Page 224 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 200

[ ] Well Programming
2.4.7

2.4.7. BHA Considerations Related to Bits

The BHA has an impact on bit performance and durability.
Logically it also has an impact on bit choice. For optimum bit perfor-
mance, the preferred rotary BHA is fully stabilized, giving several
advantages:

1. If the well is already aligned in the desired direction, a fully stabi-
lized BHA is less likely to go off course and generate directional
work to correct the wellpath.
2. Any inadequately stabilized rotary assembly will drill a slightly spi-
ral hole. This leads to high drags, reduced formation stability, and
problems running logs and casing.
3. As WOB is applied, the drill collars will tend to buckle. If they
buckle sufficiently, they may contact the wall, causing stick-slip
oscillations. Whether they may buckle under dynamic loading can
be calculated, but the significant factors are the length of unsup-
ported collars (i.e., between stabilizers), the mechanical stiffness
of the collars (related to OD, ID, and material), and the compres-
sive force exerted on the column. Buckling can also lead to fatigue
failure and twistoffs. The BHA should be designed with stabiliza-
tion that will avoid dynamic buckling under the maximum
planned WOB.
4. If the hole is enlarged, a fully stabilized BHA will still tend to buck-
le less than an unstabilized one. The situation is improved if larger
drill collars are run above the bit (in the area under greatest com-
pression); for instance, if 9 1 /2 in drill collars can be run instead of
8 in collars in a 12 1 /4 in hole.

For directional work, the bit will have to work at the parameters
imposed by that BHA. For instance, a downhole motor will dictate
ranges of possible RPM, WOB, and flow rates. If a drop in hole angle is
required using a rotary BHA, then light weights will have to be used
which may make it advantageous to run a softer formation (or PDC)
bit than might otherwise be the case.
If the assembly is not fully stabilized, then make a list of the avail-
able ranges of parameters that the driller can apply to the bit. Next,
check to see if the “ideal” bit choice can work within those parameters.
If not, the choice should be revised.

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