Enhanced Gauge Improves Rolling Cutter Bit Performance in Abrasive North Sea Sands
Authors: Scott D.E. , Zahradnik A.F. , Schmidt S.R. 
Published: October 2008

ABSTRACT
Aided by steady design improvements in bearing and seal technologies, rolling cutter bits now yield excellent reliability under the demanding drilling parameters characteristic of the North Sea. The increased reliability has resulted in the utilization of higher rotary speeds and longer runs in many North Sea applications. Unfortunately, increased life at higher rotary speeds has produced numerous under-gage bits, especially in the abrasive Paleocene sand and the Bunter sand and shale formations of the area.
The development of more abrasion-resistant tungsten carbide grades, coupled with a more durable heel row shape and increased carbide at gage, has produced a bit with substantially improved gage holding ability. This paper describes the development of the new enhanced gage bit and summarizes field experience in North Sea applications. The new design is shown to provide lower cost per meter when compared to the prior generations of bits.

INTRODUCTION
The ability of a rock bit to maintain gage while drilling abrasive formations has for years been a priority for operators and manufacturers. This concern is of particular relevance in the Central and Southern North Sea, where highly abrasive Paleocene sands are normally encountered at about 1680 - 2440 m (5,500 - 8,000 ft). Penetrating these sands frequently results in bits being pulled out of gage, resulting in increased stabilizer wear. This requires the borehole to be reamed each time a new bit is run into the well, which adds to the overall drilling costs.
With the advent of metal seal technology, the reliability of rolling cutter bits was greatly enhanced. This increased reliability resulted in the widespread use of rolling cutter bits in many applications traditionally reserved for PDC bits. The more economical rolling cutter bit was then shown to operate effectively for longer periods under the challenging drilling parameters of the North Sea.
While technology has extended the life of rolling cutter bits, the occurrences of bits pulled out of gage while drilling abrasive formations has increased. To combat this concern, design alterations such as added compacts and diamond enhanced heel rows were undertaken by various manufacturers.
In 1989, a development program was initiated to design a rolling cutter bit that would still exhibit high reliability, while maintaining gage during longer drilling hours at high rotary and/or motor speeds. The addition of more wear resistant carbide coupled with alterations in the bit design resulted in a bit with improved gage holding capabilities. A full gage bit minimizes wear to other downhole components, thereby reducing unnecessary trips, reaming operations, and contributing to lower cost/meter.

CEMENTED CARBIDE BIT DEVELOPMENT
Since the introduction of the tungsten carbide rolling cutter insert bit in 1951, metallurgists have continually strived to develop materials geared for improved insert performance. The first rolling cutter insert bits were developed for the hard and abrasive formations encountered in West Texas and Oklahoma1. Those first inserts were of the durable "bullet", or ovoid shape and were adapted from the grades used in the metal forming industry.