The paper analyzes a novel operative procedure of geological and geomechanical subdivisions of the formations, called Minimum Interval (MI). The MI concept, used in combination with other evaluation indexes, can improve consistently the evaluation of bit performances. This research compares three indexes for bit performance evaluation, i.e., cost-per-meter (CM), Specific Energy (SE) and Bit Index (BI), applied to a new type of drilling-bit database, named Formation Drillability Catalog (FDC), organized in order to implement the MI concept. Basically, the FDC is a novel classification of drilled formations conceived and prepared at Eni E&P Division (Italy), aimed to the geomechanical characterization by means of a large number of significant geological and drilling parameters, each one referred to single MI’s. So far, five wells have been identified and tested for bit performance evaluation. The FDC turned out to be functional to BI, since it discloses and organizes lithostratigraphical characters that are not easily detectable or usable from bit records or master logs, allowing for targeted bit performance evaluations. Results of this case study showed that the cross correlation between BI, CM and SE can help to identify the best performer bit for each formation and for each borehole section. The quality of bit performance analyses obtained with BI and SE (through the MI concept) seems more effective than those based on CM, performed on bit records, and can also be used as a valuable tool for drill-bit selection.

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