Page 182 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 158

[ ] Well Programming
2.3.1

tion that the bit drills. Other tools that do not affect the directional per-
formance directly but give us controlling information include wireline
run and/or retrieved survey tools, universal bottom hole orientating
(UBHO) subs, measurement while drilling subs, inclination only mea-
surement subs (‘Anderdrift’), and wireline deployed gyro tools.
In terms of directional performance, the influence at the bit of a
particular tool is greater the closer it is to the bit. For instance if the
nearbit stabilizer goes undergauge by /4 in, it will affect directional
1
performance far more than if the next stabilizer at, for example, 35 ft
up goes undergauge by the same amount. It is also the case that only
the bottom 90 ft of the BHA or the first three stabilizers (whichever
is shorter) actually affects directional performance. Undergauge sta-
bilizers are run higher up above the bottom 90 ft to minimize dynam-
ic buckling and hold the drill collars off the wall for differential stick-
ing concerns.
Following are the individual components and their effect on direc-
tional performance.
Drill bits. Some drill bit features can generate forces that produce
deflecting or stabilizing forces. A roller cone bit which is dull or dam-
aged will increase the natural tendency of rotary assemblies with roller-
cone bits to make the bit walk to the right.
Other features will help the bit to resist deflecting forces or allow
the bit to react more strongly to these forces. Long-gauge sections on
fixed-cutter bits or gauge pads on the shirttails of roller cone bits will
resist deflection forces. Parabolic or strongly convex fixed cutter bit
profiles stabilize the bit whereas a flatter bottom profile will have weak-
er or no resistance to side forces. Directional fixed cutter bits some-
times incorporate cutters around the gauge which help cut hole to the
side of the bit, helping to give stronger dogleg severities for a given bot-
tom hole assembly and drilling parameters.
Some bits are designed to cut an overgauge hole; a bi-center bit has
the center of the cutting structure offset from the centerline of the BHA
for drilling in salts and very plastic shales. If the bit is cutting over-
gauge then the gauge area has no rock to work against and would be
expected to react more strongly to deflection forces.
Stabilizers and roller reamers. A nearbit stabilizer or roller reamer
is run immediately above the bit, as its name implies. The gauge of this
stabilizer is critical to build or drop performance. An undergauge NB

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