The present disclosure relates generally to earth-boring drill bits used to drill a borehole for the ultimate recovery of oil, gas, or minerals. More particularly, the present disclosure relates to drag bits and to an improved cutting structure for such bits. Still more particularly, the present disclosure relates to cutter elements for use on drag bits and methods of manufacturing such cutter elements.
An earth-boring drill bit is connected to the lower end of a drill string and is rotated by rotating the drill string from the surface, with a downhole motor, or by both. With weight-on-bit (WOB) applied, the rotating drill bit engages the formation and proceeds to form a borehole along a predetermined path toward a target zone.
Many different types of drill bits and cutting structures for bits have been developed. Two common types of drill bits are roller cone bits and fixed cutter bits, also known as drag bits. Fixed cutter bit include a plurality of blades angularly spaced about the bit face. The blades generally project radially outward along the bit body and form flow channels there between. Cutter elements are grouped and mounted on several blades, typically arranged in rows along the leading edge of each blade. However, the configuration or layout of the cutter elements on the blades may vary widely, depending on a number of factors.
The cutter elements mounted to the blades of a fixed cutter bit are typically formed of extremely hard materials. In the typical fixed cutter bit, each cutter element comprises an elongate cylindrical tungsten carbide support member and a hard cutting layer bonded to the end of the support member. The hard cutting layer is typically made of polycrystalline diamond (PD) or other superabrasive material such as cubic boron nitride, thermally stable diamond, polycrystalline cubic boron nitride, or ultrahard tungsten carbide (meaning a tungsten carbide material having a wear-resistance that is greater than the wear-resistance of the material forming the substrate) as well as mixtures or combinations of these materials. The support member is received and secured in a pocket formed in the surface of one of the several blades with the cutting layer exposed on one end. For convenience, as used herein, reference to “PDC bit” or “PDC cutter element” refers to a fixed cutter bit or cutting element employing a hard cutting layer of polycrystalline diamond or other superabrasive material such as cubic boron nitride, thermally stable diamond, polycrystalline cubic boron nitride, or ultrahard tungsten carbide.
Conventionally, the hard cutting layer of PD is formed by subjecting a graphite powder containing catalysts such as cobalt (Co) or nickel (Ni) to high temperature and pressure conditions to transform it into a diamond powder comprising a plurality of irregularly and inconsistently-shaped crystals. The resulting diamond powder is then, for example, applied to a tungsten carbide support member by way of a sintering or other thermo-mechanical process to form a PDC cutter element.
During drilling operations, cutter elements experience relatively high temperatures due, at least in part, to the general nature of the downhole environment and friction between the cutter elements and the formation. The thermal loads result in expansion of the material components of the cutter elements. Due to differences in the coefficients of thermal expansion between the binding agent and the diamond grains, at sufficiently high temperatures, undesirable cracks may form in the PD layer. Such cracks can lead to failure of the cutter element, reduced cutting efficiency and reduced cutting effectiveness. Accordingly, prior to the diamond powder being sintered to form the PD layer, it is usually leached to remove the catalyst (e.g., Co or Ni). The PD layer may also be leached after formation to remove catalyst (e.g., Co) that infiltrated the PD layer from the tungsten carbide substrate during the sintering process. However, leaching may not remove all of the catalyst, and thus, the PD layer may be susceptible to undesirable cracking during drilling operations.