Page 658 - Air and Gas Drilling Manual

12-22 Air and Gas Drilling Manual
casing. The cased hole was kicked off at a MD of 5,557 ft so that the bottom of the
casing had a build angle of 20˚ to vertical. A 4 3/4 inch diameter PDM with a 6 1/4
inch drill bit was used to drill a medium radius curved segment of the borehole with
an arc length of 666 ft to a final angle to vertical of 88˚. Also in this well an PDM
was used to drill the horizontal segment of the borehole to a length of 1,937 ft in the
Dakota sand formation. The drill string was made up of 3 1/2 inch heavy weight
drill pipe in the vertical section of the well and API 3 1/2 inch drill pipe in the
curved and horizontal section of the well. The drilling rate in the horizontal segment
of the borehole was 25 ft/hr. Inert atmospheric air was used as the drilling fluid.
The volumetric flow rate injected into the well was 2,800 acfm (the surface location
elevation was approximately 6,200 ft). The predicted surface injection pressure was
539 psig and the actual injection pressure was 550 psig.
12.7 Conclusions
The analytic models developed in Chapter 8 for vertical wells and in this chapter
for directional wells appear to be accurate in predicting injection pressures and likely
bottomhole pressures for air (and other gases) operations.
As in preceding chapters, the demonstration calculations 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 individually 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. Allan, P. D., “Nitrogen Drilling System for Gas Drilling Applications,” SPE
28320. Presented at the SPE 69th Annual Technical Conference and Exhibition,
New Orleans, Louisiana, 25-28 September 1994.

2. Zabetakis, M. G., “Flammability Characteristics of Combustible Gases and
Vapors,” Bureau of Mines Bulletin 627, Washington D. C., 1964.

3. Coward, H. F., and Jones, G. W., “Limits of Flammability of Gases and
Vapors,” Bureau of Mines Bulletin 503, Washington D. C., 1952.
4. Marcus, R. D., Leung, L. S. , Klinzing, G. E., and Rizk, F., Pneumatic
Conveying of Solids, Chapman and Hall, 1990.