Page 87 - Air and gas Drilling Field Guide 3rd Edition

78 CHAPTER 4 Downhole Equipment

to adjust positions of the stabilizers in the BHA. These shorter drill collar lengths
are selected from the Range 1 stock of drill collars. Dimensions and mechanical
properties for API drill collars are given elsewhere [1].
Drill collars are usually fabricated from American Iron and Steel Institute (AISI)
4140 or 4145 heat-treated steel or equivalent. 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 equip-
ment 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 AISI
4140 heat-treated steel of the standard drill collars.

4.3.2 Stabilizers and Reamers
Stabilizers and rolling cutter reamers are special thick-walled drill collar subs
that areplaced in theBHA to forcethe drill collars to rotate at or near the cen-
ter 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 cen-
ter 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 blades or rolling cutters allows the air
or natural gas flow with entrained rock cuttings to return to the surface nearly
unobstructed.
Figure 4-17 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 4-18 shows two sleeve types 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 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