Pore pressure in shale is more than hydrostatic pressure; however the well does not flow because shale is in permeable. While drilling through pressured shale formation, pressure in shale causes fractures of shale due to stress crack.
Shale finally falls into the well and results in stuck pipe
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 stuck due to geo-pressured shale
• It could be happened 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 maximum allowable torque and jar down with maximum trip load.
3. If you are tripping in hole, jar up with maximum trip load without applying any torque.
4. Attempt until pipe free and circulate to clean wellbore.
1. Use proper mud weight to create over balance. You may need to weight up prior to drilling in to high pressure shale zones.
2. Minimize surge pressure and equivalent circulating density (ECD) in the wellbore.
Reference: Drilling formulas
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