The paper is aimed to calculate an innovative numerical index for bit performance evaluation called Bit Index (BI), applied on a new type of formation and bit database also known as Formation Drillability Catalogue (FDC). A dedicated research program developed by Eni E&P Div. and the University of Bologna (Italy) studied a drilling model for drill-bit performance evaluation (the BI) derived from data recorded while drilling (bit records, master log, wireline log, etc.) that includes numerical information from dull bit evaluation. This index is calculated with data collected inside the FDC, a novel classification of formations aimed to the geotechnical and geomechanical characterization and subdivisions of the formations, called Minimum Interval (MI). FDC was conceived and prepared at Eni E&P Div., and contains a large number of significant drilling parameters. Some wells have been identified inside the FDC and have been tested for bit performance evaluation. The values of BI are calculated for each bit run and are compared with the values of the cost per meter. The case study analyzes bits of the same typology and run in the same formation. The BI methodology implemented on MI classification of FDC can improve consistently the bit performances evaluation, and it helps to identify the best performer bits. Moreover, FDC turned out to be particularly functional to BI, since it discloses and organizes many formation details that are not easily detectable or usable from bit records or master logs, allowing for targeted bit performance evaluations. At this stage of development, the BI methodology proved to be economic and reliable, helping as well to make drill bit selection less erroneous and more logical.


Drill-bit performance evaluation is one of the most important issues of drilling optimization. When optimizing bit performances, drilling engineers ordinarily think in terms of factors affecting the rate of penetration, and so indirectly correlate bit performances to cost-per-meter only. This criterion, however, cannot by itself show where improvement is needed. Nowadays, drill-bit performance evaluation methods involve the development of mathematical tools along with the analysis of large number of information from various records of previously drilled wells, even though to date there is not an industry standard or a widely used methodology to address it.

Bit performance evaluation and bit selection is of paramount importance for the drilling engineer. Many attempts to model and predict drill-bit selection and to evaluate their performances have been developed for a number of years. As advancements in drilling bit technology and drilling measurement systems have increased, so have the number and the sophistication of evaluation models available. Almost all the approaches are similar in objective, to make drilling bit selection less erroneous and more logical 1 and possibly free from any personal interpretation.

Recently, the methods for the evaluation of bit performance improvements have not always kept the pace with technological advancements. Methods such as offset wells and cost per foot, specific energy, bit factor, formation drillability, design index and neural networks have been used for bit performance evaluation 2–10, although most of them still seem not completely satisfactory. The industry is unlikely to find a means to predict drilling without a means of relating it to some rock criteria.

This study illustrates the application of an innovative mathematical tool (jointly developed by Eni E&P Div. and the University of Bologna) based on Bit Index implemented on data collected in Eni's Formation Drillability Catalogue 11–12, a data collection addressed to the geological and geomechanical classification of the formations. The method proposes a new way for using the existing information under day-to-day competitive drilling circumstances, accomplishing the desired objective at the lowest cost through the use of the most effective procedures.

Finally, the paper is completed with a case study analyzing some drill-bit performances in Val d'Agri area (Southern Italy). In particular, here is presented both an application concerning the evaluation of top hole drilling in a heterogeneous formation with hard stringers, and a case of a deep and hard reservoir drilling, where a new generation of impregnated bits has been recently tested.

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