Beyond Gauge Retention: Diamond Chisel Inserts Provide Further Performance Benefits on Roller Cone Bits
Authors: Doig M.J. , Bonnett N. , Cardno D.G. 
Published: December 2007

SPE/IADC Members

Abstract
The development and successful application of the diamond enhanced insert, in roller cone bits, has been much vaunted over the past decade, in maintaining gauge. The initial difficulties in applying the original flat diamond inserts which were developed for use on PDC bits to actively cut gage on a roller cone bit had to be overcome. The beneficial durable qualities of the flat diamond enhanced inserts could only have been applied in a narrow field of operation with restricted benefits. This was a function of the shape and profile of the diamond enhanced inserts available.
As technology has advanced, the shape and profile of these inserts has progressed and the uses and benefits in the drilling process have extended beyond simple gauge retention duties. The changing geometry of the insert's face is crucial to their success. From the original flat profile cutters, the technology has made progress in building the curve of the diamond enhanced surface through slight dome structures, to hemispherical crests and beyond to asymmetric inclined chisels.
Testing began on the diamond enhanced semi round top (DESRT) insert on roller cone bits in 1984. This meant that the durable characteristics of the polycrystalline diamond enhanced surface could be used on the hole bottom in more beneficial ways than were previously possible. The technological leap, following the introduction of asymmetric diamond enhanced inclined (DEI) chisels, is apparent from field data results. These results show positive benefits in the areas of ROP, general bit durability, steerability in directional wells, enhanced gauge retention duties and bearing effectiveness, all of which reduce drilling costs. Extensive UK sector field data gathered since 1994 is discussed with reference to the effects of this technology.

Introduction
The ability to drill wells effectively, efficiently and economically can be severely impaired by the inability of a bit to hold and maintain gauge while making hole. This is caused by a combination of factors but primarily the abrasive qualities of the formation causing premature degradation of the material positioned on the bit to cut, abrade and ultimately hold gauge.
The poly crystalline diamond (PCD, hereafter diamond) coating on tungsten carbide inserts typically fails due to its lower toughness. However, Figure 1 illustrates, the superior hardness and high thermal conductivity of diamond, coupled with it's low friction coefficient in comparison to tungsten carbide making it an attractive material for wear applications, such as gauge maintenance. By optimizing the diamond composition to enhance strength and toughness and by modifying the existing insert geometry, the performance of diamond enhanced inserts in soft to medium / hard formations can significantly improve upon the previously attainable results available with tungsten carbide.
Gauge Rows. The distinction must be made between gauge and heel rows as defined by the IADC. This paper refers to the effect of changing materials and geometry specifically on the gauge row. The effectiveness of any gauge cutter on a roller cone bit is a function of: the geometrical shape; the material used; the angle of attack at the gauge; and the geological characteristics of the formation.
As formations become harder and / or more abrasive the factors and materials employed to cut and maintain the gauge encounter greater challenges. The qualities and abilities of the inserts on the gauge are distinctively different to those inserts used to cut the hole bottom. Unlike the wear resistance required on the hole wall for gauge cutting, the hole bottom cutting component of the gauge insert requires impact resistance and geometrical shapes which will gouge and scrape, especially in softer formations.
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