Page 558 - Air and Gas Drilling Manual

Chapter 10: Stable Foam drilling 10-83
back to the 9 5/8 inch casing at 7,304 ft. The 7 inch liner was run from 7,304 ft to
8,859 ft. An openhole was drilled out of the bottom of the 7 inch liner to a depth of
9,571 ft. The stable foam was used to reduce the bottomhole pressure and allow
underbalanced drilling. The openhole interval (from 8,859 ft to 9,571 ft) was drilled
with an 6 inch tri-cone roller cutter drill bit. The drill string while drilling at a
depth of approximately 9,381 ft was made up of 5 inch drill pipe from surface to
7,361 ft, 3 1/2 inch drill pipe from 7,381 ft to 8,361 ft, 3 1/2 inch heavy weight
drill pipe from 8,361 ft to 8,841 ft, and 4 3/4 inch drill collars from 8,841 ft to
9,381 ft. The incompressible fluid for the foam was 8.60 lb/gal treated water. The
treated water was injected at a rate of 45 gal/min. The drilling gas was inert
atmospheric air (specific gravity of approximately 0.97) with an injection volumetric
flow rate of approximately 1,500 acfm (surface elevation location of approximately
3,700 ft). Using the major and minor fluid flow lose calculations as described in
this chapter, the predicted surface injection pressure for the stable foam drilling fluid
was calculated to be 2,830 psig. The actual field injection pressure was
approximately 3,600 psig. The predicted bottomhole pressure was 1840 psig and
the actual bottomhole pressure was approximately 2,000 psig (surface choke back
pressure was approximately 600 psig). The predictions values are lower than the
actual field recorded values. The stable foam calculations are not as accurate as those
for air drilling operations or those for aerated drilling operations.
This comparison calculation was carried out using the same drill pipe body and
drill pipe tool joints lumped geometry approximations as those in Illustrative
Example 10.2 above. The success of these calculations depends upon the careful
inclusion of all major and minor fluid flow friction losses.
10.7 Conclusions
The discussion in this chapter has concentrated on direct circulation operations.
Stable foam drilling operations are generally restricted to direct circulation
operations. Reverse circulation stable foam operations are used to carryout oil and
natural gas well clean out (workover) maintenance.
The demonstration calcuations in this chapter have utilized lumped geometry
approximations for the drill pipe body and drill pipe tool joints. Such
approximations appear to adequately model the overall friction resistance in the
circulation system and give accurate results for bottomhole and injection pressures.
An improvement to this drill string geometry approximation technique can be made
by programming each tool joint indivdually at its proper location in the drill string.
This type of program would be best carried out using a higher level computer
language such as C++ or FORTRAN. Such a programmed solution would improve
the detail pressure versus depth accuracy of the model. However, a comparison of
this type of program gives very little change in bottomhole and injection pressures,
and in the required volumetric flow rate of gas.
References

1. Hutchison, S. O., “New Approach for Producing and Repairing Wells,”
Proceedings 40th Annual California Region Meeting of the Society of Petroleum
Engineerings of AIME, 1969.