Page 89 - Hydrocarbon

76 Drilling Problems

4.7. Drilling Problems

Drilling equipment and drilling activities have to be carried out in complex
and often hostile environments. Surface and subsurface conditions may force the
drilling rig and crew to operate close to their limits. Sometimes non-routine or
unexpected operating conditions will reach the rating of equipment and normal
drilling practices may not be adequate for a given situation. Thus, drilling problems
can and do occur.

4.7.1. Stuck pipe
This term describes a situation whereby the drill string cannot be moved up or
down or rotated. The pipe can become stuck as a result of mechanical problems
during the drilling process itself or because of the physical and chemical parameters
of the formation being drilled. Most common reasons for stuck pipe are as follows:
Excessive pressure differentials between the borehole and the formation. For
instance, if the pressure of the mud column is much higher than the formation
pressure, the drill pipe may become ‘sucked’ against the borehole wall (differential
sticking). This often happens when the pipe is stationary for some time, for
example whilst taking a deviation survey. Prevention methods include reduced
mud weights, addition of friction reducing components to the mud, continuous
rotation/moving of string, addition of centralisers or use of spiral DCs to
minimise contact area between string and formation, or a low fluid loss mud
system.
Some clay minerals may absorb some of the water contained in the drilling mud.
This will cause the clays to swell and eventually reduce the borehole size to the
point where the drill pipe becomes stuck. Prevention is through mud additives,
for example potassium salt, which prevent clay swelling.
Unstable formations or a badly worn drill bit may result in undergauged holes. An
example of an unstable formation is salt which can ‘flow’ whilst the drilling is in
progress, closing around the drill pipe. Prevention is by addition of stabilisers and
string reamers to the drilling assembly.
Residual stresses in the formation, resulting from regional tectonic forces may
cause the borehole to collapse or deform, resulting in stuck pipe. Sometimes high
mud weights may help delay deformation of the borehole.
If the well trajectory shows a severe dogleg (sudden change in angle or direction),
the movement of the string may result in a groove cut into the borehole wall by
the drill pipe. Eventually the pipe will become stuck, a process termed key seating
(Figure 4.26). The best prevention is the avoidance of doglegs, and frequent
reaming, insertion of stabilisers on top of DCs or insertion of key seat wipers in
the string (string reamers).
In many cases, the point at which the pipe is stuck can be determined by means
of a ‘free point indicator tool’, a special electrical strain gauge device run on
wireline inside the drill pipe which will measure axial and angular deformation.