 1000 ft
- 9000 ft plus , apply 3/4 to 1 turns per 1000 ft
The maximum left hand torque to be applied from surface should not exceed 80% of the drill pipe make-up torque.
The left hand torque can be applied as follows:
- Prior to apply the left hand torque, the string should be torqued to the right. The right hand torque has to be at the level of the make-up torque and transmitted down the string by performing series of pick up-slack off weight.
- The string should be set at the estimated back off weight
- Put the slips and make a mark
- Apply the left hand torque required for the back off
- Remove the slips safely and work the string up and down (Do not exceed down the marked point)
- Prior removing the slips make sure that the brake is working efficiently and it is holding.
- The string work can be performed roughly with 20 Tons pick up.
The cycle of applying left hand torque and working the string should be performed several times to make sure that the torque has moved down to the back off connection.
After applying the planned tension and left hand torque, the charges should be detonated in front of the back off connection. The weight indicator should show a loss of weight due to the loss of the stuck part of the drill string if not the job can be completed by applying left hand torque to unscrew the back off connection.
If the connection do not back off, the operation should be performed for another connection which has to be at least two connection above the previous one. Also, the torque and tension have to be re-estimated for the next tentative.
There are some safety issues related to the back off and some precautions should be taken:
- The tongs and the slips dies should be clean, sharp and in proper size.
- The slips handles have to be tied to prevent slips from being thrown out the rotary table when the pipe is breaks out
- Ensure that no torque is remained in the string before picking up the slips
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