Page 184 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 160

[ ] Well Programming
2.3.1

stabilizer, which is now in widespread use around the world. The ben-
efits of a nearbit or string stabilizer, which allows downhole size adjust-
ment, can lead to significant cost savings since trips are not needed to
change the build or drop characteristics of the BHA. The Andergauge is
available as string or nearbit types. They can be adjusted downhole
between full-gauge and one undergauge size and (at present) are avail-
1
1
able for 12 /4 in and 8 /2 in hole sizes. These are used on rotary and
steerable assemblies to control the build/hold/drop performance.
Some downhole motors can have stabilizer blades clamped on to
the body of the motor to adjust the directional characteristics. Blades
can be straight (aligned with the centerline) or spiral (wrap around in
a spiral). In very soft formations, the straight blade may tend to dig into
the side of the hole more readily than the spiral type. Spiral blades are
most common in all but the largest sizes, which is strange considering
that the largest ones are most likely to be run against soft formations!
Stabilizers may be made of steel or Monel. Monel is an alloy which
does not distort the local magnetic field and so Drill collars and
Stabilizers made of monel may be used to allow magnetic surveys to be
taken while drilling. Refer to the following section on drill collars for
positioning Monel stabilizers.
Drill collars. Drill collars are thick-walled pipes that allow weight
to be applied to the bit. Drill collars are very stiff (especially compared
to drillpipe) and have a relatively high resistance to buckling. The force
required to buckle a drill collar is determined by the material, ID, OD,
shape of the cross-section, and unsupported length. Larger ODs and
shorter distances between stabilizers in gauge hole will both increase
the compressive force required to buckle the collar.
Sometimes you want to make the collars buckle slightly because it
will tilt the bit and impose a deflection force to change direction. With
a build assembly, a nearbit stabilizer is run and a string stabilizer from
60 ft to 90 ft higher. Gravity tends to make the collars sag (assuming
the hole is already inclined) and increasing weight on the bit will
increase this deflection. Therefore in this configuration, placing more
weight on the bit will increase the build rate. The further apart the
nearbit and first string stabilizers are, the faster it will build.
Unfortunately, this increased distance also reduces azimuth control and
the bit will also turn left or right faster due to formation forces and for-
mation-stabilizer interaction. As the hole angle increases, so too will
the build rate if everything else stays the same.

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