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 formations for example are sand stone, lime, carbonate, etc.
Typically mud weight in the well is more than formation pressure. The more overbalance in the wellbore, more chance of getting differential sticking.
Poor and thick filter cake increases the chances of sticking the drill string.
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 pipe.
Stuck Identification for Differential Sticking
- 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 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
Force is in lbs.
Differential pressure is in psi.
Cross section area is in square inches.
Force = (4,500 – 3,800) x 1,500
Force = 1,050,000 lbs.
This is massive !!!
If it is assumed a coefficient friction of 0.5, axial force to over come the sticking force can be estimated by the formula below.
From the basic of physic,
F= coefficient friction x N
F is force to pull.
N is reactive force. For this calculation, the reactive force is the force by differential of hydrostatic pressure.
For this scenario, N is equal to differential force.
F = 0.5 x 1,050,000 = 525,000 lbs.
You need over pull of 525,000 lbs. to fee the pipe from this situation. This is still massive
What should you do for this situation?
- Apply torque into the drill string and jar down with the maximum allowable trip load
- Jar up without apply torque in the drill string.
- Spot light weight pill to decrease hydrostatic pressure. It is imperative that the hydrostatic pressure must be still over formation pore pressure. Otherwise, the well control situation will be accidentally introduced.
Preventive actions are as follows:
- Do not use too high of mud weight
- Do not stop moving string for a period of time, especially, when the BHA is across permeable formations.
- Keep mud properties in good shape. Low specification drilling mud will create a thick mud cake which can be a big impact for the differential sticking.
- Minimize length of BHA if possible.
- Use spiral drill collar and heavy weight drill pipe to reduce contact area.
John Mitchell Drilbert Engineering, 2002. Trouble-Free Drilling Volume 1: Stuck Pipe Prevention. Edition. Drilbert Engineering Inc.
Fanarco.net. 1999. Stuck Pipe Prevention Self-Learning Course. [ONLINE] Available at: http://www.fanarco.net/books/drilling/stuck-pipe.pdf. [Accessed 21 June 2016].