Summary
Bit whirl is well documented as a major cause of damage to
polycrystalline-diamond-compact (PDC) drill bits and results in short runs, low
rate of penetration (ROP), high cost per foot, poor hole quality, and
downhole-tool damage. Hence, consistent lateral stability is highly desirable
in PDC bits.
This paper presents a new method of producing PDC drill bits that reduce or
eliminate bit whirl. Traditionally, attempts to design laterally stable PDC
bits have assumed that the forces generated by the bit during stable drilling
cause it to begin whirling. The new approach assumes that it is the response of
the bit to forced motion off its center that causes whirl.
The new approach has been validated theoretically, in the laboratory, and in
the field. Test results from a full-scale drilling rig indicate that this
method is superior to existing methods at minimizing bit whirl.
This new method was validated successfully through an extensive field-test
program undertaken in the northwest USA, involving high-speed downhole
measurements from both conventional 7 7/8-in.-diameter PDC drill bits and bits
designed using this new method. The results demonstrated that the conventional
PDC drill bits exhibited bit whirl throughout the run. The bits designed using
the new method eliminated bit whirl completely.
The performance benefits of this new method were demonstrated in a
16-in.-bit section in a Middle Eastern field, where lateral vibrations are a
significant problem. Before testing the new-method PDC bit, bit optimization
led to the use of high imbalance force designs to minimize lateral vibration.
The first test of the new-method laterally stable PDC bit set field records for
ROP, footage, and cost per foot compared with more than 50 runs with
"antiwhirl" (Warren et al. 1990) designs, resulting in savings of more
than USD 50,000 per run.
This paper demonstrates how use of the new method can eliminate whirl in
both laboratory and field environments and deliver significant performance
improvements over existing stability techniques.
This new method represents a significant step forward in the design of PDC
bits to mitigate bit whirl, and it has been proved to reduce bit damage,
increase run lengths, increase ROP, and deliver sizeable savings for
operators.
Background
Bit whirl was first identified as a major cause of damage to PDC bits by
Brett et al. (1990). They showed that during backward whirl, the instantaneous
center of rotation moves around the face of the bit in the direction opposite
to the rotation of the bit. This can cause cutters to be accelerated sideways
and backward, causing chipping that can accelerate bit wear, reduce PDC-bit
life, and reduce ROP. In addition, bit whirl results in very high downhole
lateral acceleration, which causes damage not only to the bit but also to other
components of the bottomhole assembly (BHA), such as motors,
measurement-while-drilling tools, and rotary-steerable tools.
The effects of bit whirl are very damaging; hence, a number of methods have
been developed to evaluate and enhance the lateral stability of PDC bits in
order to minimize bit whirl. These methods can be divided roughly into three
main schools of thought:
1. Manipulation of the cut shapes created in
the rock by the bit to stabilize the bit (Clayton and Ivie 1994; Mensa-Wilmot
and Krepp 1998; Ortiz et al. 1996).
2. Manipulation of the forces generated by the PDC cutters while drilling
(Warren et al. 1990; Clegg 1992; Taylor et al. 1998).
3. Use of the bit body to stabilize the bit (Roberts 1998).
While the development of these methods has greatly improved the lateral
stability of PDC bits, bit whirl remains a problem in many applications. This
suggests one of two things:
1. The existing methods, while reducing whirl,
do not completely eliminate it.
2. There are compromises associated with each method that prevent their usage
in many applications.
This paper introduces a new method of enhancing the lateral stability of a
bit and demonstrates how it delivers improved results over conventional methods
of stabilizing PDC bits, with little or no design compromise.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
21 December 2005
- Meeting paper published:
21 February 2006
- Revised manuscript received:
22 January 2008
- Manuscript approved:
22 January 2008
- Version of record:
15 September 2008
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