Current improvements in bit performance, though normally attributed to advancements in product development1,2 , have greatly benefited from the science of formation characterization. Determination of rock mechanical properties, such as bulk compressibility, Poisson's ratio, internal friction angle, shear and compressive strengths from well log data continues to improve the bit selection and application processes. In fact, product development would not be at its present pace, without the influences of formation evaluation.

Several analytical methods3,4  have been developed by the industry for formation characterization purposes. Although these tools are effective, most of them fall short when there is a need to predict the performance of a given bit in a known field. In addition, they present a micro-scale evaluation of the formations to be drilled, making it difficult to establish and compare drillabilities in a given field or between different fields. Additional disadvantages of the current evaluation methods are that they presumptuously draw parallels between formation hardness and drilling difficulty, without establishing the relationships required to make the needed comparisons.

This paper will present a new bit performance evaluation parameter – formation Drillability (FD). It will establish a process for the determination of FD, which combines the different rock mechanical properties into a single dimensionless parameter. This approach addresses the limitations of the current evaluation processes by establishing an effective method for bit performance evaluation and prediction. FD establishes a macro-level evaluation process, which enables drillabilities to be quantified for different fields. Most importantly, FD improves operational efficiency by enhancing the development, selection and application of all types of drill bits.

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