Most real-time drilling interpretation is performed using some sort of normalized rate of penetration to infer changes in lithology and evaluate the condition of the bit. This frequently leads to ambiguous or confusing results, because it is impossible to separate all bit effects from lithology effects using only one measurement. Also, no method exists to evaluate drill bit performance such as directional responsiveness and steerability. This paper proposes a new model of bit performance evaluation and shows how the force and moment measurements can be used to separate the bit effects from the lithology effects when drilling with PDC bits.

Analytical results of rock/bit interaction modeling are based on the laboratory drilling force measurements during rock comminution with a PDC cutter. It was found that a given rock/PD C system results in the specific relationship between axial and tangential drilling forces. This empirical relationship is used to determine the distribution of drilling forces and moments on a generic conventional PDC bit. Then, it is shown how the measurements of force and moment generated during drilling can be used in bit wear and lithlogy changes evaluations. This model accounts for abrasive wear of the cutters and detects those that are exposed to more severe loads. Thus, it may be also used as a guide in bit selection and bit design modifications.

If verified in the harsh economic environment of today, field application of the method should lead to a better understanding of downhole conditions and a more accurate decision as to what direction a bit is drilling, when to pull out bits, and thus to a reduction of drilling costs.

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