Peh: Drilling Problems and Solutions Publication Information Petroleum Engineering Handbook

Mechanical Pipe Sticking

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an increase in torque, and, in some cases, no fluid return to surface. Fig. 10.3 illustrates pipe
sticking caused by wellbore instability. 
Fig. 10.3—Pipe sticking caused by wellbore
instability.
Key Seating. Key seating is a major cause of mechanical pipe sticking. The mechanics of key
seating involve wearing a small hole (groove) into the side of a full-gauge hole. This groove is
caused by the drillstring rotation with side force acting on it. Fig. 10.4 illustrates pipe sticking
caused by key seating. This condition is created either in doglegs or in undetected ledges near
washouts. The lateral force that tends to push the pipe against the wall, which causes mechanical
erosion and thus creates a key seat, is given by 
Fig. 10.4—Pipe sticking caused by key seat.
....................(10.5) 
where F
l
is the lateral force, T is the tension in the drillstring just above the key-seat area, and
ϴ
dl
is the abrupt change in hole angle (commonly referred to as dogleg angle). 

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Generally, long bit runs can cause key seats; therefore, it is common practice to make wiper trips.
Also, the use of stiffer BHAs tends to minimize severe dogleg occurrences. During tripping out of
hole, a key-seat pipe-sticking problem is indicated when several stands of pipe have been pulled
out, and then, all of a sudden, the pipe is stuck. 
Freeing mechanically stuck pipe can be undertaken in a number of ways, depending on what
caused the sticking. For example, if cuttings accumulation or hole sloughing is the suspected
cause, then rotating and reciprocating the drillstring and increasing flow rate without exceeding
the maximum allowed equivalent circulating density (ECD) is a possible remedy for freeing the
pipe. If hole narrowing as a result of plastic shale is the cause, then an increase in mud weight
may free the pipe. If hole narrowing as a result of salt is the cause, then circulating fresh water
can free the pipe. If the pipe is stuck in a key-seat area, the most likely successful solution is
backing off below the key seat and going back into the hole with an opener to drill out the key
section. This will lead to a fishing operation to retrieve the fish. The decision on how long to
continue attempting to free stuck pipe vs. back off, plug back, and then sidetrack is an economic
issue that generally is addressed by the operating company.
Lost circulation is defined as the uncontrolled flow of whole mud into a formation, sometimes
referred to as thief zone. Fig. 10.5 shows partial and total lost-circulation zones. In partial lost
circulation, mud continues to flow to surface with some loss to the formation. Total lost
circulation, however, occurs when all the mud flows into a formation with no return to surface.
If drilling continues during total lost circulation, it is referred to as blind drilling. This is not a
common practice in the field unless the formation above the thief zone is mechanically stable,
there is no production, and the fluid is clear water. Blind drilling also may continue if it is
economically feasible and safe.