Page 139 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 115

1.5.3
Directional Design [ ]

1.5.3. BHA Performance Considerations

It is better to complete the deviation work in the upper hole sec-
tions unless the target displacement is small. Aim to kickoff below sur-
face or upper intermediate casing and finish the build before setting the
next casing string. You can then use rotary locked assemblies for the
rest of the well. This approach (for a typical, one-target deviated well)
helps maximize ROP and minimize total footage drilled. The final
inclination should be over about 17° from vertical, otherwise direction
will be hard to control with rotary assemblies.
Deviating bottom hole assemblies only work predictably in an in-
gauge hole. If you are likely to have hole enlargement problems in the
kickoff part of the hole and if mud design and good drilling practices
cannot solve these instability problems, the kickoff may be hard to con-
trol, depending on how fast the wellbore enlarges and how fast you
drill. It would be better to get below these troublesome zones and case
them off before starting directional work. For example, if you preferred
to kickoff fairly shallow but had unconsolidated sand bodies at that
depth range, there might be problems not only in controlling direction
but in keeping the hole clean, problems tripping, or inadvertently side-
tracking the well when reaming in with a rotary BHA through the sand.
Directional work is better done in good wellbore conditions.
Avoid setting casing either immediately above or within the kick-
off section. This may lead to keyseating of the casing shoe, which is
tough to get out of if you get stuck in it.
Rotary tangent (locked or packed) drilling assemblies often have a
slight tendency to turn to the right (check your offset wells). This can
be compensated for by finishing your build section slightly to the left
of the planned azimuth to the target. Also, with a tangent BHA, using
high weights to maximize ROP normally gives a slight build tendency,
as does drilling into formations with dips below 40°. Leaving the well-
bore 2-3˚ below the planned inclination to the target center at the end
of the kickoff before locking up the BHA would compensate for this.
With good directional offset data you may consider aiming to finish the
build section with the wellbore pointing a bit above the lower edge of
the target, knowing that this will maximize penetration and allow
application of high WOB and capacity for a slow build and the maxi-
mum tolerance. With a circular target area, since the angle is slowly

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