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.
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.