TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractHeterogeneous formations present challenging performance issues for PDC bits, specifically in terms of ROP and durability. With current PDC bit technology, the bit runs are usually short and slow 1 , even when BHA design and drilling processes are optimized.

PDC bits are still confronted with performance challenges, especially in hard and/or abrasive formations. In most instances, the bit development or selection process compromises ROP for durability or vice versa. These solutions do not create the types of conditions needed to ensure the expected measurable gains in performance. As such effective product development, which targets the ROP/durability relationship, based on the performance optimization requirements of different applications, must be sought. This paper will discuss field proven design processes and technologies, which have successfully been used to improve PDC bit performance in harsh drilling environments. The importance and contributions of bit stabilization in this effort, as well as its effects on ROP and durability, will be discussed. As part of the product development processes that will be presented in this paper, drilling efficiency (DE) will also be discussed. In addition, the effect and influences of bit durability on drilling efficiency will be presented. Field data showing the positive impact of the new process on PDC bit performance, especially in hard and/or abrasive formations, will also be presented. Background As performance qualifiers (PQ), ROP and bit durability (BD) have the biggest effects on drilling efficiency and operational costs. In this regard, both ROP and BD must be improved, in order to achieve substantial and measurable improvements in drilling performance. This requirement, which is more critical in hard and/or abrasive formations, is hardly achieved. This situation is primarily due to the types of solutions used1, as well as the types of compromises they present. Bit features, believed to have influences on ROP2 and durability, include the following - cutter size, back rake, cutter count, and blade count. When all other design features are kept constant, these features cause strong inverse relationships between ROP and durability (Figure 1a-d). In addition some of the features, such as cutter size and cutter count are dependent on each other, and exhibit an inverse relationship (Figure 2). As mentioned in this section, these solutions do not yield the types of improvements needed due to the relationships and types of effects they have on ROP and BD. Bit stabilization3,4, achieved through the effective management, control and prediction of bit behavior, establishes the appropriate medium needed for ROP and BD improvement. Stabilization minimizes pre-mature PDC cutter failure (Figure 3), due to reduced impact loads, thereby enhancing BD. In addition, stabilization ensures efficient use of available operational parameters (RPM and WOB) for ROP maximization. Consequently, stabilization establishes the necessary and sufficient conditions needed to improve both ROP and BD. The recognition of stabilization's effects on ROP, DB and overall bit performance, has not translated into an acceptance of its achievement methodology. ROP and/or BD (needed to improve bit performance) cannot be compromised to achieve bit stabilization. Certain bit features and/or technologies, that are intended to improve bit stabilization, have negative effects on ROP and BD, and end up not having the expected effects, especially in hard and/or abrasive formations. The low depth of cut (LDOC) or managed depth of cut (MDOC) concept (Figure 4), which aims to enhance stabilization, by making bits passive, compromises both ROP and BD. In such deployments, the blade tops of PDC bits are raised to heights that are very close to the cutter tip. ROP potential is severely compromised, because the cutters can only engage the formation to a depth that is equivalent to the exposure differential between the cutter tip and the blade top (Figure 5). In addition, this type of deployment also comprises BD. Cutter exposure, usually dependent on cutter size (Figure 6), dictates the amount of diamond that can be used before blade tops come into contact with the formation being drilled.

Simulating the mechanical response of PDC drill bits contains a lot of uncertainties. Rock and fluid properties are generally poorly known, complex interactions occur downhole and physical models can hardly capture the full complexity of downhole phenomena. This paper presents a statistical approach that improves the reliability of the PDC bit design optimization process by ensuring that the expected directional behavior of the drill bit is robust over a well-defined range of drilling parameters. It is first examined how uncertainty propagates through an accurate bit/rock interaction model which simulates numerically the interaction between a given PDC drill bit geometry and a given rock formation, both represented as 3D meshed surfaces. Series of simulations have been launched with simulation parameters defined as probability density functions. The focus has been set on directional drilling simulations where the drill bit is subjected to significant variations in contact loads on gage pads along its trajectory. A global sensitivity analysis has also been performed to identify the key parameters which control drilling performance. Directional system parameters are critical in terms of steerability and tool face control, particularly in high dogleg severity applications. Based on these simulations, a statistical optimization strategy has then been implemented to ensure that the directional performance of the drill bit remains effective under a given uncertain drilling environment. Statistical analysis combined with drilling simulations indicated that ROP improvements could even be achieved without compromising steerability. A balanced bit design was selected and manufactured in an 8 1/2-in. model to drill a 714 ft section of a Kuwait field. The bit was run on a high dogleg rotary steerable system and directional assembly. The bit achieved the high steerability goals required by the application while showing a good compatibility with the directional tool. Moreover, ROP was increased by approximately 27% compared to offset wells, setting a record rate of penetration in the field. Whereas statistical analyses are commonly conducted in the field of geosciences, it has rarely been applied in the field of drilling applications. The statistical bit design optimization strategy deployed in this work has allowed to improve both the drilling performance of the drill bit and its reliability.

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PDC bit designs for directional drilling typically sacrifice penetration rate for steering performance, resulting in a higher cost per foot. This paper discusses a process for modeling the drilling behavior of PDC bits, insights gained regarding bit efficiency when steering with various motorized and non-motorized rotary steerable systems (RSS), and field experience with the resulting bit design. Prediction and analyses of PDC bit behavior used a bridgeable software platform to integrate various design applications. This allowed the combined analyses of performance objectives and criteria, including cutting structure, rock type, application, well profiles, drives, and 3D contact. Simulations were then run to examine the performance of different cutting structures relative to various drilling parameters, and matched to a specific drive system. An optimized PDC bit design was developed and manufactured, and its field performance was compared to the model and to PDC bit performance in offset wellbores. The optimized design was manufactured in a 6 1/8-in PDC bit and run on motorized and non-motorized RSS. It resulted in significant ROP increases and a lower cost per foot compared to offset wells, while retaining a high level of steering response. In one well, the ROP was increased to 48.43 ft/hr versus a target of 35 ft/hr based on offset performance. The resulting cost per foot was reduced from $11.40 KD to $8.59 KD. The paper examines bit performance and dull condition for the runs and compares them with offset runs. Field performance results validate the bit design modeling and simulation process, and emphasize the importance of integrating various performance analyses to improve bit efficiency without degrading critical steering characteristics. Extensive development of cutting structures that "drilled on paper" combined with full scale bit laboratory testing has resulted in the development of a design process that combines multiple design objectives and criteria to model and simulate bit behavior during drilling. As shown by field results, this process of balancing the design provides the means to optimize overall performance and lower costs for many different bit applications.