Pore pressure in shale is more than hydrostatic pressure; however, the well does not flow because shale is impermeable. While drilling through pressured shale formation, pressure in shale causes fractures of shale due to stress cracks.
Pressure in shale causes fractures of shale due to stress cracks.
Shale finally falls into the well and results in stuck pipe incident.
Shale falling into the well.
Warning signs of geo-pressured shale
• Shale fractures are seen at shale shakers.
• Possibly observe change in d-exponent, which means a sign of abnormal pressure, from mud logger.
• Pump pressure abnormally increases.
• Increase in rate of penetration (ROP).
• When compared to a normal trend, torque and drag trend abnormally increase.
• Background gas may increase.
Indications when you are stuck due to geo-pressured shale
• It could happen either while tripping or drilling.
• When it happens, the hole may be completely packed off; therefore, circulate is restricted or impossible in some cases.
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 proper mud weight to create over balance. You may need to weight up prior to drilling into high pressure shale zones.
2. Minimize surge pressure and equivalent circulating density (ECD) in the wellbore.
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.
