Differential Sticking Causes #Stuck Pipe

Differential Sticking is one of the most common causes of pipe stuck. It can happen when there is differential pressure (overbalance pressure) pushing a drill string & BHA into permeable formations.
Four Factors Causing the Differential Sticking
Permeable formation
Permeable formations for example are sandstone, lime, carbonate, etc.
Overbalance
Typically mud weight in the well is more than formation pressure. The more overbalance in the wellbore, the more chance of getting differential sticking.
Filter cake
Poor and thick filter cake increases the chances of sticking the drill string.
Pipe movement
If the drill string is stationary for a period of time, the filter cake will tend to develop around permeable zones and the drill string. Therefore, the potential of getting differentially stuck is increased.
Warning Signs of Getting Stuck due to Differential Sticking
There is high over balance between the wellbore and formation. Especially, when there is a highly depleted formation, the chance of getting differentially stuck is very high.
Torque, pick up and slack off weight increase when the drill string is being moved. Once it happens, it may not be able to pull or rotate a pipe.
Stuck Identification for Differential Sticking
The drill string is in a stationary position for a period of time. The differential sticking happens when there is no pipe movement for a long time.
Circulation can be established without increasing the pressure.
BHA is across the permeable zone.
How Much Differential Force Generated from Differential Stuck
The example below demonstrates how much force is generated from differential sticking.
Formation pressure = 3,800 psi
Hydrostatic pressure =4,500 psi
Cross area of stuck pipe = 1,500 square inch
You can determine how much differential force there is based on the following formula:
Force = Differential Pressure x Cross Section Area
Where
Force is in-lbs.
Differential pressure is in psi.
The cross-section area is in square inches.

Eng. Mohsen Fatahi