As the exploration and development of hydrocarbons intensifies across North America, bit designers have introduced two new products to maximize drilling performance; a conical diamond element fails the rock more efficiently at the bit center thereby increasing ROP while a polycrystalline diamond compact (PDC) cutter that rotates on its axis significantly extends run lengths by using its entire circumferential diamond edge.

Conical diamond elements improve ROP

Efforts to develop a more efficient PDC drill bit have focused on the bit’s cone area because of two inherent inefficiencies: limited rock removal at the center of the wellbore from relatively low rotational velocity of the center cutters and the potential of bit torque fluctuations from large variations in the depth of cut as center cutters bear the highest load on the bit. Both inefficiencies can limit ROP and make drill bits more susceptible to the damaging effects of lateral/torsional shocks and vibration.

image- reconfigured bit with the Stinger element

By reconfiguring the bit with the Stinger element, a column of rock is allowed to form at the center of the cutting structure, which is continuously crushed and fractured, increasing drilling efficiency. (Images courtesy of Schlumberger)

Designers theorized that removal of these central cutters would improve the overall shearing efficiency of the bit by allowing a stress-relieved rock column to develop and then be destroyed using the Stinger conical-shaped polycrystalline diamond (PCD) element. This new element has a unique conical geometry combined with an ultra-thick synthetic diamond layer that is twice as thick as those on conventional PDC cutters. Its material is engineered to provide superior resistance to abrasive wear and impact loading. To maximize drilling performance, an integrated drill bit design platform software was used to optimize the cutting structure and model cutting behavior.

The conical element is positioned at the center of the cutting structure with the conical tip pointing vertically down toward the rock. As the bit rotates, the element engages the rock column, which is continuously point-loaded with high axial compression to fracture and destroy it. This mechanism enables the bit to deliver faster ROP and reduce the potential for vibrations. Fracturing of the rock column also creates larger cuttings fragments for improved formation analysis.

Laboratory testing was conducted to verify the bottomhole profile and investigate bit behavior with the conical diamond element bits. Using a full-scale pressurized drilling simulator, different rock formations were drilled with conventional PDC drill bits and bits equipped with a conical element. The bottomhole pattern created by the different bit types was examined, confirming that the bit with the conical element had generated a small-diameter rock column at the very center of the borehole. The cuttings collected during the drilling were much larger than those generated from the conventional PDC drill bit.

Before field trials were started, engineers identified existing bit designs that were meeting expectations but concluded that further enhancements were desired to improve overall drilling performance. These designs were then modified to include the conical element and released into the field for performance evaluation. A detailed study of more than 30 individual runs in the western US demonstrated that more than 50% of the runs with the new drill bit were rated as delivering above-average ROP, with an average ROP gain of 20% over offset bit runs. In the Texas and Oklahoma Panhandle region, the average ROP improvement was 25%.

In the Williston basin in North Dakota, field trials of the conical element demonstrated even better performance with the PDC drill bit removed from the hole in good dull bit condition, exhibiting no wear on the conical element. The new drill bit was run on a steerable-motor bottomhole assembly (BHA) in similar vertical-hole applications through a highly mixed and interbedded sequence of formations, including sand-shale, salt, and limestone/dolomite/ anhydrite, with an unconfined compressive strength range from 2,000 psi to 25,000 psi.

A conventional 8?-in. PDC drill bit without the conical element from Smith Bits drilled a 1,995-m (6,583-ft) section to kickoff point at an ROP of 29.4 m/hr (97 ft/hr). In contrast, a PDC drill bit fitted with the conical element drilled similar interval lengths but with an average ROP of 51 m/hr (168 ft/hr). The runs included an operator record ROP of 61.5 m/hr (203 ft/hr) in one well. Overall, the average ROP of drill bits fitted with the conical element was 46% higher than the next best average ROP of 34.8 m/hr (115 ft/hr) reported by other drill bits used in offset wells.

Rotating PDC cutter extends bit life

Despite 40 years of prolific use, however, there still exists an inherent limitation of the fixed PDC shear element – only a relatively minor portion of the cutter’s edge contacts the formation. As a result, only a small part of the entire cutter contributes to the bit’s durability and drilling performance.

As the fixed cutters engage the formation, the cutters wear and chip due to mechanical and thermal effects. This continual degradation of the cutters’ edge causes a loss in shearing efficiency that in time terminates the bit run.

The ONYX 360 rolling PDC cutter was developed following several years of R&D to determine the ideal material, rolling mechanism design, and proper attachment for improved drilling performance. The rolling PDC cutter is mounted in the bit body and allowed to rotate freely. As the cutter revolves in its pocket during drilling, the entire circumference contacts rock, which redistributes wear. This ensures that the diamond cutting edge remains sharper longer during drilling. Because these are cooled more efficiently, the cutters survive longer downhole, leading to improved bit life. Extensive laboratory testing was conducted to evaluate the durability of the rolling cutter. Drilling a granite test sample, the new cutter demonstrated a six-fold increase in cutter life compared to premium fixed cutters. Dull analysis of the rolling cutter demonstrated that it had fully rotated, displaying even wear over its entire 360° circumference. To field-test the rolling cutter, the technology was deployed in an application where conventional PDC drill bits were experiencing heavy wear over relatively short run intervals.

The Granite Wash formation is composed of quartz and feldspar with quartz/calcite cementation. The formation is often developed using a steerable motor BHA to drill a long lateral for increased reservoir exposure.

PDC drill bits were mounted with rotating PDC cutters placed adjacent to fixed cutters to evaluate the performance and wear characteristics of the rolling cutter. These bits delivered consistent increases in footage of 20% or more in the Granite Wash formation, and dull evaluation of the cutters showed no cutter loss and 100% cutter rotation. Regular fixed PDC elements showed wear flats T1 or greater, while the rotating PDC cutters showed light uniform scarring over the entire diamond edge.

Advancing PDC technology

Through greater ROP and enhanced bit durability, conical-shaped PCD elements and rotating PDC cutters are aiding operators whose drilling programs have helped reverse the decline in US oil output.