Addressing The Drilling Challenges Of Midcontinent Chert

Drilling Woodford wells in Southern Oklahoma increases risks of PDC cutter damage caused from high chert content. This abrasive formation can lead to fast bit wear and result in broken cutters through thermal degradation. Changes in lithology and compressive strength create impact damage caused by bit and bottomhole assembly vibration. The thin strata of black and cherty shale can require a higher number of trips and damage aggressive bits beyond repair, proving costly for Midcontinent operators. Challenges drilling through the Woodford Formation have increased demand for bits and cutters with improved durability for better performance. Advances in application-specific cutter and bit designs have enabled operators to increase success in the Midcontinent area. Proprietary bits and cutters from National Oilwell Varco (NOV) have allowed operators to decrease damage, increase ROP and drill a higher average interval compared to offsets within the area, saving drilling time and contributing to operational savings.

Design process
For more than two years, bit designers and engineers at ReedHycalog, a business unit of NOV, continuously tested application-specific bits and cutters in the Woodford application, iterating and creating new designs based on performance results. The introduction of the SK713M series of bits met the initial call for durability and provided an immediate benefit in footage drilled. In initial 2016 test runs in the upper Woodford application, a customer partnering in product advancement trialed numerous runs with the initial frame and the use of existing premium cutters. 

While drilling through the chert, the frame experienced damage in the cone, prompting bit designers to alter the cutting structure to include ReedHycalog Torc Blocks in the cone. This step change coordinated with implementation of an aggressive cutter testing program, which ReedHycalog worked on in partnership with a large PDC cutter vendor. Feature additions and small modifications to the first generation of ReedHycalog bits made a positive step change in durability, with the trade-off of reduced ROP in the targeted application.

A test run using this new layout in the SKC713M reduced cone damage and eliminated core outs but resulted in a 10% reduction of ROP in the application. ROP and durability rarely coexist in most drilling applications, and that is true in the Woodford application as well. Designing bits that can maintain durability requires sacrificing certain design features that lend to high ROP. In 2018, following continuous adjustments to the cutter designs, the proprietary layout and features enabled operators to decrease the number of runs and increase footage by 50% while achieving top-tier ROP results.

Designers and engineers introduced an alternating cutter pattern that also included Torc Blocks in the cone. The current version of the layout features a mix of 3-D-shaped and full round cutters placed strategically to maximize the potential benefit of each cutter type. The ReedHycalog cutter technology offers a combination of thermal, abrasive and mechanical durability. Ion 3-D shaped cutters provide higher point-loading capabilities, resulting in a more efficient cutting structure. The mix of shaped and full round cutters maximizes the benefits from each cutter type, resulting in significant performance improvement.

The concept and success of the TKC73 and SKC713M series of bits from ReedHycalog have proven beneficial for operators drilling in the Scoop Woodford application. Compared to the standard offset in Woodford applications, the bit has enabled higher ROP with fewer trips.

Case studies
In a fourth-quarter 2018 Scoop Woodford lateral where interval drill exceeded 1,219 m (4,000 ft), the 8.75-in. TKC73-D1 drilled the fastest interval and the second longest footage drilled in a 4.8-km (3-mile) offset. The total interval drilled was 1,894 m (6,215 ft) at 17 m/hr (58 ft/hr) and the bit was pulled repairable. The best competitor offset run lacked 25% of the footage drilled and was nearly 30% slower in ROP. The top offset for footage drilled was the SK713M-K1, a design used prior the TK73. This run exhibits the continuous effort to iterate new, successful designs for this application. 

In another fourth-quarter 2018 run in Garvin County, the 8.75-in SK713M-K1 with an alternating cutter layout was run in a Woodford C lateral. The bit drilled 2,575 m (8,448 ft) at 15 m/hr (49 ft/hr), achieving the longest interval drilled in the Woodford C zone when compared to wells drilled by the operator in a 4.8-km radius since January 2018. The performance nearly doubled the best competitor offset and held great ROP, outperforming designs with lower blade counts while maintaining durability.

In the first quarter of this year, the 8.75-in. SKC713M-K1 drilled the longest Woodford interval within a 4.8-km radius compared to offset bit runs at depths greater than 4,877 m (16,000 ft). The bit drilled 1,839 m (6,033 ft) at 13.3 m/hr (43.6 ft/hr) and was pulled out in good condition at target depth. This was the top offset run by footage drilled by nearly 30% and was the second highest ROP recorded in the offset, only finishing second to the same design on a shorter offset run.

Conclusion
The latest success using bit and cutter designs from NOV opens opportunities to improve drilling performance in abrasive Woodford applications. As the market matures and operators become more knowledgeable of drilling parameters in the Woodford, bit designers at NOV will continue adjusting designs and make accommodations for higher speed motors and higher torque values. Regional design engineers will focus on field results to develop application-specific designs.

Working closely with cutter vendors, the ReedHycalog team will continue to analyze dulls and provide feedback from performances and testing results. This process with cutter vendors will drive new cutters that focus on addressing the needs of not just the application but possibly the specific position on the bit.

The locally based engineering team will continue to work with the sales team to conduct internal cutter testing, enabling trial of new shapes and efficient turnaround of products optimized for continuous improved performance and more cost-effective drilling in the Midcontinent region.