The application envelopes of PDC bits have conventionally been defined by rock hardness. Although technologies1,2,3,4 aimed at improving PDC bit efficiencies in challenging drilling environments continue to be developed, performances in such applications still lack consistency. It has become apparent, from drilling performance evaluations, that formation hardness alone can not be used to categorize PDC bits.
In some instances, PDC bit performances have varied profoundly across given fields, even when formation types and tops, lithologies and hardness have been determined to be comparable. These performance differences, which continue to baffle the industry, amplify with increasing formation heterogeneity especially in hard rock environments.
This paper will present a three part process, which has been developed to address the challenges described above. First a new approach, which characterizes rocks by their drilling difficulty, is used to establish field trends. Considering offsets, or anticipated performance requirements, bit development and/or selection is done with the aid of two new parameters - stress and durability equivalencies. Finally, operational practices aimed at enhancing the effectiveness of this new process are developed and implemented. This paper will quantify and discuss the individual and combined effects of these processes.
Analytical, laboratory and field data, showing the effectiveness of this new methodology, will be presented.
Classification of formation drillability, first as a function of lithology, followed by the hardness effect helped explain some of the PDC bit performance inconsistencies. In this regard, different lithologies of comparable hardness presented dissimilar drillability challenges. Although helpful, this approach neglected the effects of formation heterogeneity.
The emergence of heterogeneity, as a strong contributor to poor drilling performance, has had tremendous effects on PDC bit development. As will be explained in the next sections of this paper, heterogeneity in comparison to hardness has a bigger effect on drilling difficulty.
Although inefficient from an operational standpoint, in terms of resulting penetration rates (ROP), certain design features5 have sometimes been used to account for formation hardness6. These have included small PDC cutters7, high blade and cutter counts, as well as high back rake angles.
Similar methods, used in attempts to address the heterogeneity issues, have been far less satisfactory. Although the end results are similar (poor bit performances), the primary failure modes in hard or heterogeneous environments are different. Predominantly, hardness usually causes spalling and/or delamination (Figure 1) of the diamond table. In comparison, heterogeneity primarily causes PDC cutter breakage (Figure 2).
Addressing the challenges posed by heterogeneity requires more than the mere use of features that make PDC bits passive.