BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
drilling fluid properties (downhole filtrate and rheological properties) should receive special
consideration.
Other Causes of Drillpipe Sticking and Remedial Correction
Key Seating
Key seating is another possible cause of pipe sticking. Key seats are the result of sharp hole angle
changes causing the drillpipe rotation to wear a groove on one side of the hole. This groove
would be the approximate diameter of the drillpipe or tool joints. When pulling out of the hole,
the drill pipe and tool joints may pull through the groove, but the larger diameter drill collars
would become stuck.
To avoid key seating, the most important thing to do is minimize abrupt angle changes. When
drilling directional wells, the rate of deviation change (dogleg severity) should be controlled to
avoid any sharp changes in direction or angle. String reamers can be used to smooth out abrupt
angles.
The development of a key seat may not occur over the course of one bit run, but rather over a
number of bit runs. Evidence of key seat development can be observed when the drill pipe
encounters increasingly excessive drag in the same section of hole on repetitive trips.
When key seating does occur and the pipe becomes stuck, the following corrective measures are
suggested.
• Work pipe downward (if possible) while pumping.
• Avoid excessive pull on the drill string as this will in all probability compound the problem by
pulling pipe further into the key seat.
• Increase the concentration of friction reducing materials in the fluid (see Table 7-6).
• In some cases a special “driving” tool consisting of a length of casing concentric around a
second joint of smaller diameter casing with the annulus full of cement (for weight) is used to
“hammer” the drill string downward and out of the key seat. The inside length of casing has an
ID large enough to slide over a drill pipe guide and impact on a robust seat (“anvil”) mounted
on a sub inserted into the stuck drill pipe.
Foreign Objects
Foreign objects entering the wellbore may also result in stuck pipe. In this case, friction reducing
materials will probably not help. Working the pipe up and down and rotating when possible are
primary remedial actions.
Formation Caving
Formation caving can also result in stuck drillstrings. The severity of sticking is affected by
quantity and size of cavings and how well the drilling fluid is cleaning the hole. The remedial
action to be taken is to start circulating to clean the hole and run sweeps to improve cleaning
action.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-42