Page 201 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 177

2.3.4
Directional Planning [ ]

setting down weight forces steel teeth into the casing to anchor the
whipstock, setting down more weight shears the bolt. The mill follows
the concave face of the whipstock and drills into the side of the hole.
After drilling for one or two singles (sufficient for a single shot survey
to be taken in open hole away from the steel whipstock), the mill is
pulled out, and the whipstock is pulled by the BHA. A full sized bit is
then run on a suitable drilling assembly and the pilot hole is drilled
out. The process can be repeated if necessary.
A combined whipstock and mill can also be run inside casing; the
whipstock is oriented and set permanently in the casing. The whip-
stock is released by setting down weight to shear a bolt, the mill (guid-
ed by the whipstock) cuts a window in the side of the casing, and is
pulled out. A balloon mill is run to smooth off the window and then
a bit may be run to drill ahead as desired.

2.3.4. Drilling the Tangent Section

The tangent section of the well is designed to be drilled (mostly)
straight. As mentioned above, the wellbore is almost never truly
straight but will be influenced by the bit, BHA, drilling parameters,
and formation characteristics.
With a rotary assembly rotating clockwise, the forces acting on the
BHA normally generate a slight side force at the bit tending to turn it
to the right. This tendency is increased at lower rotary speeds, also as
the bit dulls or if it is crippled. Older directional hands will tell you
stories about cutting one cone off a bit to cripple it and make it turn
to the right for correcting the wellpath. In this case, they are not kid-
ding! With a sharp bit and high rotary speed, it may be possible to
reverse the normal trend so that a left-hand turn is initiated.
The tangent section generally needs to be drilled quickly, with
minimum dogleg and not spiral or overgauge so that logs and casing
can be run without problems. If any other rotary bottom hole assem-
bly is run than a fully stabilized one, the above objectives will be com-
promised to a greater or lesser extent.
Tangent section in vertical wells. In many operating companies,
vertical wells are routinely drilled with 60 ft pendulum assemblies.
This is a mistake. Of all the possible assemblies to use, this is about

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