Page 91 - Air and Gas Drilling Manual

Chapter 3: Downhole Equipment 3-19
Drill collars are usually fabricated from American Iron and Steel Institute (AISI)
4140 or 4145 heat treated steel. These are chrome-molybdenum steel alloys and
have yield stresses in excess of 100,000 psi. In addition to drill collars fabricated of
steel are special drill collars fabricated of nonmagnetic nickel alloys (e.g., usually
Monel K-500). These nickel alloy drill collars (usually three) are used at the bottom
of the drill string to allow magnetic compass like equipment to be used to survey
the borehole as the well is drilled. These nickel alloy drill collars have material
properties that are almost identical to that of the AISI 4140 heat treated steel of the
standard drill collars.
3.3.2 Stabilizers and Reamers
Stabilizers and rolling cutter reamers are special thick walled drill collar subs
that are placed in the BHA to force the drill collars to rotate at or near the center of
the borehole. By keeping the drill collars at or near the center of the borehole the
drill bit will drill on a nearly straight course projected by the center axis of the rigid
BHA. Stabilizers and rolling cutter reamers have blades or rolling cutters that
protrude from the sub wall into the annulus to near the borehole diameter. The space
between the blades or rolling cutters allows the air or natural gas flow with entrained
rock cuttings to return to the surface nearly unobstructed.
Figure 3-17 shows three rotating blade stabilizers. These three stabilizers are
respectively, the integral blade (usually a spiral blade configuration) stabilizer, the
big bear stabilizer (a larger type integral blade stabilizer), and the welded blade
(spiral blade) stabilizer. The blades on these three stabilizers are machined into
(integral) the stabilizer body, or are rigidly attached to the stabilizer body and,
therefore, rotate with the body of the stabilizer and, thus, with the drill string itself.
Figure 3-18 shows two sleeve type of blade stabilizers. These stabilizers have
replaceable sleeves (with blades). These two stabilizers are respectively, the sleeve
type stabilizer, and the rubber sleeve stabilizer. The sleeve type stabilizer has a
metal sleeve with the attached metal blades (sleeve rotates) and can be replaced on
the stabilizer body when the blades wear. The rubber sleeve stabilizer has a sleeve
that has a rubber sheath over a metal substructure (sleeve does not rotate). The
rubber sleeve can be replaced on the stabilizer body when the blades wear.
In general, the rotating blade stabilizers are shop repairable. The integral blade
stabilizers have gauge protection in the form of tungsten carbide inserts, or
replaceable wear pads. Integral blade stabilizers can be used in abrasive, hard rock
formations. When the blades are worn, the stabilizers can be returned to the machine
shop and the inserts or wear pads replaced. Welded blade stabilizers are not
recommended for use in abrasive, hard rock formations. When their blades become
worn or damaged they can be returned to the machine shop for repairs.
Non-rotating blade stabilizers can be repaired at the drilling rig location. The
worn sleeves can be removed and new ones placed on the stabilizer body. This is an
important advantage over the rotating stabilizer. The non-rotating stabilizer is most
effective in abrasive, hard rock formations since the sleeve is stationary and acts like
a drilling bushing. This action decreases wear on the metal sleeve blades.
Stabilizers are used extensively to improve the straight hole drill capability of a
BHA for both mud drilling operations and for air drilling operations. However, care
must be exercised in using stabilizers in air drilling operations. The wear rate on