Redefining Browse Basin Drilling Performance Through Drill Bit & BHA Design

Abstract The Browse Basin, offshore North-West Australia is characterised by challenging drilling conditions, with formations which result in drill bit damage from interfacial severity, bit balling, reservoir formations prone to abrasive wear, lateral and torsional vibrations. These downhole conditions often necessitate drill bit/bottom-hole assembly (BHA) trips and impede drilling performance. This case study describes how application of the Challenge the Limit™ (CTL) philosophy to drill bit design, BHA design and drilling parameter management for a subsea drilling campaign re-defined a new benchmark in drilling performance. The Challenge The Limit™ workflow begins by assessing performance limiters. Where economically practical, these limiters are designed out of the drilling system to expand operating parameter windows for Weight on Bit (WOB) and rotary speed, yielding significant drilling performance improvement compared to offset wells. Downhole WOB limiters were identified (e.g. structural drill bit limit, BHA buckling limit, jar transition, BHA weight), and each limiter was systematically challenged and re-designed, alongside surface drilling performance limiters such as the solids control system capacity. Stabilisers were re-designed and stabiliser placement adjusted to minimise lateral vibration and maximise the WOB operating envelope prior to drilling dysfunction occurring. Optimising drill bit gauge pad geometry, cutter backrake angles, and nozzle hydraulics mitigated key risks for each formation, enabling increased drill bit longevity and performance. This case study explores the trade-offs in drill bit and BHA design during the campaign, for example how increasing BHA weight to maximise available WOB is traded off against torque & drag, and the balance between drill bit cutting structure efficiency in the unworn stage verses fracture resistance when drilling through formations with interfacial severity. The importance of ensuring the re-design of one limiter does not inadvertently create a new limiter as a secondary effect is demonstrated. Prior to the case study subsea drilling campaign, the 8½″ reservoir section Browse basin drilling performance record was 16m/hr, based on over 45 exploration and development wells. Through the application of Challenge the Limit™ to drill bit and BHA design, the drilling campaign maximised the system operating window, creating a step change and setting a new reservoir section drilling performance benchmark of 31m/hr, across the typically abrasive Plover and Nome formations. The 5 × well campaign finished 79 days ahead of schedule, with 40 days saved through drilling performance alone. This case study contributes to the body of knowledge around improving drilling performance in challenging formations, defining the performance objectives for different hole sections and demonstrating how the Challenge the Limit™ philosophy, when applied to drill bit and BHA design, can enable a new performance benchmark to be defined.