Shale instability happens when shale formation becomes unstable and finally, formations break apart and fall into an annuls.
Water in the mud absorbed by shale formations causes a swelling effect on formations. When there is a lot of water, shale will not be able to hold their particles together and finally will fall apart into the well. Finally, shale particles will jam a drill string.
The shale instability is a chemical reaction which is time dependent. It means that it may not be seen on the first day but it will be able to see after drilling has been carried out for several days.
The three illustrations below will help you get more understanding about shale instability and stuck pipe.
Day#1: Water is absorbed by shale
Day#2: Shale swelling due to water and shale starts falling apart.
Day#4: A lot of shale falls down and causes stuck pipe
Warning signs of shale instability
• Torque and drag increase. An over pull may be observed.
• Mud properties became worse. You will see an increase in plastic viscosity, yield point (drilling mud becomes thicker).
• Pump pressure increases.
• Observe soft shale over shale shakers.
Indications when you stuck due to shale instability
• When it happens, you may observe very high pump pressure at a small rate and sometimes circulation may be impossible.
• Most of the time it will happen when pulling out of a hole. However, it can be possibly occurred while drilling as well.
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 is free and circulate to clean wellbore.
Preventive actions
1. For water based mud – you may need to add some salts that are compatible with a mud formula in order to reduce chemical reaction between water and shale. Moreover, you should consider adding some coating polymers to prevent water contact with formation.
2. Use oil based mud instead of water based mud because oil will not react with shale.
3. Keep good flow rate to ensure good hole cleaning.
4. Perform back reaming and/or wiper trip.
5. Keep good mud properties.
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
