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natural gas drilling can cause formation damage in low pore pressure reservoirs, but
good drilling string design can usually avoid such damage.
8.8 Conclusions
The discussion in this chapter has concentrated on direct circulation operations.
It has been tacitly assumed that there are few reverse circulation operations deeper
than the 3,000 ft depth criteria set by this book (see Chapter 5). But it should be
noted that for those rare drilling situations where reverse circulation is used to drill
beyond 3,000 ft, the discussions given above for major and minor losses, water
injection, and drilling and completions problems are, in general, applicable to
reverse circulation operations.
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. Lyons, W. C., Air and Gas Drilling Manual, First Edition, Gulf Publishing
Company, 1984.
2. Underbalanced Drilling Manual, Gas Research Institute Publication, GRI
Reference No. GRI-97/0236, 1997.

3. Angel, R. R., “Volumetric Requirements for Air or Gas Drilling,” Petroleum
Transactions, AIME, 1957.
4. Angel, R. R., Volume Requirements for Air and Gas Drilling, Gulf Publishing
Company, 1958.
5. Wolcott, D. S., and Sharma, M. P., “Analysis of Air Drilling Circulating
Systems with Application to Volume Requirement Estimations,” SPE Paper
No. 15950, Proceedings of SPE Eastern Regional Meeting, Columbus, Ohio,
November 12-14, 1986.
6. Ikoku, C. U., and Williams, C. R., “Drill Cuttings Transport in Vertical Annuli
by Air, Mist and Foam in Aerated Drilling Operations,” Contracts for Field
Projects and Supporting research on Enhanced Oil Recovery and Improved
Drilling Technology: Progress Review No. 24, February 1981.