Failing the rock and removal of failed rock are the two primary components of controlling drillbit efficiency. The main driver of efficiency varies across rock types; in sandstone, for example, failing the rock is more important, while in shales removing the rock is critical. A proper design will ideally address both requirements but focus on whichever of the two is the major efficiency driver. In response to this, National Oilwell Varco (NOV) developed ION shaped cutter technology, where the geometry of the PDC cutter is designed according to the rock type.

Differences in rock failing

Shale exhibits ductility under confining pressure and cuttings form in the shape of long ribbons that extrude up the face of the cutter, resulting in energy losses due to friction at the cutter-to-cuttings interface. The long shale ribbons are hard to evacuate, resulting in the creation of a cuttings bed in deviated and horizontal wells. In some cases the long ribbons coalesce to form a ball on the face of the bit and junk slot area, which causes a significant drop in ROP and costs the operators a trip to clean the bit.

Failing the rock itself carries a bigger weight than the removal of cuttings in sandstone as the rock mostly fails in brittle mode, and cuttings are in the form of small chips. In carbonates both cuttings creation and removal need to be addressed as the rock strength is medium to high and rock fails in ductile mode under pressure. However, the rock ductility is lower than shale.

Salt is another rock that is typically drilled with PDC bits. Fracture initiation and propagation were investigated by triaxial testing and compared with pressurized drilling tests. Researchers concluded that the rock fails in ductile mode and continues to carry the load even after the initial failure, most likely due to the microstructural bond between salt particles.

Customized geometries

ION shaped cutters combine NOV’s PDC cutter technology, enhanced with the latest leaching techniques, with unique application-based geometries. All the new geometries were optimized for maximum rock-cutting efficiency while maintaining cutter durability via analytical and numerical cutter-rock-cutting interaction modeling, finite element analysis, computational fluid dynamics and pressurized drilling tests on different rocks, including limestone, sandstone, shale and salt.

The first geometry developed was the ION 2D for use in shale applications. Adding a large chamfer to the noncutting side of the cutter reduced contact surface between cutter and cuttings. This chamfer directs the flow toward that interface to help with breakage of cuttings ribbons. Pressurized laboratory testing indicated a 20% improvement in ROP using these cutters. More importantly, a reduction in the typical size of cuttings ribbons from 3 in. to 4 in. to less than 1 in. This reduction resulted in improved hole cleaning and mitigation of global bit balling.

NOV also developed ION 3D and ION 4D cutters for better fracture initiation and propagation. Both cutters are designed to increase stress concentration in the formation by point loading. Point loading from an ION 4D cutter comes from a chisel-shaped tip combined with a plow-shaped nonplanar geometry that also includes a chamfer on the noncutting side to maximize point loading while enhancing cuttings evacuation. Pressurized laboratory testing in different rocks including halite, Torrey Buff sandstone, Carthage marble and Catoosa shale indicated not only up to a 61% increase in ROP for the same weight-on-bit but also up to a 34% increase in ROP when the torque is kept constant. The latter is especially crucial to improve efficiency and aim for higher ROP in applications where surface torque is a limiter. Some examples of such applications include the salt section in the deepwater Gulf of Mexico and long laterals in unconventional reservoirs where frictional energy losses in the drillstring consume the majority of available torque at the surface.

Case studies

In the San Juan Basin, an operator ran two 8¾-in. ReedHycalog TK56 bits, one utilizing ION 2D shaped cutters. The first bit drilled 1,273.4 m (4,178 ft) in 18 hours for an average ROP of 70.7 m/hr (232 ft/hr). With input from the ReedHycalog team, the operator elected to run the TK56 with ION 2D cutters for improved cuttings evacuation with the high instantaneous ROP that they were able to achieve. The bit drilled 1,174.4 m (3,853 ft) in 12 hours for an average ROP of 97.8 m/hr (321 ft/hr), resulting in a new record 8¾-in. section for the operator.

In a 12¼-in. section in Oman, an operator was drilling an interbedded formation with lithologies ranging from limestone and sandstone to dolomite and shale. A TK66 bit was designed for the application and equipped with ION 3D cutters after close collaboration between the operator and NOV team. The section, which was normally drilled in two runs with average ROP below 14.9 m/hr (49 ft/hr), was drilled in one run with ROP more than 29.8 m/hr (98 ft/hr).

ION 4D is the newest NOV shaped cutter and is in field testing. An operator in Pinedale, Wyo., decided to use ION 4D cutters in an interbedded application that consisted of sandstone, limestone, shale and salt with unconfined compressive strength ranging from 10 ksi to 30 ksi. An 8¾-in. six-blade bit with ION 4D cutters drilled 601.6 m (1,974 ft) in 12.27 hours, resulting in an overall average ROP of 49 m/hr (160.9 ft/hr)—45% faster than the average offset ROP of 33.9 m/hr (111.3 ft/hr). This also was 15% faster than the fastest offset run regardless of bit design. Also, when compared with the same bit design without ION 4D cutter, this bit was 19% faster.

As there continues to be a need to drive down drilling costs, the use of more efficient PDC cutters is inevitable. The current pricing environment and the growth of unconventional shale has made the development of new shaped PDC cutters a necessity. Supported by extensive field testing and R&D, NOV’s new cutters advance both cutter-rock interaction modeling and testing and cutter material and manufacturing technology. By adopting such technologies, the industry will see the type of performance improvements needed to maximize value in challenging drilling environments.