1. Harold Vance Department of Petroleum Engineering
Lesson 14
Air, Gas, and Mist Drilling
Read: UDM Chapter
pages
Harold Vance Department of Petroleum Engineering

2. Air, Gas, and Mist Drilling
Circulating Pressures
Harold Vance Department of Petroleum Engineering

3. Circulating Pressures
Calculating standpipe pressure starts with predicting the pressure just below the bit, and working your way back to the surface
Harold Vance Department of Petroleum Engineering

4. Harold Vance Department of Petroleum Engineering
Bit Pressure Drop
As air flows through the jets, it expands in response to the decrease in pressure and it’s velocity increases
Once the pressure drop exceeds a certain level, the air velocity reaches the prevailing speed of sound.
Harold Vance Department of Petroleum Engineering

5. Harold Vance Department of Petroleum Engineering
Bit Pressure Drop
At this point, the air cannot expand any faster and the upstream pressure becomes independent of the downstream pressure.
This implies that under sonic discharge conditions the standpipe pressure is independent of the annular pressure
Harold Vance Department of Petroleum Engineering

6. Harold Vance Department of Petroleum Engineering
Bit Pressure Drop
Harold Vance Department of Petroleum Engineering

7. Harold Vance Department of Petroleum Engineering
Bit Pressure Drop
If the upstream pressure is more than 1.89 times the annulus pressure beneath the bit, flow through the bit will be sonic.
Harold Vance Department of Petroleum Engineering

8. Upstream bit pressure - sonic flow
Harold Vance Department of Petroleum Engineering

9. Upstream bit pressure - sonic flow
Harold Vance Department of Petroleum Engineering

10. Upstream bit pressure sub-sonic flow
Harold Vance Department of Petroleum Engineering

11. Upstream bit pressure sub-sonic flow
Harold Vance Department of Petroleum Engineering

12. Upstream bit pressure sub-sonic flow
Harold Vance Department of Petroleum Engineering

13. Harold Vance Department of Petroleum Engineering
Standpipe pressure
Harold Vance Department of Petroleum Engineering

14. Steps to predicting standpipe pressure
Assess whether flow through bit is sonic or sub-sonic
If sonic, the pressure above the bit is determined with equation 2.21
This value is used in equation 2.25 to predict standpipe pressure
Harold Vance Department of Petroleum Engineering

15. Steps to predicting standpipe pressure
If flow is sub-sonic, the annulus pressure below the bit must be first predicted (Angel’s analysis, etc) using equation 2.12.
The pressure above the bit is determined by equation 2.23
This value is used in Equation 2.25 to determine standpipe pressure
Harold Vance Department of Petroleum Engineering

16. Harold Vance Department of Petroleum Engineering
Important point
When air drilling, large changes in annulus pressure may result in smaller changes in standpipe pressure, or in the case of sonic flow through the bit, no change in standpipe pressure at all.
Hole problems that lead to an increase in annulus pressure may be indicated by small changes in standpipe pressure.
Harold Vance Department of Petroleum Engineering

17. Harold Vance Department of Petroleum Engineering
Important point
It is very important to monitor the standpipe pressure closely and react promptly to unanticipated changes.
It is important to know if flow through the bit is sonic or not.
If flow is sonic, standpipe pressure will not change with changes in annulus pressure
Harold Vance Department of Petroleum Engineering

18. Harold Vance Department of Petroleum Engineering
Example
8.1/2” hole at 6000’ drilled with 4 1/2” drillpipe air rate is 1400 scfm. Penetration rate ranges up to 300 ft/hr. Bit has no nozzles in one example and 3- 14’s in the other.
Harold Vance Department of Petroleum Engineering

19. Harold Vance Department of Petroleum Engineering