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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractCurrent improvements in bit performance, though normally attributed to advancements in product development 1,2 , 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.

Improvements in drilling efficiency and economics are continuously being achieved with PDC bits, specifically in medium-hard and non-abrasive formations - normal drilling. These gains have been aided by advancements in PDC bit technology, their selection practices and BHA design. However, PDC bit performance in harsh environments - hard, abrasive and heterogeneous formations - still lacks consistency and predictability. The problems associated with these applications are amplified when large hole sizes (diameters greater than 12") are drilled. Performance qualifiers such as footage, and especially rate of penetration (ROP), tend to be considerably lower in such instances. Considering the depths at which small hole sizes (diameters less than 9") are usually drilled, the likelihood of such hole sizes being drilled in harsh environments tends to be very high. It is evident that huge operational savings will be achieved if the performances of PDC bits are improved. This paper will discuss the effects formation hardness, abrasiveness and heterogeneity have on PDC bit performance. In addition, it will establish the influences different hole sizes have on PDC bits, especially in harsh environments. Drilling efficiency will be discussed in terms of operational parameters, vibrational behavior, durability equivalency (DEQ) and lithology differences. An engineered approach to PDC bit development and selection, that enhances performance in harsh environments and in large hole sizes, will be discussed. Background Researchers have identified stabilization1,2,3,4, durability5, directional efficiency6,7,8 and rate of penetration (ROP) as critical performance qualifiers (PQ) for PDC bits. Although the contributions of these qualifiers to operational success have normally been discussed in isolation, it is known that they exhibit strong dependencies. The importance of the different PQ's to PDC bit efficiency depends on the mechanical properties of the formations being drilled. Formation drillability9,10 (FD), characterized through drilling difficulty establishes the effects rock mechanical properties such as bulk compressibility, abrasivity, compressive strength and lithological composition have on the drilling process. Environments become harsh as their drilling difficulties increase. In some instances, high pore pressures, and associated higher mud weights, depositional depths and hole size compound the problems normally associated with harsh environments. These different PQs assume diverse roles when it comes to normal and harsh environment drilling. Most importantly, ROP's influence on operational efficiency in normal and harsh environments can be classified as direct and indirect respectively. This distinction establishes the groundwork needed to improve PDC bit performance in the challenging environments that will be described in this paper. Introduction Inaccurate evaluations of PDC bit dull grades have contributed to the known inefficiencies of these bits in harsh environments. In addition, inefficient interpretations of drilling parameters11,12, such as weight on bit (WOB), rotary speed (RPM), flow rate (Q) and HSI have also contributed to the performance inconsistencies of these bits.

Drilling efficiency must be improved, in order to drive down operational costs. This requirement must not be viewed in isolation, considering the vast number of activities that determine a drilling program's success. In most instances, discussions relating to drilling efficiency have centered on rate of penetration (ROP)1,2. As a result, ROP is either equated to drilling efficiency, or seen as the parameter that establishes drilling efficiency. These positions, in addition to being flawed, are highly inconsistent with field results. ROP must not be equated to drilling efficiency. Rather, ROP needs to be seen as one of several parameters that influence drilling efficiency. The industry's most common performance quantifying metrics - cost per foot (CPF), feet per day (FPD), mechanical specific energy (MSE)3,4, etc - are strongly influenced by ROP. These relationships complicated earlier efforts that sought to establish the appropriate dependencies, between drilling efficiency and ROP. To drive down operational costs, while continuing to push drilling activities into harsher and more challenging environments, emphasis must clearly be placed on drilling efficiency. Thus, in order to achieve this goal, drilling efficiency and ROP must both be defined. In addition, the factors influencing ROP and drilling efficiency must be identified. Most importantly, drilling efficiency's different influencing factors5, which include but are not limited to ROP, must always be analyzed based on specific project objectives. This paper, in addition to addressing the issues listed above, will also establish a clear distinction between drilling efficiency and ROP. In addition the contributions and impact of both ROP and drilling efficiency, as it relates to value creation will be discussed. The positions, to be presented and argued, will be supported with field and operational data.

Technological advancements, together with improved selection and application practices, have enhanced Polycrystalline Diamond Compact (PDC) bit performance. In addition to drilling faster, these bits are now drilling longer and extending their application envelopes. However in hard, abrasive and highly heterogeneous formations, PDC bit performance still lacks consistency and must be improved. An important qualifier that needs to be optimized in order to enhance PDC bit performance in the harsh environments listed above is durability. However, the engineering efforts needed to define, quantify and explain the concept of PDC bit durability have been minimal. Current industry methods of improving durability have been very ineffective, because they focus on maximization and are based on "trial and error" processes. These approaches compromise the efficient shearing rock removal mechanism of PDC bits, thus their rate of penetration (ROP). They also neglect the relationships and influences formation drillability and vibration characteristics have on durability. This paper will establish the differences and effects of durability maximization (DM) and optimization (DP) on PDC bit performance. It will establish the necessary medium for durability enhancement, and define the processes necessary for its quantification. The influences that formation drillability and vibration behavior have on durability optimization will also be discussed. Laboratory analysis and field data showing DP's effects on drilling efficiency and operational costs will also be presented. The paper will show how durability optimization enhances PDC bit performance in harsh environments. Background PDC bits, by virtue of their shearing rock dislocation mechanism, drill faster at identical loading conditions when compared to other bit types. Although the effects of this advantage on operational efficiency have clearly been established in the industry1,2, the economic impact is consistent only in soft and medium hard formations. If PDC bits could drill hard, abrasive and heterogeneous formations effectively, especially in larger hole sizes and at great depths while maintaining their ROP superiority, tremendous savings could be realized by operators. To achieve this goal, industry R&D efforts have focused on the development of processes and technologies that will enhance PDC bit performance and economics in harsh environments. The improvements that have been made in PDC bit performance, especially in the soft and medium hard formations can be attributed to several factors. Formation drillability analysis, cutting structure (cutter layout) developments, vibration characterization and PDC cutter technology have played key roles in this regard. PDC bit development and selection has benefited from the clear distinction that has been established between formation hardness and drilling difficulty3. Cutting structures with effective vibration prevention and mitigation characteristics have been developed4,5,6. PDC cutters have been engineered to exhibit superior impact, abrasion, thermal and fatique qualities7,8,9. Although these improvements have been beneficial, they have not been effective at extending the application envelope of PDC bits into harsher environments.