Overburden stress increases over depth (the more a well is drilled, the more overburden stress will be seen). When mud weight is not enough to support the overburden, the stress from the overburden will create shale fractures, which will then fall down into the wellbore. Finally, shale fractures will pack the wellbore and cause a stuck pipe incident,
Mud weight is high enough to overcome overburden stress.
Mud weight is not high enough to overcome overburden stress.
Warning signs of overburden stress shale
• Torque and drag increase.
• Pump pressure increase.
• Abnormal amount of shale at shale shakers
• Caving shape of shale at shakers
Indications when you stuck due to overburden stress shale
• It could happen either while tripping or drilling (most likely while drilling).
• When it happens, the hole may be completely packed off or bridged off; therefore, circulation is very difficult or impossible to establish.
What should you do for this situation?
1. Attempt to circulate with low pressure (300-400 psi). Do not use high pump pressure because the annulus will be packed harder and you will not be able to free the pipe anymore.
2. If you are drilling or POOH, apply the maximum allowable torque and jar down with the maximum trip load.
3. If you are tripping in a hole, jar up with the maximum trip load without applying any torque.
4. Attempt until the pipe is freed and circulate to clean wellbore.
Preventive actions:
1. Use drilling mud that is heavy enough to stabilize overburden stress.
2. Weight up the mud prior to drilling into stressed shale zones.
References
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
Steve Devereux, 2012. Drilling Technology in Nontechnical Language, 2d Ed.. 2 Edition. PennWell Corp.
