Fig. 9.30. Pressure differential sticking of drill pipe. Stuck pipe results when it becomes motionless against permeable bed, due to hole-formation Ap acting on isolated area.
Fig. 9.31. Procedure for calculating free point of stuck pipe.
Consider Figure 9.31, which shows the pipe stuck at some depth, L. The following procedure is carried out:
- 1) An upward force Fx is applied to the pipe. This must be greater than the total pipe weight, to insure that the entire string is in tension.
- 2) A reference point is marked on the pipe at the surface — normally this is done at the top of the rotary table.
- 3) A larger upward force F2 is applied causing the free portion of the drill string to stretch by an amount e. The stretch is measured as the movement of the reference point in (2). F2 is of course limited by the yield strength (elastic limit) of the pipe.
These measurements facilitate calculation of L, as is shown by the following derivation.
Recall the definition of Young's modulus for steel:
S = unit stretch or strain, length per length
where e = inches of stretch measured L = depth to stuck point, ft
The stress causing the differential stretch e is
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