Key Seat: Key seats occur when the drillstring rubs against the formation on the inside of a dogleg. The drillstring is held against the wellbore by tension as pipe rotation and movement wear a narrow groove in the side of the hole. The longer the interval below the dogleg and the more severe the dogleg, the greater the side load and the faster the development of a key seat. Key seat sticking occurs when the pipe becomes wedge into the narrow slot of the key seat as it is pulled up and the sticking coming up when the pipe is moving on. Pipe stuck in a key seat often can be freed by jarring downward, especially if sticking occurs while picking up.
Generally, the higher the change in well angle or direction, the higher the risk of mechanically sticking the pipe. «S»- shaped wells are even worsen and put an additional risk of pipe sticking due to increases in friction and drag.
Casing failures: Casing-related failures can stick the drillstring. The casing will collapse if external pressure exceed the casing strength. This situation happens often opposite plastic formations. Salt formations become increasingly plastic with pressure and temperature, and are often associated with collapsed casing.
If the casing is not cemented properly, the bottom joint or joints can be unscrewed by drillstring rotation. If this occurs, the casing below the unscrewed connection may drop and turn at an angle in the hole, catching the drillstring.