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Differential-pressure pipe sticking
Differential-pressure pipe sticking occurs when a portion of the drillstring becomes embedded in a mudcake (an impermeable film of fine solids) that forms on the wall of a permeable formation during drilling.
If the mud pressure, pm , which acts on the outside wall of the pipe, is greater than the formation-fluid pressure, pff , which generally is the case (with the exception of underbalanced drilling), then the pipe is said to be differentially stuck (see Fig. 1).
The differential pressure acting on the portion of the drillpipe that is embedded in the mudcake can be expressed as:
The pull force, Fp, required to free the stuck pipe is a function of the differential pressure, Δp; the coefficient of friction, f; and the area of contact, Ac, between the pipe and mudcake surfaces.
From Ref. 1.,
In this formula, Lep is the length of the permeable zone, Dop is the outside diameter of the pipe, Dh is the diameter of the hole, and hmc is the mudcake thickness. The dimensionless coefficient of friction, f, can vary from less than 0.04 for oil-based mud to as much as 0.35 for weighted water-based mud with no added lubricants.
Pipe sticking force
Eqs. 2 and 3 show controllable parameters that will cause higher pipe-sticking force and the potential inability of freeing the stuck pipe. These parameters are:
- Unnecessarily high differential pressure
- Thick mudcake (high continuous fluid loss to formation)
- Low-lubricity mudcake (high coefficient of friction)
- Excessive embedded pipe length in mudcake (delay of time in freeing operations)
Although hole and pipe diameters and hole angle play a role in the pipe-sticking force, they are uncontrollable variables once they are selected to meet well design objectives. However, the shape of drill collars, such as square, or the use of drill collars with spiral grooves and external-upset tool joints can minimize the sticking force.
Indicators of differential stuck pipe
Some of the indicators of differential-pressure-stuck pipe while drilling permeable zones or known depleted-pressure zones are:
- An increase in torque and drag
- An inability to reciprocate the drillstring and, in some cases, to rotate it
- Uninterrupted drilling-fluid circulation
Prevention or mitigation of differential stuck pipe
Differential-pressure pipe sticking can be prevented or its occurrence mitigated if some or all of the following precautions are taken:
- Maintain the lowest continuous fluid loss adhering to the project economic objectives.
- Maintain the lowest level of drilled solids in the mud system, or, if economical, remove all drilled solids.
- Use the lowest differential pressure with allowance for swab and surge pressures during tripping operations.
- Select a mud system that will yield smooth mudcake (low coefficient of friction).
- Maintain drillstring rotation at all times, if possible.
Differential-pressure-pipe-sticking problems may not be totally prevented. If sticking does occur, common field practices for freeing the stuck pipe include:
- Mud-hydrostatic-pressure reduction in the annulus
- Oil spotting around the stuck portion of the drillstring
- Washing over the stuck pipe
Some of the methods used to reduce the hydrostatic pressure in the annulus include:
- Reducing mud weight by dilution
- Reducing mud weight by gasifying with nitrogen
- Placing a packer in the hole above the stuck point
- Bourgoyne, A.T., Millheim, K.K., Chenevert , M.E. et al. 1986. Applied Drilling Engineering. Richardson, Texas: Textbook Series, SPE.