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Abstract
One concept to avoid downhole vibrations with PDC bits requires a net side force that rotates with the bit and pushes a low friction gage pad against the borehole (anti-whirl bit). When drilling with an angle build assembly or in an angled hole, a non-rotating side load is applied to the bit. When drilling with a steerable assembly in the rotate mode, the bit axis is tilted relative to the profile of the bottom of the hole. This axis tilt affects the bit cutting forces and may consequently affect bit stability. This paper investigates these non-axisymetric loads for their effects on the stability characteristics of anti-whirl designs as they compare with standard PDC bits.
A second hypothesis holds that standard PDC bits occasionally whirl on directional bottomhole assemblies, which leads to overgage holes and ledging. The build rates, hole diameter, and ledging characteristics of anti-whirl bits are compared to standard PDC bits.
Comparisons were established in lab tests using standard and anti-whirl PDC bits both in an anisotropic formation (Green River shale) and in an isotropic formation (Carthage limestone) with a nonaxisymetric loading.
Further, a comparison is made between two directional wells drilled at a field research facility using the same surface hole. One well was drilled with a standard PDC bit; the other with an anti-whirl PDC bit. The two wells are compared for surface vibration level, rate of penetration, build rate, dull condition of the bits, and hole diameter and ledging from calipers of the wellbore.
Introduction
Suppression of the bit whirl phenomena has dramatically changed the design and operation of PDC drill bits. One of the methods used to control whirl involves designing the cutter locations to generate a resultant imbalance force that is directed toward a low friction gage pad. This design technique stabilizes the bit against the hole wall.
In a recent study, three non-axisymetric load cases that may affect stability were examined. When drilling with directional assemblies, or in high angle wells, an external side load is applied to the bit. In anisotropic rock, the cutting forces change as the bit rotates, thereby changing the imbalance force vector of the bit with time. Lastly, when drilling with steerable assemblies, the bit axis is tilted relative to the direction of bit motion. This modifies the cutting forces that are generated, thus modifying the resultant imbalance force.
Since bit stability was uncertain in the above situations, early field tests of anti-whirl designs were restricted to straight, low-angle holes, drilled with conventional rotary assemblies. In order to eliminate these restrictions and understand the effects of loading as previously described, lab and field comparisons were undertaken on conventional and anti-whirl PDC bits.
Lab tests were conducted in both anisotropic and isotropic rocks with non-axisymetric loading. The resulting loads and displacements were then recorded.
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