Page 655 - Air and Gas Drilling Manual

Chapter 12: Directional Drilling Operations 12-19
top of the lumped geometry tool joints that is above the BHA (i.e., approximately
150 ft from the bottom of the openhole section of the well).
Sliding the Drill String
As was discussed above, when the drill string is slid in the openhole (when a
directional correction must be made), the drill string will lay on the low side of the
horizontal openhole. If the flow area at the top of the drill string in the horizontal
segment of the openhole is assumed to have the geometry discussed in Illustrative
Example 12.1, then the flow velocity and the kinetic energy per unit volume will be
high in this space. Thus, the minimum kinetic energy per unit volume will shift to
a new critical annulus cross-section. This new critical annulus cross-section is just
up-hole from the top of the lumped geometry tool joints in the cased section of the
well (i.e., approximately 350 ft from the bottom of the cased section of the well).
The approximate volumetric flow rate of inert atmospheric air that will give an
3
annulus kinetic energy per unit volume of 3.0 ft-lb/ft at this new critical cross-
section is 1,350 acfm.

Figure 12-8: Directional well profile for Illustrative Examples 12.2 and 12.3.