Peh: Drilling Problems and Solutions Publication Information Petroleum Engineering Handbook

Fig. 10.1—Differential-pressure sticking.
....................(10.1) 
Pipe Sticking
Differential-Pressure Pipe Sticking

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The pull force, F
p
, required to free the stuck pipe is a function of the differential pressure, Δp; the
coefficient of friction, f; and the area of contact, A
c
, between the pipe and mudcake surfaces. 
....................(10.2) 
From Bourgoyne
[1]
, 
....................(10.3) 
where 
....................(10.4) 
In this formula, L
ep
is the length of the permeable zone, D
op
is the outside diameter of the pipe,
D
h
is the diameter of the hole, and h
mc
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. 
Eqs. 10.2 and 10.3 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), and 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. 
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; and uninterrupted drilling-fluid circulation.
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, and washing over the stuck

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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, and placing a
packer in the hole above the stuck point.
The causes of mechanical pipe sticking are inadequate removal of drilled cuttings from the
annulus; borehole instabilities, such as hole caving, sloughing, or collapse; plastic shale or salt
sections squeezing (creeping); and key seating.