| CPC E21B 44/02 (2013.01) [E21B 45/00 (2013.01)] | 17 Claims |

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1. An apparatus comprising:
a processor; and
a machine-readable medium having program code executable by the processor to cause the apparatus to,
determine a first parameter based on a derivative of a walk force to a rate of lateral penetration of a drill bit while the drill bit is rotating in a bit rotation direction when drilling a borehole,
generate a whirl index value that comprises a ratio of drilling parameters, wherein the drilling parameters include the first parameter and weight on bit,
based on a determination that the whirl index value exceeds a whirl index threshold, determine that a drilling operation induces backward whirl, wherein the drilling operation is characterized by the drilling parameters, and
adjust a drilling parameter of the drill bit during the drilling operation in response to the determination that the whirl index value exceeds the whirl index threshold.
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