High Penetration Rates and Extended Bit Life Through Revolutionary Hydraulic and Mechanical Design in PDC Drill Bit Development
- M.R. Taylor (Hycalog) | A.D. Murdock (Hycalog) | S.M. Evans (Hycalog)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 1999
- Document Type
- Journal Paper
- 34 - 41
- 1999. Society of Petroleum Engineers
- 1.6 Drilling Operations, 1.5.1 Bit Design, 1.5 Drill Bits, 4.3.4 Scale
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PDC drill bit developments have been made to achieve higher penetration rates and longer life, involving a compromise between open, light set, bits for speed, and heavy set ones for durability. Developments are described which provided the benefits of both in a revolutionary hydraulic and mechanical design. The hydraulic design causes mud to flow first towards the bit center and then outwards. It was extensively flow tested using high-speed photography to ensure that bit balling was prevented. It includes features to address bit whirl which were demonstrated in full scale laboratory testing to reduce the bit's vibration level. The mechanical design maximizes open-face volume, known to benefit penetration rate, by using very high blades. However, the heights attainable can be limited by the bit body's mechanical strength. Steel was chosen to maximize blade strength and was coated with a newly developed hardfacing to improve erosion resistance. A program of fatigue testing assured adequate strength.
It is well-recognized that each generation of PDC bits has produced better performance than its forebears, with one author claiming 60% improvement in rate of penetration (ROP) and 115% improvement in life between 1989 and 1995.1 These improvements have fueled the growth in use of this bit technology. However, this capability has in turn fed a demand for ever more difficult well programs which continue to demand improvements in bit performance. The economic benefits of such well designs have been demonstrated by Barr and Clegg.2 One of the biggest challenges for the bit is to drill from shoe to total depth (TD) through a range of formations from soft to hard and sticky to abrasive, or a homogeneous formation of increasing hardness. Once this has been achieved, the design is developed to improve the penetration rate through the interval. Typically, a bit design is optimized for a particular application.
The bit designer has many ways of optimizing a particular design for a given application. A feedback loop from analysis of field performance and dull condition has been shown many times to be a successful strategy in obtaining improvements. From this has come a growing recognition of important parameters. These include cutter count, hydraulic efficiency, and open face volume.3 Unfortunately, these can conflict. For example, increasing the cutter count extends bit life but is difficult to achieve without reducing the hydraulic efficiency of the design and thus the rate of penetration. Many bit designs have been produced which alter the balance between these considerations but few attempt to resolve the inherent conflict.
The basic constraints within which the bit is designed are defined by the materials of which it is made. Most PDC bits today are manufactured from steel or an infiltrated matrix of tungsten carbide powder. These have very different properties. Matrix provides excellent surface properties, with an exceptional ability to resist fluid erosion. Fluid erosion is caused by loss of the soft binder alloy which then loosens the tungsten carbide powder grains from the matrix. The erosion resistance is increased by increasing the volume percentage of tungsten carbide but this in turn leads to problems. If this percentage becomes too high the poor transverse rupture strength of the tungsten carbide limits the mechanical stresses that the material will sustain, particularly in shock load conditions. In terms of bit design, this means that a matrix-bodied bit will tend to have lower and thicker blades, reducing the open-face volume that can be achieved. It could, therefore, be argued that matrix-bodied bits are best suited to long intervals of relatively slow drilling with correspondingly long periods of mud circulation and in which shock loads are avoided.
Steel, in contrast, has quite different mechanical properties. Its erosion resistance is relatively poor, particularly in fluid velocities above 100 ft/s or when the fluid contains highly abrasive solids such as haemetite. However, its transverse rupture and impact strength are much higher than those of the matrix. These properties allow steel bits to have higher, thinner, blades than matrix bits, with the consequent benefits to the rate of penetration. This suggests that steel-bodied bits are best suited to applications where the open-face volume can contribute to high penetration rates. The number of circulating hours to which the bit is exposed before completion of the interval is limited by the high rate of penetration, a virtuous circle.
The demands of today's well programs mean that it is neither practical nor desirable to distinguish applications so crudely. Accordingly, bit manufacturers have sought to address the weaknesses of the matrix by incorporating reinforcement into the matrix casting; to improve the surface properties of steel bits by cladding, or to seek alternative materials.
Cladding and reinforcing are alternative embodiments of the same basic concept—the structural strength of steel is combined with the surface properties of the matrix. However, cladding is inherently more attractive because the coating of erosion-resistant material can be thinner, thereby providing the bit designer with greater freedom to maximize the open-face volume. Many claddings have been developed but two weaknesses are apparent in most. First, the density of tungsten carbide powder is much lower than in the infiltrated matrix, limiting the erosion resistance of the bit body. Second, the cladding does not intimately contact the PDC cutters because of a meniscus effect when it is laid down.
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