STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Bits and Downhole Tools 827
Outer Rubber
Cushion
Cushion Stabilizer
A
B
Figure 4-1 82. Sleeve-type stabilizers. (A) Rotating sleave-type stabilizer
(Servco). (B) Grant cushion stabilizers (nonrotating sleave-type stabilizer).
(Courtesy Smith International and Masco Tech Inc.)
To employ pendulum effect in directional drilling, usually one stabilizer is
placed in the optimum position in the drill collar string. The position is
determined by the hole size, drill collar size, angle of deviation and the weight
on the bit. A properly placed stabilizer extends the suspended portion of the
drilling string (that portion between the bit and the point of contact with the
low side of the hole). The force of gravity working on this extended portion
results in a stronger force directing the bit toward vertical so the well trajectory
returns to vertical.
To employ fulcrum effect one stabilizer is placed just above the bit and
additional weight is applied to the bit. The configuration acts as a fulcrum
forcing the bit to the high side of the hole. The angle of hole deviation increases
(buildup) as more weight is applied.
To employ a restricted fulcrum effect one stabilizer is placed just above the
bit while second stabilizer is placed above the nonmagnetic drill collar. The
hookup allows a gradual buildup of inclination with no abrupt changes.
To prevent key-seating one stabilizer is placed directly above the top of drill
collars. The configuration prevents drill collars wedging into a key seat during
tripping out of the hole.
To prevent differential sticking across depleted sands stabilizers are placed
throughout the drill collar string. The area of contact between drill collars and
hole is reduced, thus reducing the sticking force.