Abstract

This manuscript is submitted as an addendum to SPE/IADC 79797¹, which discussed emerging successes using a new fixed cutter bit to drill hard rock applications conventionally drilled with roller cone bits. The new bit design using thermostable polycrystalline (TSP) diamond cutters and stability enhancing features has produced results, particularly in the Carthage field in Panola County, which are revolutionizing hard rock drilling in several areas of East Texas. The paper focuses on how teamwork, selective testing programs, and product improvements have combined to create step change improvements in hard-rock drilling capabilities. The paper discusses new information on optimal operating parameters and includes case history examples.

Background

The use of fixed cutter Polycrystalline Diamond Compact (PDC) drill bits in the hard rock applications of East Texas has been limited. These sandstone formations, conventionally drilled with IADC 647-837 roller cone insert bits, are typically too hard, abrasive, and heterogeneous for PDC bits to be a practical drilling option. Significant amounts of pyrite and chert found in the formations further limit drill bit selection.

SPE/IADC 79797 discussed how a new bit design featuring synthetic diamond cutters and a 360° full-contact gauge for stability demonstrated potential cost-per-foot savings in some East Texas fields. However, due to limited PDC bit design experience and less than optimal running parameters in these variable and challenging formations, only marginal economic improvements were realized. Since SPE/IADC 79797 was published, continued experimentation has succeeded in defining optimum operating parameters.

This operational knowledge, including a lower-vibration running technique (specifically an RPM increase) was the single-most important improvement in performance. Contrary to conventional wisdom, applying higher rotary parameters (in this case, raising the average RPM from 60 to 90) resulted in a significant improvement in penetration rate and footage drilled. Secondary parameter changes involving weight on bit (WOB) and torque control also played a role in optimizing the running parameters.

Selective testing throughout East Texas using these optimized parameters has subsequently set numerous field records and resulted in significant financial savings.

Introduction

Since its initial testing phase, the stabilized ring bit equipped with enhanced NPI cutters¹ has been widely used throughout the East Texas-North Louisiana region. The earliest runs showed performance improvements over insert bits in intervals where standard heavy-set PDC bits had not been successful. However, due to lower rates of penetration in many hard rock applications, the savings varied from a marginal advantage to none at all. Still, these early runs demonstrated that PDC bits could be cost-effectively employed in hard and abrasive applications. The next challenge was to find new application techniques that would justify commercial usage of the bits.

Design and Technology Advances

Several attempts have been made to drill the hard and abrasive intervals of the region's Travis Peak and Cotton Valley formations using fixed cutter PDC bits. Most of the efforts were not cost effective and often resulted in high levels of abrasive wear or major impact damage from bit whirl or slip-stick conditions.