Page 193 - Drilling Technology in Nontechnical Language

184 Drilling Technology in Nontechnical Language Second Edition

Stabilizers are positioned so that drill collar sag tends to push the bit
into the low side of the hole or to push the bit up to the high side of the hole,
or to have very little or no side force. This is achieved by selecting both the
positions of the stabilizers and whether the stabilizers are the same size as
the drill bit (full gauge) or smaller (undergauge).
For a rotary build assembly to work, the hole needs some inclination
(over about 12°) to create the sag. The higher the inclination, the more
side force is generated with a particular rotary build assembly. The near
bit stabilizer (NB stab) acts as a fulcrum, so the drill collars above can be
thought of like a lever—push down on the upper part of the lever, and the
opposite side pushes up. If the NB stab is undergauge, it will develop less
upwards side force, and so the stabilizer gauge has an effect. If the next
stabilizer up is (or becomes) undergauge, this will increase the build effect
because the upper part of the lever will be moved downwards.

The weight applied to the bit also has a large effect. With sag in the
drill collars, applying weight from above will increase the level of sag as
the drill collar starts to buckle. Therefore the more weight that is applied
to a build assembly, the faster it will build.
To drop angle, no NB stab is run. There will be either 60 ft or 90 ft

of drill collars between the bit and the first stabilizer. The sag, minus the
NB stab fulcrum, pushes the bit to the low side and therefore will tend to
drop angle.
To get the drop started, light weight on bit is run. Once the trend
is established, it can be increased by adding some more weight on bit.
However, as this type of assembly is poorly stabilized by its nature, it
will tend to follow any natural formation trends and often will wander off
sideways as well as dropping. Azimuth cannot be well maintained, and this
then might require a trip out of the hole to change the BHA configuration

in order to correct the errant well path. A drop assembly is also known as

a pendulum assembly (fig. 8–7).
The drill bit also has an effect. A drill bit can be designed so it has
some side cutting action. This will increase the effectiveness of the build
or drop assembly. On the other hand, if it is necessary to deviate as little as
possible, a bit could be selected that had no side cutting action and would
therefore try to drill straight.

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