Page 202 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 178

[ ] Well Programming
2.3.5

the worst—with the exceptions of a 90 ft pendulum or a completely
slick assembly. The problems with using a pendulum assembly in a
vertical well include drilling a spiral hole (seriously bad for increasing
drags, problems running logs and casing, spiral communication paths
in cement outside casing, casing wear, etc.), inadvertently kicking off
in streaky formations or on boulders, and seriously restricting the
maximum weight on bit.
If the well is pointing in the right direction, whether it is vertical
or deviated, then lock it up and save yourself a lot of problems. If you
have concerns about those stabilizers causing excessive drags then use
one or two roller reamers to replace the NB and first string stabilizers.
In fact, unless you are likely to drill a spiral hole with your locked BHA,
or if you have seriously mobile formations, you should not see exces-
sive drags and torques with the packed BHA. In either case, the roller
reamers should handle the problem.
If balling is your concern then revise your drilling fluids program.
Modern water-based muds can largely eliminate balling due to shale
hydration. Stabilizers above the first three can be run a little bit under-
gauge to minimize dynamic buckling and differential sticking without
a large contact area with the formation.

2.3.5. Dropping Hole Angle

Of course there will be occasions when you need to drop angle. In
this case, either a pendulum assembly or some kind of motor assembly
is required.
In most cases, a rotary assembly will be preferred. The problem
with a motor is that when the toolface is aligned to the low side of the
hole, the tool will tend to flip over once reactive torque is applied as
the bit contacts the bottom. It can be quite tricky to hold the motor in
this position if the hole is deep and at a high angle. A steerable assem-
bly is preferred to a motor-bent sub combination since the steerable has
a smaller bend, close to the bit.
Drilling parameters are also important with a pendulum assembly.
Until a drop trend is established, very low bit weights will be required.
Once the drop trend is established (that is, once the second stabilizer
is in the section of hole that is dropping), it should be possible to bring

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