Improvements in polycrystalline diamond compact (PDC) cutter technology have contributed immensely to the industry's acceptance of PDC bits as effective drilling tools. These cutters are being engineered to address the needs and requirements of different drilling programs. Extensive research and development efforts have been dedicated to the analysis of the diamond table/tungsten carbide interface.

Development of non-planar interface geometries has enhanced PDC cutter performance. Additionally, the identification, quantification and optimization of PDC cutter characteristics, such as impact and abrasion resistance, thermal stability, interface shear strength and fatigue life has improved performance.
Conventionally, the cutter characteristics listed exhibit inverse relationships. As such, improvement in any particular characteristic has always been made at the expense of the others. To further improve PDC bit performance and extend applications to harsher environments, it is critical that new relationships are established between the performance characteristics of PDC cutters.
PDC cutter development
For many years, the diamond table thickness of PDC cutters has been limited to about 0.6mm, or 0.025in. (Figure 1). This is because researchers established a relationship between a cutter's damaging residual stresses and its diamond table thickness. During sintering, residual stresses are created in PDC cutters, the magnitudes of which increase with diamond table thickness. These stresses initiate chipping, spalling and delamination of the diamond tables, thus reducing PDC cutter life and bit efficiency.
Non-planar interface technology redefined the relationship between diamond table thickness and a cutter's stress levels. When effectively engineered, this technology reduces the magnitude of these stresses (Figure 2). This new geometry relocates the damaging stresses away from the diamond table periphery and reduces the stress gradients in the PDC cutter. This technology permitted diamond table thicknesses to be increased without creating the stress relationships previously mentioned. These cutters, which have diamond tables of about 2mm (0.08in.), are classified as standard premium (SP) cutters (Figure 3). Even though these premium cutters have much thicker diamond tables, their specific non-planar interface geometries define limiting diamond table thicknesses. If the thickness of the diamond is increased beyond that required for a specific interface geometry, gained stress reduction benefits previously achieved are lost.
To improve the performance of the industry's premium cutters, a new series of PDC cutters - GeoMax (GM) - has been developed. GM cutters have been engineered to address the specific needs of different applications, lithologies and drilling environments. These cutters are redefining the characteristics of the PDC cutters and setting new standards in PDC bit performance.
The GM series
The GM series is made up of three uniquely engineered cutter types: SonicMax (SMX), GridMax (GMX) and TecMax (TMX). These cutters have dissimilar interface geometries, material technologies and sintering processes (Figure 4). They have the thickest diamond tables in the industry (without compromises to the PDC cutter stress regimes) and have been engineered to improve PDC bit development and performance.
The GM series enables bits to be developed in accordance with the drillability requirements of different formations. Detailed engineering analysis has been incorporated into the bit development process, instead of the conventional trial-and-error approach. Product development requires bit profile and cutter size to be defined in terms of drilling environments and formation drillability, respectively. In addition, cutting structures are being established in accordance with the operator's performance objectives and requirements.
The GM cutters enhance the performance of existing bits, especially rate of penetration (ROP) and durability. The industry notion that PDC bit durability depended on cutter size and blade count is no longer valid. Durability depends on cutter type, not cutter count.
The limitation on diamond table thickness of premium cutters does not hold for the TecMax (Figure 6). This cutter has two diamond tables, a primary and a secondary. The primary diamond table functions in a manner similar to the diamond table on a GM cutter. As wear of the TecMax cutter's primary table progresses during drilling, the secondary diamond table is exposed, which then increases the cutter's diamond volume and extends bit life. The TMX cutter's developmental philosophy is based on the fact that when the impact and abrasion resistance of a cutter are optimized (primary diamond table), any increase in diamond volume (secondary diamond table), extends bit life. The diamond tables on GM series cutters are 70% to 100% thicker than that of the industry's standard premium cutters (Figure 7).
Operational challenges
A project was initiated with an operator in the UK central North Sea to establish the effect of GM cutters in field applications. The selected field was Elgin, which lies in Block 22/30c, and is surrounded by Shearwater (22/30b), Franklin (29/5b) and Erskine (23/26b).
All the fields are of high-pressure, high-temperature (HPHT) nature and have traditionally presented challenging operational problems. The first trial of the GM series cutters was in the 8½in. interval of Franklin's drilling program. Due to the HPHT environment, mud weights are usually high, in excess of 17ppg, while flow rates are kept low at about 280 gpm. The 8½in. section starts at about 17,500ft along hole measured depth (AHMD), in the last member of the Upper Cretaceous group (Hidra formation), or the first member of the Lower Cretaceous group (Rodby formation). The section is about 2,300ft long, and ends in the Franklin and Pentland reservoir sandstones, which are of Jurassic age.
The upper formations in the section - Hidra, Rodby, Sola and Valhall - are easier to drill due to their lithologic composition (Marl limestones and claystones). However, these upper formations pose operational and bit-cleaning challenges due to the hydraulics regime previously discussed. The Jurassic members are the Kimmeridge, Heather, Franklin and Pentland formations. The last two members, the Franklin and Pentland, are hard and abrasive and present bit durability concerns at the high mud weights and lower flow rates of the operation. Technically, two bit types were needed: a lighter set bit for the upper formations and a heavier set bit for the lower reservoir sandstones.
Offset performance
Four PDC bits had been used to finish the 8½in. section on an offset Franklin well. Three of the bits had eight blades and 13mm cutters and used standard industry premium PDC cutters (Figure 8). This was a GeoDiamond product (Type A). The other bit used on this well was a competitor PDC bit, which had eight primary blades and eight secondary blades as backups (Type B) to maximize durability. The (Type B) had 13mm cutters, all of which were of the standard premium type. The premium cutter types used in bit types A and B were different and proprietary to GeoDiamond and the other competitor.
Franklin field comparison
On the second Franklin well, an existing GeoDiamond PDC bit (Figure 9) was fitted with GM series cutters (Type C). The bit was much lighter set, and had six blades and 13mm cutters. This bit drilled faster and longer than any of the heavier set bits previously used on the first well. An additional bit (Type A) was used to finish the section.
On a third well, with a relatively shorter 8½in. section (2,119ft AHMD), a single bit (Type C) with GM cutters finished the whole interval. This bit recorded the fastest Franklin field ROP in the 8½in. section. The fourth well was relatively longer when compared to the third and needed two bits (Type C) with GM cutters to finish the interval. The fifth well needed one bit (Type C) with GM cutters to finish the interval.
Performance comparison in terms of footage, number of bits used per 8½in. section, footage per bit and associated cutter type, ROP and dull grading for the bits used on the Franklin wells is shown in Table 1.
Shearwater field comparison
The experiences gained in the Franklin field's 8½in. section were extended to the adjacent Shearwater field. The challenges of Shearwater are similar to that of Franklin in terms of depth, interval length, formation types, drilling difficulty and hydraulic limitations. On Shearwater well A-6, the manufacturer of the Type B bit supplied a new PDC bit (Type D). The engineering principles previously used to maximize durability of the previous heavy set bit (Type B) were evidently incorporated into (Type D) - high cutter and blade count. Like the previous bit (Type B), the new bit (Type D) had eight primary blades, an additional eight backup blades and was fitted with proprietary premium cutters. This new bit drilled 420ft of the Pentland formation at 7.1 ft/hr. The much lighter set bit (Type C) from GeoDiamond, which had been fitted with GM series cutters, was run to finish the interval. The GeoDiamond bit (Type C) drilled the remainder of the hard and abrasive Pentland formation (1,055ft) at 15.4 ft/hr. Table 2 shows the performance comparison of the two bits used on the Shearwater application.
The concepts used enhanced PDC performance in these challenging applications. Lighter sets with new generation GeoMax series cutters are used worldwide. Outstanding results similar to those recorded on the Franklin and Shearwater drilling programs are being recorded.