Advanced Cutting Structure Improves PDC Bit Performance in Hard and Abrasive Drilling Environments

Abstract

Abstract The performances of PDC bits in hard formations, especially when these formations are also abrasive, still lack effectiveness and consistency. In such applications, the industry has usually resorted to the use of "insert type" roller cone (RC) and impregnated diamond bits (ID) - due primarily to their performance consistencies. Impregnated bits, usually, exhibit the durability characteristics needed to drill long footages in hard and abrasive formations. However, due to their grinding rock removal action, penetration rates (ROP) are typically low, commonly in the (3 - 9 fph) range. These low ROPs experience rapid deteriorations, once argillaceous and/or shale bearing formations are encountered. These conditions reduce the operational efficiency of impregnated bits, driving up drilling costs. From an ROP perspective, roller cone bits perform better (6 - 12 fph) than impregnated bits in hard and abrasive formations. In addition, the ROPs of RC bits experience minimal decay in argillaceous and/or shale bearing formations. However RC bits are commonly tripped on hours and, do not exhibit the longevity needed for hard and abrasive environments. Considering the depths at which such hard and abrasive formations are usually encountered, trips increase operational hours, HSE risk and thus substantially add to drilling cost. Due to their depositional depths, hard and abrasive formations are sometimes encountered in HP/HT environments. This condition often necessitates the use of high mud weights, or BHA's that incorporate high speed drive mechanisms - PDMs or turbo-drills. Such conditions, necessary to improve operational efficiency, further complicate the drilling environment - affecting the ROP and/or durability of all drill bits. This paper discusses an innovative PDC bit, which has specifically been developed to improve operational efficiency in hard and abrasive formations. This new generation PDC bit effectively addresses the ROP limitations of impregnated bits, as well as the footage deficiencies of roller cone bits. In addition, these bits have been developed to be effective, even when hard abrasive formations are drilled with high mud weights and/or with high speed drive mechanisms. Background The industry currently defaults to roller cone and impregnated bits, when it comes to hard and abrasive formation drilling. This situation has been predicated by past PDC bit experiences in these applications, which have consistently been poor. Heavy set PDC bits1 (Figure 1) - having high blade counts, small PDC cutting elements and high cutter counts - have typically been used in these applications. These bits have high total diamond volume, which was expected to improve durability, and thus longevity and run length. Such a gain, when combined with the known ROP potential of PDC bits, was expected to improve drilling performance in hard and abrasive formations. However, without a proper understanding and interpretation of the conditions needed for success in such applications, the performances of such "heavy set" bits always left much to be desired. As has been shown by their ineffective performances, heavy set bits do not necessarily posses the attributes needed to be effective in hard and abrasive formations. Specific characteristics and functional behaviors have to be exhibited by PDC bits, to make them effective in these applications. Research efforts, focusing on this specific application, have identified the requirements and conditions needed to improve PDC bit performance. The effects of operational parameters, specifically WOB and RPM, have been isolated and analyzed. The relationships between ROP, WOB and wear flat generation have also been quantified. In addition, the effects of RPM and time on wear flat generation rate have also been evaluated. The positives, as well as limitations of the resulting operational medium, from a vibration perspective, have also been analyzed.