Abstract

Historically, the hardest and most abrasive rock formations, such as the Travis Peak, Hosston, and Bossier Sands areas of East Texas and North Louisiana, were drilled almost exclusively by roller-cone insert bits. With some exceptions, the only polycrystalline diamond compact (PDC) runs were small-diameter holes where the formation was predominantly comprised of shale and limestone. Significant advances in design and cutter technology in PDC bits during the past 5-10 years have made it possible for PDC to be used more frequently on the fringes of these hard rock intervals.

Combining advanced PDC bit designs with state-of-the-art cutter technology has opened up many new applications for PDC bits that were once only appropriate for IADC Series 6, 7, and 8 insert drill bits. Because of this new technology, PDC has become a practical alternative for drilling hard rock, providing a significant reduction in cost per foot. Combining new, highly abrasion-resistant PDC cutters with competent stable bit designs that provide both low torque and dynamically stable characteristics was the key to this development.

Several case histories of hard rock drilling using PDC bits will be discussed in this paper, illustrating the reduction in costs. These case histories demonstrate, through descriptions of trial and error, the problems associated with these new PDC applications and how they were overcome.

Background

Until recently, insert bits were mandatory when drilling hard rock intervals in areas of East Texas and North Louisiana that have significant sandstone sections. This paper will discuss trial applications of PDC bits designed for drilling such hard and abrasive formations as the Travis Peak, Hosston, Cotton Valley, and Bossier in this area. The areas and intervals discussed in this paper have historically been drilled best with insert bits; however, because of advancements in cutter technology and drill bit design, operators have recently been testing PDC bits. In some cases, the PDC bit option offers considerable savings.

In the past 10 years, PDC bits were widely used in these areas for drilling the softer and less abrasive shallow sections to depths of 6,000-9,000 ft. These formations were found to be very PDC drillable, being composed of shale, limestone, and anhydrite, with minor amounts of sand. Upon reaching the hard and abrasive sandstone sections, however, insert bits became necessary in most areas. This interval may extend to 3,000 ft or more, as in the Travis Peak and Cotton Valley. This section alone may represent 60-80% of the drilling costs for an entire well.

The use of PDC bits has consistently and significantly reduced drilling costs, but this was usually in softer formations. Occasional attempts to drill with a PDC bit in the hard rock intervals typically resulted in poor rate of penetration (ROP) and footage results, with dulls exhibiting extensive bit wear. However, in some areas, such as the first 700 ft of the Travis Peak in Panola County, PDC has been used successfully.¹ These areas tend to have formations composed more of shale, with sands being less consolidated in nature. Another successful example is the use of PDC for intervals with slimhole size. Because of the smaller radial diameter of slimhole size bits, less bit wear is a direct result of the slower rotational velocities, which tend to wear less on the cutting structure. The decreased wear is typically reduced in the outer diameter radius area only, where abrasive wear is lessened in the small hole versus the same formation being drilled in a significantly larger hole size.

Impact damage to the PDC cutting structure caused by bit vibration is a common problem associated with drilling hard rock with PDC. In heterogeneous lithologies, cutting structure failure can result from vibration forces generated when the bit is subjected to variations in acceleration. Because of the cutting mechanism involved, PDC bits are more likely to be affected by these forces than roller-cone bits. (There is much literature available on the subject of vibration, which includes bit whirl and slip stick. These phenomena are collectively referred to as bit and drillstring vibrations. They include lateral, torsion, and axial vibrations of the bit and drillstring.) ^2,3