In normal overbalanced drilling conditions, a pressure differential exists between the mud column and the formation pressure. In situations where the formation pressure is significantly lower than that exerted by the mud column, the drillstring may be forced against the wellbore wall and, if the pressure difference is large enough and acts over a significantly large area of the pipe, the pipe may become differentially stuck.
The force required (F) exerted against the pipe may be calculated through:
F= Dp x A x Cof.
Dp = differential pressure between the pressure exerted by the mud column and the formation fluid pressure
A = the area of pipe surface in contact with the filter cake
Cof = the coefficient of friction between the pipe and the filter cake.
This force acts in addition to the normal gravitational force acting against the pipe in a deviated hole.
Indications of Differential Rather than Mechanical Sticking
- The pipe was stationary before sticking
- Full circulation is possible
- Permeable formations are known to be open
- There is a significantly overbalanced condition.