Page 185 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 161

2.3.1
Directional Planning [ ]

This deflection, which is due to the weight of the collars, is also
used to reduce hole angle. A pendulum assembly will have a bit, drill
collar(s), and the first stabilizer between 45 ft and 90 ft higher. The
weight of the lowest section of collars pushes the bit against the bot-
tom of the hole and this will make the bit drill lower angles. Pendulum
assemblies also tend to walk sideways and/or drill spiral holes; weight
on bit needs to be kept low, otherwise directional performance can
become completely unpredictable and so rate of penetration suffers
with pendulums. Once a drop trend is established, increasing the
weight slightly can help increase the speed of drop. As the hole
becomes more vertical, the rate of drop decreases. It is very hard to
drop to within a degree or two of vertical using rotary pendulum
assemblies. A less severe pendulum could be set up like a 60 ft build,
but use an undergauge nearbit stabilizer to drop the angle.
If uncontrolled drill collar buckling takes place, fatigue failures are
likely to occur—usually a few threads down from the box-end shoulder.
Apart from failure of the steel, this dynamic buckling is likely to lead to
contact between the wall and the drill collar OD. This will lead to “stick-
slip” oscillations where the string stops downhole (rotary table torques
up and slows down) then the sticking is released, and the BHA spins
very fast (rotary table speeds up; the string may overtake the rotary and
the kelly bushing will bang against the drive holes). Stick-slip oscilla-
tions are seriously detrimental to bit performance and life. Sufficient
stabilizers must be run so that dynamic buckling does not occur. A com-
puterized analysis can be done or refer to the general BHA configura-
tions at the start of this section for rule-of-thumb BHA design.
Drill collars are available in a square cross section. They are very
stiff mechanically (i.e., resist bending or buckling better than round)
and may be called for in severe crooked hole country.
The outer surface of round drill collars can be plain but mostly have
spiral grooves machined in them. This minimizes the surface area in con-
tact with filter cake and should reduce differential sticking tendency.
If magnetic survey tools will be used (magnetic single shot or mag-
netic multishot) then a section of the BHA has to be of a metal that does
not distort the Earth’s magnetic field. Monel drill collars and stabilizers
can be run such that at the depth of the survey tool, magnetic interfer-
ence from the rest of the BHA is acceptably low. The length of monel to
be run depends on several factors, such as latitude of the drilling loca-
tion, inclination of the wellbore, and azimuth (see Fig. 2.6).

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