Page 90 - Air and gas Drilling Field Guide 3rd Edition
P. 90
4.3 Bottom Hole Assembly 81
FIGURE 4-19. Three-point rolling cutter reamer (courtesy of Smith International).
under the drill bit gauge or at the drill bit gauge. The reamers provide the near-bit
stabilization needed for straight drilling in abrasive formations.
4.3.3 Downhole Surveying Equipment
Bottom hole assemblies for drill strings used to drill deep vertical wells are usually
fitted with several nonmagnetic drill collars. These drill collars are usually near
the bottom of the BHA (just above the drill bit). These nonmagnetic collars are
needed to carry out the downhole surveys required by most natural resource reg-
ulatory agencies. Downhole surveys are used to describe the deviation of a drilled
borehole from the ideal vertical centerline of the intended well. Knowledge of
where an actual well has been drilled is needed to ensure economic and environ-
mentally safe recovery of mineral resources.
The survey is usually accomplished by using a magnetic single-shot instrument.
This instrument is usually part of the equipment inventory of a typical double and
